midterm 1 Flashcards
1
Q
do earthquakes occur at predicted/pattern frequencies?
A
no
lots of randomness
where smaller ones happen often and major/great earthquakes happen less frequently
Great (8 or higher)
freq per year = ~1
return period = ~1 year
Major (7–7.9)
freq per year = ~10
return period = ~1 month
Strong 6–6.9
freq per year = ~100
return period = ~½ week
Moderate 5–5.9
freq per year = ~1,000
return period = ~8 hours
Light 4–4.9
freq per year = ~10,000
return period = ~1 hour
Minor 3–3.9
freq per year = ~100,000
return period = ~5 minutes
Very minor 2–2.9
freq per year = ~1,000,000 return period = ~½ minute
2
Q
equation for hazard, vulnerability, and risk
A
risk = hazard x vulnerability
3
Q
what is a “natural hazard”?
A
natural event that is potentially dangerous
4
Q
what is a “natural disaster” ?
A
when a hazardous event causes loss of life, injury, damage to property or infrastructure, or economic losses
5
Q
A
6
Q
how did earth form, how long did it take (roughly), and how long ago ?
A
accretion and differentiation
around 30 million years
∼4.55 billion years ago
moon around same time
7
Q
what compound formed mantle ?
A
SiO2
metal sinks and silicate rises
8
Q
how did differentiation occur ?
A
formed into distinct layers – SiO2 formed mantle
metal sinks and silicate rises
9
Q
meteorites
A
fragments of protoplanets
look similar to rocks found on earth
easy to spot in deserts or frozen deserts
DIFFERENT TYPES:
*CHONDRITES
- 75-90% silica (SiO2) + 10-25% nickel-iron alloy
- bubbles prove its never been molten
- oldest rock in entire solar system
- chondrules
–> representation of how earth
and other planets formed
–> represent primitive material
and protoplanetary disk
*ACHONDRITES
- 75-90% silica (SiO2) + 10-25% nickel-iron alloy
- NO chondrules
- outer silica mantle
–> represent molten differentiation of protoplanets
*STONY-IRONS
- 50% silica + 50% nickel-iron alloy
- boundary between outer silica mantle and core
- molten iron and molten silica are unmissable (dont mix)
–> represent differentiated cores
of these bodies
*IRON meteorites
- 100% nickel-iron alloy
- earliest sources of iron
- origin in cores of protoplanets
–> represent differentiated cores
of these bodies
10
Q
radiometric dating
A
Chondrites
~4.56 billion yrs ago
Achondrites,
Stony-irons, and
Irons
- Processes took up to 30 million years
80% of geological
time: no animals
- humans: 0.004%
earths mantle much hotter when formed than it is today
11
Q
Impact events
- energy, return periods
A
amount of energy released is enormous
avg return is proportional to size of impacter
inversely proportional to size
earths surface is constantly changing by erosion and tectonics - therefore less traceable with events than the moon
have caused mass extinctions in past
12
Q
asteroids
A
rocky bodies up to several hundred kilometers in diameter which orbit the Sun in the Asteroid Belt between Mars and Jupiter, or in the Kuiper Belt beyond Neptune
too small to be considered planets
- mostly in asteroid belt and kuiper belt
13
Q
Comets
A
bodies composed of rock, dust and ice up to ∼60 km in diameter
normally orbit the sun
vaporize causing tail
different than asteroids
bc also contain ice
water, methane, and ammonia
tails can be really long
“mostly originate from the Kuiper Belt or the distant Oort Cloud and which orbit the Sun in highly elliptical orbits”
14
Q
impact craters on earth
A
just under two hundred confirmed impact craters on Earth,
amongst the largest of which are the
∼214 million year-old Manicouagan, Quebec
and ∼1.85 billion year-old Sudbury, Ontario
15
Q
Earth’s structure
A
density increases inwards
Sedimentary rocks:
~2 g/cm3
Granite:
~2.8 g/cm3
Basalt:
~3.0 g/cm3
16
Q
Earth’s structure – the mantle (components)
A
Olivine
Spinel
Perovskite
17
Q
earthquake waves
A
- can be used to infer depths of the boundaries
- know outer core is still molten since s waves dont pass thru liquids
P-waves: faster,
can travel through liquid
S-waves:
slower, cannot travel through liquid
18
Q
Earth’s physical and chemical structure
A
Crust: (light colour, low
density rock)
~0.5% of Earth mass
Temp: ~0–1000 ºC
Silicate rocks (Al, Na)
Mantle: (solid, rocky)
~67% of Earth mass
Temp: ~1000–3000 ºC
Silicate rocks (Fe, Mg)
Core: outer core = liq iron
inner core = solid iron
~32% of Earth mass
Temp: ~4000–6000 ºC
Fe-Ni alloy (metal)
19
Q
Earth’s rheological structure
A
atmosphere (gas)
hydrosphere (liquid)
lithosphere (solid)
asthenosphere (“soft plastic”)
mesosphere (“stiff plastic”)
outer core (liquid)
inner core (solid)
20
Q
Rheology – stress and strain
A
stress = force per unit area
strain = deformation of material occurs under stress
(unitless)
shear = one side goes one way, one goes another
elastic, plastic, ductile, brittle
Rheology is dependent on time, temperature, and pressure (and composition)
21
Q
mechanisms of heat transfer
A
conduction
- particles themselves dont move, just transfer of heat thru electrons
convection
- heat transfer thru particles in a fluid –> v little gradient in temp from top to bottom
radiation
22
Q
The geothermal gradient
A
3 different melt states
3 distinct layers on outer part of earth
solid, partially molten, liquid
mesosphere
= no melting
- stiff, plastic, solid
asthenosphere
= partial melting
- soft, ductile
lithosphere
- no melting
- brittle
23
Q
Oceanic and continental crust
A
Oceanic crust:
= basalt
~3.0 g/cm3
- iron and magnesium rich rock
Mantle:
= peridotite
~3.2 g/cm3
Continental crust:
= granite
~2.8 g/cm3
- sodium and aluminum
- lighter than basalt
24
Q
Earth’s magnetic field
A
north magnetic pole and south magnetic pole are not same as geographic north and south poles
25
Solar wind
caused by suns corona
- rim of sun that can be seen during solar eclipse
earths magnetosphere is asymmetric bc of solar wind
long tail on night side
26
Coronal Mass Ejection
larger release of energetic particles
- huge bubbles of gas with magnetic field lines
- solar wind flows continuously around the earth w large tail
- severely compresses magnetic field filling it w plasma
27
Geomagnetic storms
would cause problems in technology and radio communications
- power transmission lines and transformer problems
- pipelines and flow meters
28
29
plate tectonics (general)
earths outer lithosphere is divided into rigid plates that move relative to one another, driven by convection of the mantle
generating peaks and trophs in ocean seafloor - leading to geological hazards - tsunamis, volcanoes etc
30
Geological evidence for Pangaea
one clue for continental drift = continents fitting together
another clue = geological evidence
mountains on each side of earth
- same age and share other geological charateristics
- matching plants and animals — ones that couldnt swim across
31
Glacial evidence for Pangaea
direction of glacial striations across land masses
ice flowing onto the land, which is weird —> this only makes sense when continents are combined
most are north south - show that glaciers flowed a certain way - toward juan de fuca
32
steps of continental drift
1. 180 Ma
- northern = Laurasia
(europe and asia)
- southern = Gondwanaland
(south america, africa, antarctica, india, australia)
separated by Tethys Sea
2. 135 Ma
- rifting propogated from south to north
- india on the move northward
3. 65 Ma
- drifting to more present areas
- when collided, forms mountain ranges and oceans/seas
4. present
33
plate tectonics
earths outer lithosphere divided into rigid plates that move relative to one another --> driven by convection of the mantle
(asthenosphere and mesosphere)
subduction zones are where one plate subducts under another (lithosphere moves into mantle)
-- downwelling currents associated w destruction of old lithosphere and its re-integration into deep mantle subduction zones
mantle convection is ordered into discreet cells
mid ocean ridges are associated w upwelling currents of mantle
mantle convection is driven by the transfer of primodial heat from earths super hot metallic core to its cool outer surface
34
Plate boundaries
plates are very rigid by move alongside one another along boundaries
characterized by direction of relative motion by plates on either side
if the boundary is oceanic, its known as a mid ocean ridge
-- ex// mid atlantic ridge
OR Juan de fuca ridge
if boundary is continental, its known as a continental rift
-- ex// East African rift
35
Why does melting occur at mid-ocean ridges?
Due to lower mantle pressure
36
The Yellowstone hot spot track suggests that the North America plate is moving in which direction?
Towards the south-west.
37
How can hot spots be used as evidence for plate tectonics?
They indicate how the plate has moved over the stationary mantle plume.
38
What is a passive continental margin?
A transition from oceanic to continental crust within the same plate.
39
What type of plate boundary does the San Andreas fault represent?
A continental strike-slip boundary.
40
Victoria is situated closest to the boundary between which tectonic plates?
The boundary between the North American and Juan de Fuca plates.
41
Which ocean is characterized by passive continental margins on both sides?
The Atlantic Ocean.
42
Why does melting occur at subduction zones?
Due to higher water content of the mantle.
43
Why does melting occur at hot-spots?
Due to higher mantle temperatures.
44
What is the relationship between the theories of continental drift and plate tectonics?
Plate tectonics provides the physical mechanism to explain continental drift.
45
46
47
two types of convergent boundary
subduction zones and continental collision zones.
48
Convergent plate boundaries definition
where the two plates are moving towards each other, resulting in lithosphere being consumed or thickened
49
Triple junctions definition
where three plate boundaries meet at a point
there are many different configurations depending on the types and relative rates of the three boundaries
closest example to us is the Queen Charlotte triple junction north-west of Vancouver Island:
-- where the Juan de Fuca ridge, the Queen Charlotte fault, and the Cascadia subduction zone all meet.
50
Hot spots definition
rich in seismic and volcanic activity, yet not a plate boundary per se
situated above upwelling mantle plumes
- where anomalously high temperatures coupled with decompression give rise to melting and volcanism
Hot spots can be located under oceanic plates
(e.g. Hawai’i) or continental ones (e.g. Yellowstone)
are occasionally conincident with plate boundaries
(e.g. Iceland, which also lies upon a mid-ocean ridge).
51
Isostasy definition
a gravitational equilibrium by which solids float upon underlying fluids at a level governed by their density contrast
like wood or polystyrene floating on water, icebergs on the ocean, or continental or oceanic crust on the asthenospheric mantle
Isostasy explains why tall mountain ranges and plateaus are underlain by thicker crust than continents lying at or just above sea- level; why denser oceanic crust (∼3.0 g/cm3) is thinner still; and why the oceanic crust sit at lower elevations than the continents, resulting in ocean basins.
52
What type of plate boundary is the Nazca–South America plate boundary?
This is an ocean-continent convergent boundary.
This is a subduction zone.
53
Knowing that the Pacific plate is moving northwestwards and given what you've observed about the Nazca plate, what type of plate boundary do you infer the Pacific-Nazca plate boundary to be?
This plate boundary is a mid-ocean ridge.
This plate boundary is a divergent ocean-ocean boundary.
54
55
Earthquake faulting
56
what is a surface rupture ?
surface rupture is where the rupture area interrupts the earths surface, not all large earthquakes generate these, and small ones rarely do
pronounced linear trend of defamation
surface ruptures are not characterized by chasm in earths crust - common misconception
- adjacent blocks of earths crust on either side of the rupture have slipped past eachother
- offset caused by slip of faults
- surface rupture where the fault intersects w earths surface
57
do earthquakes occur at points ?
no,
earthquakes involve slip along faults
58
corrations and striations
analogous to glacial striations
fault striations caused by rock mass of one fault scratching other - striations align in the direction of slip
smooth and polished by more earthquake slips
59
diagram of earthquake faulting (epicenter, hypocenter, rupture area, slip, fault line)
large ones typically
reach depths of 10-20km
-- this defines seismogetic zone
in subduction zones, the seismogenic zone can extend to depths much greater than 20 km
not ALL of the fault plane has to rupture in an earthquake, the rupture area is the part of fault plane that slips in the earthquake
large earthquakes = large rupture areas, smaller earthquakes only involve smaller areas
epicenter is point on surface directly above hypocenter — these are shown as points on map
hypocenter point in fault plane where slip initiates - normally at several km depths, rarely at surface
60
3 types of faults
REVERSE (thrust) faults
- crustal thickening
- shortening (contraction)
- low dips
- associated w convergent plate boundaries
- mainly subduction zones and continental collision zones
- from horizontal compressive stress
NORMAL faults
- crustal thinning
- extension
- fault plain usually visible bc no overhang
- from horizontal tension stress
- divergent plate boundaries, mid ocean ridges, subducting slabs and continental rifts
STRIKE-SLIP faults
- left lateral or right lateral strike slip
- two sides move laterally past one another --> horizontal motions
- in response to simple sheer stress
- dip 90 degrees
- Continental shear zones, Continental collision zones, and Subduction fore-arcs
61
Reverse (thrust) faults
- crustal thickening
- shortening (contraction)
ONE SIDE UP AND ONE SIDE DOWN - w overhang
creates overhang - can collapse into slope of debris -- FAULT SCARP
in response to horizontal compressive stress
one side thrust over other, as 2 blocks move together - leading to shortening or contraction, and crustal thickening
dip angle (angle between foreplane and surface) can vary up to about 30 degrees (gentle dip)
one w low dips (only a few degrees) are classified by thrust faults
LOCATION: mainly
- subduction zones
- continental collision zones
examples:
- chi chi earthquake Taiwan (1999)
- Main Himalayan thrust, Nepal -- responsible for raising Himalayan mountains
- cascadia megathrust fault
62
Normal faults
- crustal thinning
- extension
ONE SIDE UP AND ONE SIDE DOWN w exposed fault
response to horizontal tension stress
one side of fault slides down other, as 2 blocks move apart, leading to horizontal extension and crustal thinning
faults dip angle deeper - abt 60 degrees
LOCATION:
- divergent plate boundaries
- mid ocean ridges
- continental rifts
- subducting slabs (convergent plate boundaries) -- intraslab
examples:
- 1959 Hebgen Lake, Montana (7.3)
- 2016 Norcia, Italy (6.6)
63
Strike-slip faults
- left-lateral strike slip
- right-lateral strike slip
- oblique strike slip
two sides moving past eachother
- horizonal movements
due to simple shear stress
dip abt 90 degrees
can be detrimental to infrastructure like pipes, roads, aqueducts etc
LOCATION:
- continental shear zones
- continental collision zones
- subduction fore-arcs
examples:
- San Andreas
- 2016 Kaikoura, New Zealand
64
the earthquake cycle
important note !!!!! this is cyclical !!!
large earthquakes do not create new faults, but reactivate existing faults
Interseismic phase
- same as “stick” part of experiement
- inter = between earthquakes
- build up of strain
- steady motion away from fault
Coseismic phase
- "slip" part
- co=during
— during an earthquake
- fast-reverses the arc-tangent strain
65
interseismic phase
(The megathrust earthquake cycle)
during interseismic phase, oceanic plate slowly converges w the overriding plate, but the megathrust fault (in grey) is stuck
ongoing convergence causes the overriding plate to become squeezed
at the trench, the overriding plate is pushed backwards (like the loading of a spring) - further back, the squeezing causes the overriding plate to buldge and lift
66
Coseismic phase
(The megathrust earthquake cycle)
during the earthquake (coseismic phase), the long term motions are reversed in a matter of seconds - friction on locked megathrust fault is overcome and overriding plate slips over subducting plate, rebounding to original position, the buldge in overriding plate is relaxed causing subsidence at the coastline, further out to sea, the seafloor is uplifted, raising water and generating a tsunami
67
Seismic waves (2 categories)
Surface waves
- travel around surface of earth
Body waves
- pass thru body of the earth
- faster
- sped up w increasing depth in mantle
- p waves and S waves are 2 types of body waves
seismic wave velocity depends on stiffness of material that its passing thru
waves in deep mantle travel fastest, waves in upper mantle travel slower, and waves at surface travel slowest (seismic wave velocity is slowest at surface) - these waves travel at 2-3km/second
take away points:
1) body waves faster than surface. they reach a point faster than surface waves
2) path of body wave within earths interior forms a curve - steep first, flattens at depth, surfaces at steep angle
we can see this based on how material properties change with different depths
*******surface waves have much larger sizes (amplitude) than body waves, makes surface waves more damaging, but also slower
68
p waves
type of body wave
comprise Primary compressional P-waves
fastest waves - always arriving first at location
69
p waves
type of body wave
Secondary Shear S-waves
second fastest, arriving second
- move side to side perpendicular to direction wave travels in
- particles sheared as waves passes thru
- s waves also called shear waves
- unlike p waves, s waves cannot pass thru fluids
70
love waves
type of surface waves
particles move side to side - perpendicular to the direction they're travelling in
- similar to shear body waves
particles on surface move more than those at depth
- reflecting that they are surface waves
71
Rayleigh waves
type of surface waves
first forward in direction of wave propagation, then upward, then backward, then downward —> pattern known as retrograde elipse
these waves are often called “ground roll”
like the love wave, particles at surface move more than at depth - reflecting that they are surface waves
72
Seismic array
seismic array = dense network of seismic stations
73
Imaging Earth’s interior
seismic waves manifest at the earths surface
**stations between 105 and 140 dont display p waves or s waves
p waves are refracted across the core mantle boundary bc of the abrupt drop of p wave velocity between the lower mantle and the outer core
—> resulting in a p-wave shadow zone between 105 and 140 degrees epicenteral distance from the
earthquake (within
this range, it wont display clear p waves - but would at greater than 140 degrees - when they passed thru the core)
epicenteral distance is the angle between the earthquake, the center of the earth, and the distance seismometer
74
on seismogram, how to detect P vs S waves ?
P waves are the first sign of activity
S waves are the start of the biggest activity
75
what waves cause "ground roll" of earths surface ?
Rayleigh waves
76
which waves can pass through liquid of earths body ?
P waves
S or secondary shear waves cannot pass thru liquid
77
How do earthquake seismograms recorded at larger epicentral distances differ from those recorded at shorter epicentral distances?
At larger epicentral distances, the P wave, S wave and surface wave arrivals will be more separated on the seismogram.
78
What is the earthquake cycle?
The cyclic build-up and release of stress and strain on a fault.
79
At which of the following types of plate boundary are reverse/thrust faults most common?
- Subduction zones and continental collision zones.
- Mid-ocean ridges and continental rifts.
- Continental shear zones.
Subduction zones and continental collision zones.
80
What type of strain do normal faults accommodate?
Extension
81
What best describes earthquake body waves, which travel through the Earth's interior, in relation to surface waves, which travel around the Earth's surface?
Body waves are faster and larger amplitude.
Body waves are faster and smaller amplitude.
They travel at the same velocity, but body waves are larger amplitude.
Body waves are slower and larger amplitude.
Body waves are slower and smaller amplitude.
Body waves are faster and smaller amplitude.
82
What is the name given to faults that accommodate both vertical and lateral slip?
- Oblique slip faults
- Mixed mode faults
- Splay faults
- Bilateral faults
Oblique slip faults
83
What best describes particle motion during the passage of a P-wave?
Compression/tension in the direction that the P-wave travels.
Compression/tension perpendicular to the direction that the P-wave travels.
Shear in the direction that the P-wave travels.
Shear perpendicular to the direction that the P-wave travels.
Compression/tension in the direction that the P-wave travels.
84
What type of faulting results in crustal thickening?
Reverse/thrust faulting.
85
86
definition of magnitude
physical size of the earthquake
87
definition of intensity
strength of shaking the earthquake generates
88
differences between foreshocks, mainshocks, and aftershocks
mainshock is always the biggest shock, anything before that is foreshock, anything smaller after main shock is aftershock
89
Richter magnitude scale
this scale corrected for the phenomenon of wave amplitude decaying as distance from earthquake increases
but maxed out at 6.0
so instead called "local magnitude scale" and used for small earthquakes in California
may people accidentally say "Richter magnitude" instead of correct term "moment magnitude"
90
seismic moment
(definition and equation - and what components of equation mean)
seismic moment is the truest measure of the earthquakes energy
moment (Nm) = Rupture area (m^2) x slip (m) x shear modulus (N/m^2)
shear modulus = stiffness
91
what is the major benefit to the moment magnitude scale compares to the Richter scale ?
it does not saturate at a particular size
digital calculations make it simple to calculate moment magnitude
92
what is the increase in moment from each unit increase ?
(ex// 6.0 to 7.0)
10 ^1.5
= 31.6 times
2 unit increase = 10^3
etc...
7.2 has twice the seismic moment as a 7.0
93
is rupture area unlimited ?
no, rupture area is restricted due to the size of the fault plain
rupture area determines seismic moment and moment magnitude
-- size matters when it comes to earthquakes
examples:
- Mw 6.0 earthquake
= ~5-20km rupture length
= ~20-50cm avg slip
- Mw 7.0 earthquake
= ~30-100km rupture length
= ~1-2m avg sip
94
most dangerous 2 faults in North America
Cascadia Subduction Megafault
- forms Juan de fuca and North America plate boundary between offshore Vancouver island and offshore Northern California
San Andreas strike slip fault
- forms the main Pacific North America plate boundary, South of the Mendocino Triple Junction
95
which of Cascadia subduction megathrust fault versus San Andreas strike slip fault is capable of hosting the biggest earthquake?
length of these is relatively the same (around 1000km)
rupture widths very different
- cascadia abt 100km or more
- san andreas 10-20km
cascadia rupture area 5X or more larger
cascadia said to be capable of magnitude 9.0 earthquake
an analog of "worst case" is 2011 Tohoku Japan earthquake -- but unlikely bc cascadia has small tremor earthquakes often (releasing pressure)
96
what is the largest possible earthquake and where and why ?
magnitude 10.0
Nazca Plate is longest at 6000 km long
97
Earthquake magnitude vs frequency
for each unit increase in magnitude, there is a 10 fold decrease in frequency
98
do all earthquakes account for same amount of seismic release ?
largest earthquakes dominate cumulative seismic releases
the biggest one in chile accounts for A LOT of the energy release
99
what does the Mercalli scale measure and what is the range on scale ?
Intensity
I (instrumental)
II (weak)
III (slight)
IV (moderate)
V (rather strong)
VI (strong)
VII (very strong)
VIII (destructive)
IX (violent)
X (intense)
XI (extreme)
XII (cataclysmic)
this is different depending on location and distance from earthquake
100
Measuring intensities
(1) Accelerometers
(2) Felt reports
101
What controls intensity?
waves less as distance increases
places can suffer more/less from directivity,
slip going in one direction — amplifies waves in this direction
geology of the area can impact this as well
- a more broken up area (older) will feel stronger earthquake intensity but travel less distance
- younger (more together rock) will feel weaker earthquake intensity but travel further
102
how does geology effect earthquake intensity ?
- active plate boundary
- crust "broken up" by dense faulting
- strong attenuation
- travels less distance
- tectonically inactive for ~200 mill years
- fewer active faults
- weak attenuation
- travels further
103
what differed from the 2019 Mw 7.1 Ridgecrest, California earthquake versus the 2018 Mw 7.1 Anchorage, Alaska earthquake ?
2019 Mw 7.1 Ridgecrest, California
= peak intensities higher
= smaller area felt
= shallower depth
2018 Mw 7.1 Anchorage, Alaska
= lower peak intensities
= felt across wider distances
= deeper
104
what is basin amplification ?
looser ground = higher amplification = worse for buildings
makes for potential of the strongest shaking not right at earthquake, but further from it
ex//
2017 Mw 7.1 Puebla, Mexico
cocos plate
bc of geological history
- lake drained where Mexico City was built
--- longer period waves here make stronger and more damaging movements
105
liquefaction
potential where soft sediment
alaska
vancouver more likely than vic
106
which type is most hazardous to coastal BC?
subduction zone, shallow crustal, and deep earthquakes
shallow crustal earthquakes would pose more local threat to cascadia
offshore megathrust would cause more cumulative damage as they would reach larger areas
107
Global earthquake hazards
- subduction megathrust earthquakes vs continental earthquakes
subduction megathrust earthquakes can be larger,
but more fatalities due to continental earthquakes
108
Mw scale (great earthquakes etc)
2.0 - 3.0 = small earthquakes
4.0 - 5.0 = moderate
6.0 - 7.0 = large earthquakes
8.0 - 9.0 = great earthquakes
109
where do the largest earthquakes almost exclusively occur ?
subduction zones --> subduction megathrust faults
long and provide large area for earthquake to grow
110
continental earthquakes are usually up to ____ Mw
less than 8.0
but often more fatalities due to population density, geology, and the difference between magnitude and intensity
111
do shallower or deeper earthquakes generate stronger surface shaking ?
shallower
112
What factors could influence the shaking intensity felt during an earthquake at a given location?
The magnitude of the earthquake.
The type of rock in that location (e.g. soft sediment, hard bedrock)
The depth of the earthquake.
The distance to the earthquake epicenter.
All of these factors would influence the felt intensity.
All of these factors would influence the felt intensity.
113
Given two earthquakes of the same magnitude and depth, one in western North America and one in eastern North America, which will likely be felt over a wider area and why?
The earthquake in eastern North America will be felt over a wider area because of the stronger (less fractured) geology underlying the eastern half of the continent
114
The largest magnitude earthquakes are:
Subduction megathrust earthquakes.
115
How can the dip angle of the fault influence the magnitude of an earthquake?
Shallow-dipping faults rupture more slowly than steeply-dipping faults, generating smaller magnitude earthquakes.
Shallow-dipping faults have a larger surface area within the brittle upper crust than steeply-dipping faults, giving them greater potential for larger magnitude earthquakes.
Shallow-dipping faults naturally slip more than steeply-dipping faults, generating larger magnitude earthquakes.
Shallow-dipping faults naturally slip less than steeply-dipping faults, generating smaller magnitude earthquakes.
Shallow-dipping faults have a larger surface area within the brittle upper crust than steeply-dipping faults, giving them greater potential for larger magnitude earthquakes.
116
In the equation for seismic moment, what does the shear modulus describe?
It describes a material's resistance to deformation by shear stress — or put more simply, its stiffness.
117
Why do sedimentary basins amplify incoming seismic waves (basin amplification)?
Because seismic waves slow down as they pass into loose sedimentary rocks.
Because seismic waves speed up as they pass into loose sedimentary rocks.
Because water-saturated sedimentary rocks liquefy during ground shaking.
Because seismic waves slow down as they pass into loose sedimentary rocks.
118
Which statement best describes earthquake frequency-magnitude relations?
For each unit increase in moment magnitude, there are about ten times more earthquakes.
For each unit increase in moment magnitude, there are about twice as many earthquakes.
For each unit increase in moment magnitude, there are about half as many earthquakes.
For each unit increase in moment magnitude, there are about ten times fewer earthquakes.
For each unit increase in moment magnitude, there are about ten times fewer earthquakes.
119
How much more seismic moment is there in a Mw 6.0 earthquake than in a Mw 5.0 earthquake?
10^ 1.5 = 32 times as much.
120
What is the truest measure of the energy released by an earthquake?
Its maximum intensity on the Mercalli scale.
It's seismic moment.
Its Richter scale magnitude.
Its rupture velocity (the speed at which the earthquake propagates along the fault).
Its seismic moment.
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what is an intraplate earthquake ?
earthquakes occurring within tectonic plates, rather than along boundaries
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Subduction zone earthquakes
3 most important types dangerous here in Cascadia
- Crustal earthquakes
- Megathrust events
- Intermediate depth/ intraslab (normal faulting)
CRUSTAL earthquakes within the forearc
- tend to be smaller in size (magnitudes of abt 7), but can occur at shallow depths directly beneath continents (where ppl live) have potential for great damage
- can involve a variety of fault types (reverse, normal, or strike-slip - depending on subduction zone
MEGATHRUST events along the shallow plate interface
- the shallow
fault forming
the plate interface = the subduction megathrust
— megathrust earthquakes here
- can be 100’s or 1000’s km long and typically 100’s of km wide - gently inclined at dip angle of a few degrees - so huge area where earthquake slip can propagate and so these type can have greatest moment magnitudes (exceeding 8 or 9)
* "Outer rise”
normal faulting
from plate flexure
- the outer rise refers to a buldge in the incoming oceanic lithosphere where it starts to flex bc of the weight of the overriding plate. this bending can lead to extension and normal faulting (similar to what happens w interslab earthquakes - but outer rise earthquakes happen at much shallower depths)
- usually too far offshore to cause shaking damage.
but larger ones can create tsunamis. asterisk here shows that doesnt happen in all subduction zones
Intermediate depth (50–
300 km) intraslab events,
often normal faulting
- third most impotant category of earthquakes in subduction zones are intermediate depth earthquakes (defined by focal depths within range 50-300km. occur within downgoing slab and also called intraslab earthquakes. normal faulting - related to extension of the slab as it bends or flexes downwards. can also reach magnitudes of abt 7 (like crustal events)
* Deep earthquakes
(300–700 km) within
the subducting slab
- so far from surface so rarely damaging (except in v largest cases)
- at depths of 300-700 km, the juan de fuca plate cant act as brittle
- deep earthquakes are not observed in cascadia as the subducting part of the juan de fuca plate is relatively young (formed a few million years ago at the
juan de fuca ridge (so still rather warm
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crustal
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megathrust
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The megathrust earthquake cycle
initial condition =
oceanic plate subducting under overriding plate
interseismic =
- oceanic plate converges w overriding plate
- megathrust fault stuck or locked
- ongoing convergence causes overriding plate to become squeezed
- at the trench, the overriding plate gets pushed backwards like a loaded spring
- bulges up causing uplift
Coseismic (during)
- longterm motions suddenly reversed
- friction on locked megathrust fault is overcome and the overriding plate slips over, rebounding to its original position
- the previous bulge in overriding plate is suddenly relaxed, causing subsidence at the coastline
further out, the seafloor is uplifted, raising the water column and generating a tsunami
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Tectonic setting -
2011 Mw 9.0 Tohoku earthquake
this earthquake ruptured the japan trench megathrust where the pacific plate subducts westwards beneath the japan volcanic arc under japans largest island
3 subduction zones offshore japan
PACIFIC plate and PHILLIPINE sea plate subduct westwards between japan volcanic arc
the PACIFIC plate subducts under northern Japan under the Japan trench
PHILIPPINE sea plate subducts under southern japan along the nankai trench
there is a triple junction where the PACIFIC and
PHILIPPINE sea plate meet the overriding Japan arc
however, the PACIFIC and PHILIPPINE sea plate subduct in slightly different directions, such that the these 2 OCEANIC PLATES ARE ACTUALLY CONVERGING
where 2 oceanic plates converge, it will always be the older, colder, and denser one of the two plates that will subduct
the denser plate is the pacific plate, the younger more buoyant one is the philippine sea plate (occurs along izu-bonin trench)
the east coast of island also uplifts interseismically as the volcanic arc bulges upwards. the gps vectors here only show horizontal motions
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The Japan Trench
accretionary wedge (mud)
- shallowest part of fault sits near accretionary wedge (a trianglar shape of marine sediments -mostly mud)
- wedge sits above shallowest part of megathrust fault
- since the sediments of the wedge are loose, it cannot stick to oceanic - they slide over it in a slow and steady motion known as fault creep - the shallowest part of the fault avoids the slip motion bc of the creeping. this area is labelled “aseismic” meaning without earthquakes bc of lack of slip
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fault creep definition
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The 2011 Mw 9.0 Tohoku earthquake - actual event
ruptured all the way to sea floor, so generated a very large tsunami wave
- (max height 39m)
large portion involved for this earthquake - ruptured from abt 40km depth to the seafloor of the trench
slip extends from about 40 km depth to about the sea floor at japan trench - peak slip here is abt 40m. some 50-60 m of slip
the shallow megathrust turned out to not be aseismic as previously thought, but conditionally stable —> can creep but also can be involved in v large earthquakes
after this, looking at layers of sediment showed could've expected smt like this
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can the cascadian megathrust fault perform similarly to the japan one ?
- the shallowest part of the cascadia megathrust also lies beneath a wedge of unconsolidated sediment
- this accretionary wedge is shown in this image offshore tofino - could the shallow part here slip in a great cascadia earthquake ?
- the answer to this could govern the tsunami runoff heights - rn we dont know but area of active research
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How will Cascadia behave?
3 scenarios
worst case (like Japan)
- peak slip occurs at trench
- causing 10m tsunami
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Continental plate boundary zones
Continental collision zones
involve shallow earthquakes
with a variety of mechanisms
- reverse OR thrust faults, strike slip faults, normal faults (like tibet)
Continental rifts involve
shallow normal faulting
earthquakes
Continental shear-zones
are characterized by
shallow strike-slip faults
***COMMON FOR ALL 3 TYPES is UPPER CRUSTAL ONLY
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what type of earthquakes and mechanisms do Continental collision zones involve ?
shallow earthquakes
variety of mechanisms
- reverse (thrust) faults
- strike slip faults
- normal faults
examples:
India/Eurasian collision
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what type of earthquakes and mechanisms do Continental shear zones involve ?
characterized by shallow strike-slip faults
example:
California San Andreas fault
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what type of earthquakes and mechanisms do Continental rifts involve ?
shallow normal faulting earthquakes
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The Alpine–Himalayan belt
these plate boundaries are diffuse — makes it challenging to map out all hazardous faults
formed from 3 separate collision zones all connected w eurasia plate
shallow crusting
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The Arabia-Eurasia collision zone (Iran/ Afghanistan)
shorten from south to north
shortening here
PROMINENTLY reverse or thrust faulting - although sometimes strike slip (mostly in eastern iran)
northsouth oriented right lateral shear between central iran and afghanistan
direct links between plate motions, earthquakes and mountainous topography
entire country is one large diffuse boundary zone
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2003 Mw 6.6 Bam earthquake
Eastern Iran
abt half of population died
90% of older adobe building collapsed
the town of Bam doesnt lie on mapped fault line — so the fault line responsible was to be tracked
2 faults invovled —> one main right lateral strike slip fault in the west, and secondary reverse fault in the east
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Mapping earthquakes with InSAR
Large antenna simulated by combining multiple
images along track
(Synthetic Aperture Radar)
finer spatial resolution of earths surface
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why was the Bam earthquake so damaging ?
direct bullseye on the towns
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what is the fatal attraction w humans and earthquakes ?
usually cities etc along places dangerous for earthquakes (like along mountain ranges/coastlines)
active faults
example: Tehran
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examples of intraplate earthquakes in Eastern Canada
Labrador Sea
- failed continental rift
St Lawrence Rift
- failed continental rift
Atlantic Passive margin
- full seafloor spreading
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Post-glacial earthquakes
as weight of ice is removed, return flow of the aesthenosphere causes the lithosphere to flex upwards — stressing and reactivating ancient crustal faults
most of these happened at end of pleistocene, but still ongoing hazard
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Induced earthquakes
human triggers can cause stress —> causing “induced earthquakes”
activities shown here
- subsurface fluid induction —> fracking***
—> when pressurized fluids come in contact w pressurized thing, can make it happen
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Induced earthquakes in the U.S. - what specific place ?
OKLAHOMA
fracking here
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intraplate earthquake types
failed rifts or passive continental margins
postglacial earthquakes - high latitudes
induced earthquakes - manmade
***** prevalent in Eastern Canada
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Which aspects of the 2011 Mw 9.0 2011 Tohoku earthquake took scientists by surprise?
Its rupture area, average slip, and moment magnitude were all much greater than expected.
It generated a larger tsunami than was anticipated.
It slipped all the way to the surface (the sea-floor at the trench).
All of the above.
All of the above.
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During the interseismic period preceding the 2011 Mw 9.0 Tohoku
earthquake, in which direction did the eastern coastline of the main Japanese island of Honshu move?
Westwards and upwards (uplift).
Eastwards and downwards (subsidence).
Eastwards and upwards (uplift).
Northwards and downwards (subsidence).
Westwards and upwards (uplift).
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Intraplate earthquakes in eastern Canada are mostly associated with which geological feature(s)?
The Canadian Shield.
Failed rifts and the North Atlantic passive margin.
Faults and folds of the Appalachian mountain belt.
The Sudbury and Manicouagan impact craters.
Failed rifts and the North Atlantic passive margin.
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What is a postglacial earthquake?
An earthquake caused by bending stresses following the melting of ice sheets and glaciers.
Any earthquake that occurred during the Holocene (i.e., since the end of the last Ice Age).
A fracture caused by thawing of the ground surface following the melting of ice sheets and glaciers.
An earthquake caused by shearing stresses from the flow of glaciers over the Earth surface.
An earthquake caused by bending stresses following the melting of ice sheets and glaciers.
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Which of the following statements about the Iranian capital city of Tehran is true?
Tehran was badly damaged in the 2003 Mw 6.6 Bam earthquake.
Fortunately, Tehran lies within an area of Iran that is not thought to be at risk from earthquakes.
Tehran has no historic record of earthquakes, but modern seismic instrumentation detects abundant seismicity there.
Tehran has experienced several large earthquakes in its recorded history, but none since it grew into a large metropolis.
Tehran has experienced several large earthquakes in its recorded history, but none since it grew into a large metropolis.
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Which of the following regions within Cascadia is most closely associated with intermediate depth "intraslab" earthquakes?
Northern Vancouver Island.
The Lower Mainland region of BC.
Southern Vancouver Island.
The Puget Sound region in Washington State.
The Puget Sound region in Washington State.
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Which of the following U.S. States is closely associated with induced seismicity?
Oklahoma.
California.
Florida.
Alaska.
Hawaii.
Oklahoma.
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Along some parts of some faults, the two sides move past each other slowly, and continuously, without the stick-slip behaviour associated with earthquakes. What is the name given to this slow and steady fault motion?
Fault creep.
Poro-elastic rebound.
Visco-elastic relaxation.
Interseismic deformation.
Fault creep.
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Iran occupies the boundary zone between which two tectonic plates?
Africa and Eurasia.
Arabia and Africa.
Arabia and Eurasia.
India and Eurasia.
Arabia and Eurasia.
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During the 2011 Mw 9.0 Tohoku earthquake, in which direction did the eastern coastline of the main Japanese island of Honshu move?
Eastwards and downwards (subsidence).
Northwards and upwards (uplift).
Westwards and downwards (subsidence).
Westwards and upwards (uplift).
Eastwards and downwards (subsidence).
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159
space for stuff from notes (class 9)
160
What is the engineering term for decoupling/disconnecting a building from the ground, in order to reduce its vibration in an earthquake?
base isolation
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Complete the sentence: earthquake prediction is...
... currently impossible.
... currently possible, but only for earthquakes with foreshocks.
... currently possible, but only in certain parts of the world.
... currently possible, but only for the largest earthquakes.
... currently impossible.
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'There is a ~12% to ~33% chance of a megathrust earthquake on the Cascadia subduction zone in the next fifty years.' This statement is an example of what?
An earthquake early warning.
An earthquake forecast.
A seismic retrofit.
An earthquake prediction.
An earthquake forecast.
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What is the central aim of the U.S. Geological Survey's PAGER program?
To map damage in each major earthquake from across the United States using satellite radar measurements
To map damage in each major earthquake from around the world using satellite radar measurements
To issue to users (e.g. scientists, emergency responders) a rapid alert to each earthquake above a threshold magnitude of 3 from across the United States
To issue to users (e.g. scientists, emergency responders) a rapid alert to each earthquake above a threshold magnitude of 4 from around the world
To estimate very rapidly the damage (in terms of both fatalities and economic costs) caused by each major earthquake from across the United States
To estimate very rapidly the damage (in terms of both fatalities and economic costs) caused by each major earthquake from around the world
To estimate very rapidly the damage (in terms of both fatalities and economic costs) caused by each major earthquake from around the world
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If the BC government built an earthquake early warning system on Vancouver Island, what would happen to the seismic hazard and seismic risk in Victoria?
The hazard would stay the same, but the risk would decrease.
The hazard would decrease, but the risk would stay the same.
Both the hazard and the risk would stay the same.
Both the hazard and the risk would decrease.
The hazard would stay the same, but the risk would decrease.
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Which of these statements is true?
Immediately after a large earthquake, authorities and media often underestimate the number of fatalities compared to expert estimates.
Immediately after a large earthquake, authorities and media usually overestimate the number of fatalities compared to expert estimates.
Immediately after a large earthquake, authorities and media are highly accurate at estimating the number of fatalities.
Immediately after a large earthquake, authorities and media often underestimate the number of fatalities compared to expert estimates.
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In what way did damage to Mexico City differ between the 1985 Mw 8.0 Michoacán and 2017 Mw 7.1 Puebla earthquakes?
The Michoacán earthquake destroyed all of the oldest buildings in the city, so that the Puebla earthquake damaged only modern buildings.
Damage from the Puebla earthquake was greater, despite its smaller magnitude.
Damage from the Michoacán earthquake occurred mostly to larger buildings, whereas damage from the Puebla earthquake occurred mostly to smaller buildings.
Damage from the Michoacán earthquake occurred mostly to larger buildings, whereas damage from the Puebla earthquake occurred mostly to smaller buildings.
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Which continent has suffered the most fatalities from earthquakes?
Europe
Asia
South America
Africa
Asia
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Soft first storey collapse is a common mode of building failure in earthquakes, because...
...many buildings are constructed to withstand vertical forces, but not strong horizontal accelerations.
...many buildings are built from masonry, rather than with stronger steel or wood frames.
...many buildings are constructed upon soft sediment that can liquify during ground shaking.
...many buildings are constructed to withstand vertical forces, but not strong horizontal accelerations.
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activity class 10
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Over the course of the clip, three seismologists repeatedly shout out the earthquake magnitude as it climbs gradually from a 6.5 to a 7.9. Is this theoretically possible?
Yes. Earthquakes grow in rupture area and thus magnitude as slip propagates across the fault, such that with enough instrumentation and computing resources, one could watch the magnitude grow in near real-time.
No. Earthquakes slip instantaneously across the entire rupture area, so there is no growth in rupture area or magnitude to even observe.
Yes. Earthquakes grow in rupture area and thus magnitude as slip propagates across the fault, such that with enough instrumentation and computing resources, one could watch the magnitude grow in near real-time.
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How does the speed at which an earthquake rupture propagates compare to that of a train?
Use 100 km/h as a typical train speed. (Hint: earthquake ruptures propagate at about the same speed as seismic surface waves).
Earthquake ruptures propagate about 30–50 times slower than a train
Earthquake ruptures propagate at about the same speed as a train
Earthquake ruptures propagate about 2–3 times faster than a train
Earthquake ruptures propagate about 20–30 times faster than a train
Earthquake ruptures propagate about 100 times faster than a train
Earthquake ruptures propagate about 100 times faster than a train
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174
space for stuff from notes (class 11)
175
volcanic eruptions are associated w what hazards (not extensive list) ?
lava flows
volcanic bombs
lahars
pyroclastic flows
ash
avalanches
large volumes of gasses
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are all volcanoes conical ?
no, can take number of forms
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definition of volcano
opening in earths crust for which molten rock, rock fragments, and gasses escape in a volcanic eruption
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what is lava ? what is the difference between lava and magma ?
lava is molten or partially molten rock at earths surface
- erupted thru a volcanic vent as a flow or as airborne particles
magma is molten or partially molten rock situated below the earths surface,
- may sit within magma chamber (a large body of molten rock at depth, or travel upwards along
a volcanic conduit)
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what are volatiles and how do they act under different conditions ?
chemical compounds w low boiling points
- most abundant volatile = water
dissolved in magma
under normal conditions would be gasses
- but in magma they're pressurized by weight of overlying rock - so kept in solution (dissolved)
examples :
- carbon monoxide
- carbon dioxide
- methane
- sulfur dioxide
- hydrogen sulfide
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what happens to magma as it travels up ?
the magma will become destabilized
closer to the surface the pressure has decreased and the volatiles come out of solution (exsolution)
the gas produces bubbles in liquid magma like opening a bottle of coke --> pressure suddenly released (reducing density of magma)
in explosive types of eruption, the rapid exsolution overwhelms the magma fragmenting it into pieces called pyroclasts
chunks of surrounding bedrock can also be entrained, super heated gas jet explodes out of volcanic vent, carrying the the pyroclasts and other rocky fragments with it
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what happens in eruption after magma comes up ?
in the most explosive eruptions, the momentum of the gas jets can feed an eruption column, or plume, which transports pyroclastic material into the atmosphere
- surrounding air becomes mixed into the plume, heats up and expands
- reduces the density of the column to below that of the surrounding atmosphere, driving the column higher.
- as it rises higher, eventually it will match the desnity of the surrounding atmosphere
in the mutual buoyancy region, the eruption column ascent will slow and start to spread out sideways - the umbrella region and lies within troposphere
although the larger eruption columns can extend even larger, into the stratosphere
the tallest eruption noted reached about 50 km
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igneous rocks
(definition and types)
when lava cools and solidifies, particles arrange themselves in crystals in form of igneous rock
type depends on composition of magma and size of crystals
almost all igneous rocks are silicate rocks - built around a framework of silicon dioxide or silica (different igneous rocks have different proportions of silica)
- different secondary elements like iron and aluminum
Basalt
- oceanic crust
- erupted from volcano
- fine grained
- mafic
Gabbro
- oceanic crust
- plutonic (formed in magma chamber)
- coarse grained
-mafic
Andesite
- intermediate rock
- erupted from volcano
- fine grained
- felsic
Diorite
- intermediate rock
- plutonic (formed in magma chamber)
- coarse grained
Rhyolite
- continental crust
- erupted from volcano
- fine grained
- felsic
Granite
- continental crust
- plutonic (formed in magma chamber)
- coarse grained
- felsic
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what 3 characteristics does viscosity depend on ?
temperature
crystal content
silica content
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hotter =
low viscosity (thinner)
185
higher crystal content =
higher viscosity (thicker)
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higher silica content =
higher viscosity (thicker)
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viscosity
(basalt, andesite, rhyolite)
basalt
~50% silica
- flows easily
- low H2O content
andesite
- intermediate
- intermediate H2O content
rhyolite & granite
~ 70% silica
- slow moving
- high H2O content
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volatile content
(basalt, andesite, rhyolite)
basalt
- low H2O content
- fewer gas bubbles form
- least viscous
- bubbles form and escape easily
andesite
- intermediate
rhyolite
- high H2O content
- lots of gas bubbles as reduced
- harder for bubbles to escape bc high viscosity - leads to more explosiveness
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Magma at mid-ocean ridges
main part of upper mantle = peridotite = undergoes decompression melting forming mafic magma
(high temp. low water content and low silica content = low visc)
- either cools and crystalizes slowly, a little below the surface, generating gabro
OR erupted as lava and solidifies as pillow basalts
- mid ocean ridges account for abt 80% of lava erupted globally
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what type of melting at mid ocean ridges
decompression melting of mantle asthenosphere
The basaltic magma is high temperature but has low volatile content and low viscosity, leading to a peaceful eruptive style that produces characteristic pillow basalts on the sea-floor.
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Magma at volcanic arcs (subducting)
at volcanic arcs, melting occurs for a different reason — the subducting the subducting oceanic plate carries wet sediments and hydrated minerals downwards into the asthenosphere
- the water is released into the slab into the overlying asthenosphere
to erupt, this magma still rises thru the silica rich crust of the volcanic arc
- as it rises, it melts surrounding rocks, the melts combine to make a composition of andesite and rhyolite
- these magmas have high volatile contents and high viscosities, so the gasses cannot escape easily
- explaining why eruptions at arc volcanoes tend to be much more explosive than those at mid ocean ridges
- volcanic arcs account for about 10% of magma erupted globally, compared to 80% at mid ocean ridges
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Volcanic materials – glass
skin has glassy appearance, bc it is glass
- non crystalline amorphous solids generated by rapid quenching of a melt
when lava comes in contact w cold water, crystallization cannot happen bc solidifies so quickly
the most common volcanic glass = obsidian
- atoms disordered instead of being ordered into crytals
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Types of pyroclast
PUMICE
- frothy magmas full of bubbles
- mostly produced by ryolithic lavas
(most volatile content) - some andesitic
- by volume, 90% of pumice is cavities, extremely light, can float on water, forming pumice rafts
cinder/scoria
- gas bubbles in magma
- mainly associated w basaltic and andesitic lavas rather than ryolithic
- darker (red or black)
- "lava rock"
blocks = solid while airborne (solidified before being ejected from volcano) usually more uniform shapes
- bombs = still liquid while airborne
ash
- dust or sand size particles - but still can be very dangerous
- smaller the particle, more likely to stay airborne
- can be transported over much larger distances
- can be sorted into different layers of grain sizes
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pyroclastic flow
If the ash cloud it is too dense to be transported up into the plume, it collapses under its own weight into a pyroclastic flow.
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how are volcanic eruptions classified ?
1. viscosity
2. volatile content
3. volume of the magma erupted
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types of volcanic eruptions
1. Icelandic-type eruptions
2. Hawaiian-type eruptions
3. Strombolian-type eruptions
4. Vulcanic-type eruptions
5. Plinian-type eruptions
Ultra-Plinian-type
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1. Icelandic-type eruptions
*lowest viscosity
*lowest volatiles
- mafic
(magnesium and iron)
∼50% silica (SiO2)
*very large volume
- flood basalts
mid ocean ridge
but above sea level
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2. Hawaiian-type eruptions
*low viscosity
*low volatiles
*very large volume
Hawaiian type
= Shield volcanoes
- gentle sloping profiles
- v wide
- slightly more explosive
1. pahoehoe
(smooth unbroken flow)
2. A'a
(stony rough lava)
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Arc volcanism
3 categories of eruption that are characteristic of volcanic arcs:
1. Stromboli
= first and least explosive category - tiranian sea
2. Vulcano
3. Vesuvius
= most explosive (named after guy who noted this explosion of this volcano on italian peninsula near Naples) - famously destroyed town of Pompeii
volcanic arc in southern italy Calabrian Arc
—> formed by northwards subduction of oceanic lithosphere belonging to the Northern part of the African plate
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3. Strombolian-type eruptions
*low/med viscosity
*medium volatiles
*small volume
Strombolian type
= scoria cones
- small mounds of debris piled up next to the volcanic vent
basaltic to andesitic magma w slightly higher viscosity and volatile content
mildly explosive
famous example:
Tseax Cone
- Canadas worst natural disaster on record
- about 1700 CE
- about 2000 people died from noxous gasses thru asphyxiation
- eruption of basaltic lava flows
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4. Vulcanic-type eruptions
*med/high viscosity
* med/high volatiles
*large volume
Vulcanian type
= stratovolcanoes
- tall, steep-sided, symmetrical volcanic peaks built of alternating ash/rock and viscous lava flows
andositic to rhyolithic magma
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5. Plinian-type eruptions
*high viscosity
*high volatiles
*very large volume
Plinian type
= Calderas
- also associated w strathovolcanoes but involve larger eruption volumes
- cone collapses over the emptied magma chamber forming a caldera
ryolithic magma
ex// mt pinatubo (1991)
ex// mt st helens
- most famous and documented plinian type eruption
- May 1980
- giant explosion
- strong directional focus towards north — big reason for lots of fatalities
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6. Ultra-plinian type eruptions
*very high viscosity
*very high volatiles
*massive calderas
Ultra-Plinian-type
= Massive Calderas
ryolithic magma w highest level of viscosity and volatile content - forming massive calderas and
huge volumes of pyroclastic material ejected
204
what caused most fatalities in 1991 Mt Pinatubo eruption ?
ash fallout collapsing homes due to weight and heavy rainfall, more than eruption itself
205
how do plinian type differ from vulcanian type eruptions in a few key ways ?
1. involve the highest viscosity of ryolithic magma - which makes them very explosive
2. bc of greater explosivity, greater volumes of magma are ejected
3. results in emptying of magma chamber beneath the stratovolcano whos cone then collapses into a large crater called a cordera
- this collapse generates even more pyroclastic material
only a few of these types of eruptions occur per century (compares to 3-5 per year for vulcanian types)
206
1980 mt st helens
(what happened, what type, etc)
plinian type eruption
blasted out northern flank of volcano first (by vulcanian type eruption)
then plinian type came thru and can empty out and collapse cone (into caldera)
207
What is a volcanic igneous rock (e.g. basalt, andesite, rhyolite)?
Rock made from magma that solidified below the surface.
Rock made from lava that solidified at the surface.
Rock formed from fall-out of airborne pyroclasts like ash.
Rock made from lava that solidified at the surface.
208
Which of the following volcanic eruption styles is associated with flood basalts?
Stombolian-type.
Plinean-type.
Icelandic-type.
Vulcanian-type.
Icelandic-type.
209
What accounts for the initial melting in a subduction zone beneath a volcanic arc?
Compression melting, as pressure increases as the subducting slab moves to greater depths in the mantle.
Release of water from subducted wet sediments and hydrated minerals.
Friction on the subduction megathrust fault. Radioactivity in the mantle.
Release of water from subducted wet sediments and hydrated minerals.
210
What type of volcano can be found in parts of central and northern British Columbia?
Cinder cones (or scoria cones).
Stratovolcanoes.
Shield volcanoes.
Cinder cones (or scoria cones).
211
Which component of silicate minerals is responsible for raising the viscosity of magma (making it less runny)?
Magnesium sulfate (MgSO4).
Calcium carbonate (CaCO3).
Silica (SiO2).
Water (H2O).
Iron carbono-potassiumate (FeCK).
Silica (SiO2).
212
Which of the following volcanic eruption styles is associated with shield volcanoes?
Hawaiian-type.
Vulcanian-type.
Icelandic-type.
Stombolian-type.
Plinian-type.
Hawaiian-type.
213
What is the volcanic equivalent of granite?
Rhyolite.
Basalt.
Andesite.
Gabbro.
Rhyolite.
214
Which of the following types of volcano is smallest?
Cinder cones (or scoria cones).
Stratovolcanoes.
Shield volcanoes.
Cinder cones (or scoria cones).
215
Vulcanian-type eruptions are associated with which type of volcanic landform?
Tall, gently-sloped shield volcanoes.
Small, steep cinder cones (or scoria cones).
Tall, steep stratovolcanoes.
Stacks of flat flood basalts.
Tall, steep stratovolcanoes.
216
When magma rises, and become less pressurized, what process occurs that is a key factor in driving explosive volcanism?
The magma becomes less viscous (more runny).
The magma becomes hotter, and therefore less dense, and more buoyant.
Dissolved volatiles come out of solution ("exsolution"), forming gas bubbles.
Dissolved volatiles come out of solution ("exsolution"), forming gas bubbles.
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218
2018 eruption of Kīlauea
the one we watched diff flows from
1. pahoehoe
(smooth unbroken flow)
2. A'a
(stony rough lava)
LAVA BOMBS
- the old guy putting out fires
active shield volcano on big island of Hawaii
- lies southeast of Mauna Loa
- at present much smaller than this, but more active of the 2
— this is expected as the pacific plate moves northwestwards over the stationary hawaiian mantle plume, the volcanic activity expected to migrate slowly southeastwards
this eruption occurred in the eastern part of Kilauea, in the East Rift Zone
- starting on May 3rd and continued for several weeks, lava spilled out of several fissures
obliterated residential neighbourhood - 800 mill damages, no one killed
- residential neighbourhoods built on top of old lava flows… why ???
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types of hazards
1. LAVA FLOWS
- typical of basaltic magma erupted sub-aerially
- such as in Iceland and Hawai’i
- can be extremely destructive but rarely deadly since the viscous magma is usually slow-moving.
2. SHOCKWAVES
- explosive eruptions may generate strong atmospheric pressure waves
3. PYROCLASTIC FLOWS
- gravity currents made of pyroclastic debris and entrained air
- sourced either by a lateral blast, the collapse of a volcanic dome, spillover of pyroclastic debris from a crater, or collapse of a plume under its own weight.
- two parts:
* the basal flow hugs the ground and contains larger, coarse boulders and rock fragments
* hot ash plume lofts above it fed by turbulent mixing of cooler, overlying air.
*** extremely high temps
EXAMPLE: Mt Unzen, Japan (1991)
4. PHREATIC ERUPTIONS
- sudden contact of hot magma with ground water, surface water, snow or ice
5. VOLCANIC GASSES
(LIMBIC ERUPTIONS)
- Tseax volcano
* CO2 released during a minor Strombolian-type eruption killed ∼2,000 people in adjacent valleys in what is Canada’s worst known natural disaster
- A limnic eruption is a rare type of natural disaster in which volcanic gases dissolved in lake waters — mostly CO2 and methane — suddenly comes out of solution, forming a gas cloud capable of suffocating wildlife, livestock, and humans.
6. LAHARS
- are channelized flows of pyroclastic debris mixed with water that can devastate valleys surrounding the erupting volcano, up to hundreds of km away.
- high fatalities
7. ASH FALL
- can asphyxiate people and collapse buildings
- most dangerous near the volcano
example : volcano is located directly under the north atlantic jet stream
- turbulant westly winds here
- at the time, jet stream pointing towards europe
8. CLIMATIC EFFECTS
- acid rain
- can cause communication problems etc
9. CLIMATE MODULATION
- caused invention of bikes !
In April 1815, Mt. Tambora erupted in the largest volcanic explosion in recorded history, killing tens of thousands of people on the Indonesian islands of Sumbawa and Lombok. Huge volumes of SO2 were released into the stratosphere, which quickly spread across the globe and oxidized into sulphate aerosols, which reduce net shortwave radiation causing widespread surface cooling. Earth’s mean surface temperature dropped ∼1–3 degrees ◦C for over one year, and in Europe 1816 became known as the “year without a summer”.
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what is a limbic eruption and where are they most prevalent ?
*****abrupt overturn of the lake waters
A limnic eruption is a rare type of natural disaster in which volcanic gases dissolved in lake waters — mostly CO2 and methane — suddenly comes out of solution, forming a gas cloud capable of suffocating wildlife, livestock, and humans.
Africa
*****abrupt overturn of the lake waters
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The Cascade volcanic arc
comprises of abt 50 stratovolcanoes and lots of other small volcanoes
between Lassen Peak in northern California and Mt. Baker in northern Washington.
plinian type eruptions
Mt Jefferson
Mt Rainier - tallest
Mt Hood
Mt St. Helens
- most active/dangerous
Mt Adams
Middle Sister
North Sister
South Sister
fed by northeastward subduction of the oceanic juan de fuca plate beneath the north american continental plate
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Mt St Helens eruption
1980
Mw 5.1 vulcanian type eruption earthquake collapsed north flank
Firstly, it depressurized the magma chamber below, causing volatiles to come out of solution and drive a violent eruption.
Secondly, it directed this initial blast northwards rather than upwards: a sideways explosion known as a lateral blast.
generated massive lahars on all sides of the volcano
Finally, pressure release in the magma chamber drove a ∼9 hour Plinian-style eruption
The vertical plume reached ∼20 km elevation
deposited ∼500 million tonnes of ash fall over a large swath of the northwestern U.S.
****economic cost - at more than 3 billion dollars
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types of hazards responsible for largest fatalities:
1. largest = pyroclastic flows
2. Tsumani
3. Lahars
4. climatic effects
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how do death tolls of earthquakes vs volcanoes ?
compare death tolls of worst volcanic disasters with those from worst earthquakes
volcanoes = as few as 6 killed more than 10 000 ppl
***whereas 30 earthquakes killed more than this in the 20th century alone****
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Volcano Disaster Assistance Program
one explanation for this difference is the gradual buildup of preceeded volcanic activity often over several days or weeks —> giving ppl time to prepare
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Which of the following volcanic hazards was not responsible for fatalities in the 1980 Mount St. Helens eruption?
The lateral blast.
Lava flows.
Lahars.
Pyroclastic flows.
Lava flows.
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Which best summarizes the human impacts of the 2018 eruption of Kīlauea volcano, Hawai'i?
Trans-Pacific air travel was disrupted for many days by ash dispersed in the atmosphere.
Lava flows and volcanic bombs destroyed much property, but nobody was killed.
Several tourists were killed by volcanic blocks and bombs.
Lahars wiped out roads and bridges, completely cutting off communities in southern Hawai'i.
Lava flows and volcanic bombs destroyed much property, but nobody was killed.
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Order the following volcanic processes according to speed, starting with the slowest:
A’a lava flow < Lahar < Pyroclastic flow
Lahar < Pyroclastic flow < A’a lava flow
A’a lava flow < Pyroclastic flow < Lahar
Pyroclastic flow < A’a lava flow < Lahar
A’a lava flow < Lahar < Pyroclastic flow
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Limnic eruptions are mostly associated with which continent?
Africa
Asia
South America
North America
Africa
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Which was the deadliest volcanic eruption in North American recorded history?
The 2018 eruption of Kīlauea, Hawai'i.
The 1980 eruption of Mount St. Helens, Washington.
The ~1700 CE eruption of Tseax Cone, BC.
The ~1700 CE eruption of Tseax Cone, BC.
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Over the past 4,000 years, which has been the most active of the Cascade arc volcanoes?
Mount Rainier, Washington.
Mount St. Helens, Washington.
Mount Baker, Washington.
Lassen Peak, California.
Crater Lake, Oregon.
Mount St. Helens, Washington.
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In the 1980 Mount St. Helens eruption, what was responsible for the strong northward directivity (lateral blast)?
A landslide collapsed the northern flank of the volcano.
The underlying subducting slab dips towards the North.
Prevailing winds from the South.
Intensive forestry in the North, which destabilized the Northern flank of the volcano.
A landslide collapsed the northern flank of the volcano.
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In which of these eruptions did around 30,000 people die in a pyroclastic flow?
The 1902 eruption of Mount Pelée, Martinique.
The 1980 eruption of Mount St. Helens, USA.
The 1985 eruption of Nevado del Ruiz, Colombia.
The 1991 eruption of Mount Unzen, Japan.
The 1991 eruption of Mount Pinatubo, Philippines.
The 1902 eruption of Mount Pelée, Martinique.
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What is a limnic eruption?
A sudden overturn of deep lake waters that releases large volumes of dissolved volcanic gases.
An explosion caused by hot magma or lava coming into contact with ground water, surface water, snow or ice.
An eruption that is intermediary between a Strombolian-type eruption and a Vulcanian-type eruption.
The ejection of pressurized mud from a mud volcano.
A sudden overturn of deep lake waters that releases large volumes of dissolved volcanic gases.
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How can volcanic eruptions often be anticipated in advance?
Earthquake swarms indicate moving magma at depth.
The volcano “bulges” due to rising magma, which can be observed at the surface.
Both of the above.
Neither of the above: volcanic eruptions can never be anticipated.
Both of the above.
