midterm 1 Flashcards
do earthquakes occur at predicted/pattern frequencies?
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
equation for hazard, vulnerability, and risk
risk = hazard x vulnerability
what is a “natural hazard”?
natural event that is potentially dangerous
what is a “natural disaster” ?
when a hazardous event causes loss of life, injury, damage to property or infrastructure, or economic losses
how did earth form, how long did it take (roughly), and how long ago ?
accretion and differentiation
around 30 million years
∼4.55 billion years ago
moon around same time
what compound formed mantle ?
SiO2
metal sinks and silicate rises
how did differentiation occur ?
formed into distinct layers – SiO2 formed mantle
metal sinks and silicate rises
meteorites
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
radiometric dating
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
Impact events
- energy, return periods
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
asteroids
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
Comets
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”
impact craters on earth
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
Earth’s structure
density increases inwards
Sedimentary rocks:
~2 g/cm3
Granite:
~2.8 g/cm3
Basalt:
~3.0 g/cm3
Earth’s structure – the mantle (components)
Olivine
Spinel
Perovskite
earthquake waves
- 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
Earth’s physical and chemical structure
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)
Earth’s rheological structure
atmosphere (gas)
hydrosphere (liquid)
lithosphere (solid)
asthenosphere (“soft plastic”)
mesosphere (“stiff plastic”)
outer core (liquid)
inner core (solid)
Rheology – stress and strain
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)
mechanisms of heat transfer
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
The geothermal gradient
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
Oceanic and continental crust
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
Earth’s magnetic field
north magnetic pole and south magnetic pole are not same as geographic north and south poles
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
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
Geomagnetic storms
would cause problems in technology and radio communications
- power transmission lines and transformer problems
- pipelines and flow meters
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
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
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
steps of continental drift
- 180 Ma
- northern = Laurasia
(europe and asia)
- southern = Gondwanaland
(south america, africa, antarctica, india, australia)
separated by Tethys Sea
- 135 Ma
- rifting propogated from south to north
- india on the move northward - 65 Ma
- drifting to more present areas
- when collided, forms mountain ranges and oceans/seas - present
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
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
Why does melting occur at mid-ocean ridges?
Due to lower mantle pressure
The Yellowstone hot spot track suggests that the North America plate is moving in which direction?
Towards the south-west.
How can hot spots be used as evidence for plate tectonics?
They indicate how the plate has moved over the stationary mantle plume.
What is a passive continental margin?
A transition from oceanic to continental crust within the same plate.
What type of plate boundary does the San Andreas fault represent?
A continental strike-slip boundary.
Victoria is situated closest to the boundary between which tectonic plates?
The boundary between the North American and Juan de Fuca plates.
Which ocean is characterized by passive continental margins on both sides?
The Atlantic Ocean.
Why does melting occur at subduction zones?
Due to higher water content of the mantle.
Why does melting occur at hot-spots?
Due to higher mantle temperatures.
What is the relationship between the theories of continental drift and plate tectonics?
Plate tectonics provides the physical mechanism to explain continental drift.
two types of convergent boundary
subduction zones and continental collision zones.
Convergent plate boundaries definition
where the two plates are moving towards each other, resulting in lithosphere being consumed or thickened
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.
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).
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.
What type of plate boundary is the Nazca–South America plate boundary?
This is an ocean-continent convergent boundary.
This is a subduction zone.
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.
Earthquake faulting
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
do earthquakes occur at points ?
no,
earthquakes involve slip along faults
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
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
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
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
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)
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
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
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
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
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
p waves
type of body wave
comprise Primary compressional P-waves
fastest waves - always arriving first at location
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
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
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
Seismic array
seismic array = dense network of seismic stations
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
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
what waves cause “ground roll” of earths surface ?
Rayleigh waves
which waves can pass through liquid of earths body ?
P waves
S or secondary shear waves cannot pass thru liquid
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.
What is the earthquake cycle?
The cyclic build-up and release of stress and strain on a fault.
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.
What type of strain do normal faults accommodate?
Extension
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.
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
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.
What type of faulting results in crustal thickening?
Reverse/thrust faulting.
definition of magnitude
physical size of the earthquake
definition of intensity
strength of shaking the earthquake generates
differences between foreshocks, mainshocks, and aftershocks
mainshock is always the biggest shock, anything before that is foreshock, anything smaller after main shock is aftershock
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”
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
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
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
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
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