Earthquakes Flashcards
What is an Earthquake?
Very strong underground explosion
Earth shaking caused by a rapid release of energy
How destructive are Earthquakes?
They destroy buildings and kill people
3.5 million deaths in the last 2,000 years
What causes earthquakes?
tectonic stresses that causes rocks to break
Energy moving outward as an expanding sphere of waves
What is seismicty
Earthquake activity
What is seismicty due to?
motion along a newly formed crustal fracture (fault)
motion on a existing fault
A sudden change in mineral structure
Inflation of a magma chamber
volcanic eruption
giant landslides
meteorite impacts
nuclear detonations
Where do most earthquakes occur?
Along faults
What are faults?
crustal faults that move rock masses
What is displacement?
The amount of movement
What is offset (slip)
another term for displacement
How do faults move? (What is stick-slip behavior)
move in jumps
quickly stops due to friction
strain will build up again, causing failure
What is stick
where friction prevents motion
What is slip
friction briefly overwhelmed by motion- violent and quick
What is elasticity
A property of materials that results in wave propagation and earthquakes
Bends
the capacity to return to the og configuration after being distorted
What’re the conditions necessary for periodic motion in the form of traveling waves?
Elasticity
Source of energy
What is focus?
The spot underground where earthquake waves originate
usually works on a fault surface
earthquake waves expand outward thousands of miles from the focus
What is the epicenter
land surface directly above the focus
Elastic rebound theory
explanation of how energy is spread during earthquakes
Stores a lot of energy in a rock, it acts like a rubber band
What are fault motions
result from rocks breaking and stored elastic strain is released
The energy, as waves, generates vibrations
Vibrations cause motions
Foreshocks and aftershocks are ofen
Types of seismic waves?
Body Waves
Surface waves
What’re body waves?
waves that pass through the Earth’s interior
What’re the two types of body waves?
Shear/ Secondary (S) transverse waves
Compressional or Primary (P) longitudinal waves`
Secondary waves
travels only through solids: not liquids
can’t feel in water
slower than compressional waves
“Shaking motions”
alternating transverse motion (perpendicular to the pirection of propagation and the raypath)
moves like a wave that crowds do
Compressional or Primary Waves
Push-pull (compress and expand) motion
travels through solids, liquids, and gasses
fastest
alternating compression (push pull), directed in the same direction as the wave is propagating (along the raypath)
Particle motion is parallel to the direction of propagation
What are the types of surface waves
Love waves
Rayleigh waves
What’re love waves
S-waves intersecting the surface
move like a writing snake
violent, destructive waves that push the rock sideways
Transverse horizontal motion, perpendicular to the direction
slower than S waves
What’re rayleigh waves
P-waves intersecting the surface
move like ripples on a pond
Particle motion consists of elliptical motions
most destructive, turbulence
the slowest
What’re surface waves
Waves that travel along Earth’s surface
What is seismology
The study of earthquake waves
how long it last
the vertical movements
the horizontal movements
reveals the size and location of earthquakes
What is seismographs
instruments that record seismicity
they detect earthquakes anywhere on earth
drum always rotating
Weight stays still
preserves a record of wave arrivals
1st wave causes frame to sink (pen goes up)
next vibration causes opposite motion
How do you determine the distance of station to epicenter?
(arrival time of )S - P(arrival time of)
s=S waves
p=P waves
The arrival time increases based off how far they are from the epicenter
What does seismograph measures?
Wave arrival times
magnitude of ground motion
How to locate and epicenter?
data from 3 stations
a circle is drawn around each station
the radius is equal to the distance to the epicenter
circles around 3 or more stations will intersect
the point of the intersection is the epicenter
What is seismograph operation?
Waves arrive in a sequence
P-waves arrives first
S-waves arrive second
What’re the two means of describing earthquakes?
Intensity
Magnitude
What is intensity
The degree of shaking based on damage
Mercalli intensity scale
diminishes with distance
damage occurs in zones
roman numeral assigned to diff levels of damage
What is the magnitude?
the amount of energy released
What’re the magnitude scales
richter magnitude scale
seismic moment magnitude scale
scales are logarithmic
increases 1 unit= 10 fold- increase in ground motion
increases of 1 unit = 33- fold increase in energy
What is the mercalli intensity scale
measures the intensity of shaking and damage at a specific location
depends on distance to earthquake and strength of earthquake
What is the richter magnitude scale
richter assigned each earthquake a number as a measure of its size
depends on the amplitude (size) of the ground movement caused by seismic waves
log 10
1
log 100
2
log 1000
3
log 10000
4
log 100000
5
log 1000000
6
log 10000000
7
log 100000000
8
log 1000000000
9
What is the seismic moment?
(the amount of slip) x (length of rupture) x (depth of rupture) x (rock strength)
Richter magnitude, Energy
The increased energy released as seismic waves with increasing magnitude of an earthquake is a factor of 33!!!! for each magnitude unit
Ex. 2-4=2
33^2= 33 x 33 =1089
Richter magnitude, Amplitude
Two earthquakes that differ in size of ground motion by a factor of 10, differ in magnitude by 1 richter unit
Mg 2 is 10 times stronger than Mg 1
Ex. 4-2
10^2 = 100
Mg 4 is 100 times stronger than Mg 2
Earthquake order
1st ground shaking and displacement
2nd earthquake waves arrive in diff sequences
- p-waves first to come
create a rapid up and down movement - s-waves around next
produce back and forth motion
stronger than p-waves
cause extensive damage - surface waves can come after s-waves
- love waves firsts
- ground moves like a snake - r-waves are the last to arrive
- last longer than others
- extensive damage
- love waves firsts
how do landslides and avalanches happen
shaking causes slopes to fail
ancient slope failures
follow earthquakes in uplands
an earthquake started the landslide that uncorked Mt. St. Helens on May 18,1980
What is liquefaction
waves liquefy H2O-filled sediments
high pore pressure reducing forces grains apart reducing friction
Water-saturated sediments turn into a mobile fluid
How do liquified sediments flow
like a slurry
What does sand become through liquefaction
quicksand
What does clay become through liquefaction
quickclay
What’re trunamis
waves generated by a disturbance in the sea or lake
generated by abrupt changes in water level
Ocean floor is unevenly uplifted or downdropped during an earthquake
motion on a fold on the ocean floor
harbor waves
Normal faulting drops the seabed; thrusting raises it
-displaces the volume of overlying water
giant mound (trough) forms on sea surface (enormous- 10,000mi^2 area)
-mound collapses creating waves that race rapidly away
wavelengths 10s-100s of meters
wave height and length unaffected by windspeed
wave velocity 100s of kph
-jetliner speed
water comes ashore
How frequent are tsunamis
1 every year
94 in the last 100 years
51,000 victims
are inevitable
Wind waves
wind driven waves that contain a small volume of water and do not submerge higher areas
influence the upper 100m
wavelengths 10s-100s of meters
wave height and length affected by windspeed
wave velocity maximum 10s of kph
waves break in shallow water and expand all stored energy
What is tsunami behavior
low wave height
long wavelength
frictional drag when water shallows
-waves grow in height, reaching 10-15m or more
What does tsunami destruction depend on
offshore bathymetry
-increase amplitude but limit wave energy
quick deep-to-shallow transition is the deadlies condition
waves have maximum energy
heights are modest
water pours into land as a sheet
broad, low land allows for maximum damage
steep rise of land- less damage
Tsunami prediction
scientific modeling predicts tsunami behavior
detection is expanding
detectors placed on ocean floor
senses pressure increase from changes in sea thickness
