Earthquakes Flashcards
where do earthquakes generally occur? where do they rarely occur?
-along linear belts which are usually parallel plate boundaries
-within plates
earthquakes can have different focal depths. what are they?
-deep (> 300 km)
-intermediate (50-300 km)
-shallow (< 50 km)
what are earthquakes?
shaking or vibration of the ground
earthquakes are the release of built-up stress. How does the stress build up?
stress is built up when rocks that are being deformed suddenly break along a fault -> the energy comes from the fact that friction prevented the two rocks from sliding past each other easily
what is a fault?
the fracture between two blocks of rock
what is the fault offset?
horizontal displacement between points on either side of the fault which can range from millimeters to kilometers
greatest evidence for fault offset?
features like fences/roads that have been displaced
What are the 3 main types of fault movements that cause earthquakes & the stresses that cause them?
- dip-slip due to tensile stress (rocks pulled apart)
- thrust faults due to compressive stress (rocks pushed together)
- strike-slip faults due to shearing stress
fault plane (def.)
the fracture itself
fault scarp (def.)
part of the fault plane that is exposed to the surface
fault trace (def.)
where the fault plane meets the earth surface
focus (def.)
the point on a fault where the 1st break happen during the earthquake
epicenter (def.)
the point on the earth’s surface that is directly above the focus
what are seismic waves?
intense vibrations that travel outwards from the focus when the rocks slip during earthquake
what is elastic rebound?
when after the displacement and release of stress, rocks snap back elastically to their previous dimensions
displacement of rocks during earthquake is called what?
slip
Two types of seismic waves
1) body wave
2) surface wave
what are body waves?
waves that travel through the interior of the earth
what are the two types of body waves and type of movement they display?
- P waves (compression waves - like sound waves)
- S waves (shear waves but they only pass through rock and not magma)
which are slower, P or S waves?
S waves
what are surface waves?
travels along the surface
which type of wave causes the most ground displacement, body or surface waves?
surface
which type of wave results in the most earthquake damage, body or surface waves?
body
why do we need to understand seismic waves?
to build earthquake resistant structures
what are the two types of surface waves?
-rayleigh waves (particle motion is like a circle/like ocean)
-love waves (particle motion is side-to-side)
which waves can cause the most damage, love or rayleigh waves?
love wave (side-to-side motion (like a snake wriggling) causes the ground to twist from side to side)
how does the density of rock will affect the travel time for seismic waves?
waves move fast through high density rocks
waves move slow through low density rocks
what does a seismograph do?
detects ground movement; useful in calculating the location of epicenter
how does seismograph calculate location of epicenter?
it records arrival of different seismic waves. the interval of time between the first arrivals of P and S waves is a function of distance to the epicenter (P waves arrives first)
it requires how many seismographs to locate an earthquake?
3
magnitude (def.)
the amount of ground motion related to an earthquake
intensity (def.)
effect on humans, and their structures, caused by the energy released by an earthquake
energy released by earthquake is related to two characteristics?
the strength of rock and area of break (length x depth)
how was the original richter magnitude measured?
-measure difference between arrival of S and P waves
-measure maximum amplitude
-intersection = magnitude
richter scale breaks down when?
8.6 (use moment magnitude)
the damage an earthquake causes can be measured in two ways (2)
1) magnitude
2) intensity
what are the two types of magnitude?
1) richter = measures amount of shaking at epicentre (logarithmic)
2) moment = takes into account area of break on fault surface, displacement, and strength of rock; measure what happened at earthquake source not at distant point
what does mercalli intensity measure?
the impact of an earthquake on humans and surface features; used in US and Canada; reported as roman numerals (I- NOT FELT, XII - DAMAGE TOTAL)
what are some of the advantages of mercalli intensity?
-can be used to measure ancient earthquakes from old texts and folkore
-modern technologies allow general pop’n to send observations and measurements to seismologists
earthquakes start at two depths. what are they?
-shallow: spreading ridges, associated with divergent boundaries and transform faults, continents
-deep: subduction zones
san andreas fault produces what kind of earthquake?
-it is a transform fault; so it is a shallow EQ
Earthquakes that are not associated with plate margins are usual. Why do they occur?
-occur on old faults that were once part of ancient plate boundaries (south carolina) or isostatic rebound (eastern canada)
4 ways earthquakes cause damage
1) Ground motion (Ground shaking & fault movement)
2) Ground failure
3) Fire
4) Tsunamis
What can determine the amount of ground shaking caused by EQ?
quality of bedrock
Types of ground failure
1) Landslides
2) Cracks
3) Liquefaction
What is liquefaction?
strength & stiffness of soil reduced by earthquake shaking or other rapid loading
What does liquefaction at depth cause?
collapse of surface
how can fire start after EQ?
broken gas lines
How can EQ cause tsunamis?
1) EQ cause seafloor to be uplifted and down-dropped pushing water up and down
2) potential energy transferred to kinetic energy of wave
3) column of water splits into two (distant and local)
4) local travels to coast and distant travel to deep ocean (deep ocean travels faster -> speed varies as squar root of water depth)
5)local tsunami amplitude increases over continental slope; wave length decreases
6) tsunami runup occurs = fast tides
tsunami that form vertical walls = ____
bores
most of the damage caused by tsunamis are caused by ______
strong currents and floating debris
How to migate earthquake damage?
-avoidance (better to build away from earthquake zones)
-engineering (use better construction designs and materials)
-predicting/giving warnings
things like tsunami barriers do what?
they may not really stop damage, false sense of security
learning from past EQ: columns
introducing columns with more vertical and wrapping rebar; preventing shear failure
Predication and Forecasting of EQ examples
1) Geological field work (ancient EQ)
2) Seismic gaps (active fault that hasn’t slipped in a long time)
3) Precursors
4) Prediction
5) Forecasting
4 causes of induced EQ
1) Fluid injection & withdrawal
2) Hydraulic fracturing
3) Dams (loading)
4) Engineered EQ? -> mining , fracking, geothermal energy
animals + EQ
stabled animals are fidgety before EQ, free range are fine
