Sept 14 Lecture (earthquakes) Flashcards
where do eq usually occur?
- along linear belts
- belts generally parallel plate boundaries (some eq within the plates)
eq focal depths are:
deep, intermediate, shallow
define eq
= a shaking/ vibration of the ground
- release of built-up stress (occurs when rocks being deformed suddenly break along a fault)
- energy arises b/c friction prevents the 2 opposing rocks on either side of a fault from sliding past each other easily
3 main types of fault movements that initiate eq:
- Dip-slip= due to tensile stress
- Thrust faults= due to compressive stress
- Strike slip= due to shearing stress
focus =
the point on a fault where the first movement/ break occurs during an eq
epicenter=
the point on earth’s surface directly above the focus
when blocks slip suddenly at the time of the eq, intense vibrations called ___ ___ travel outwards from the focus
seismic waves
explain “elastic rebound”
with sudden displacement and release of stress, the rocks snap back elastically to their previous dimensions. The distance of displacement= the “slip”
2 types of seismic waves=
- body wave
2. surface wave
define body seismic waves
travels through the interior of the earth
- P-waves= compression waves
- S-waves= shear waves but pass only through solid rock (not magma)
define surface seismic waves
travels along the surface
- larger ground displacement than body waves
- results in most eq damage
___ of rock will affect the travel time for seismic waves
- waves move fast through ___ ___ rocks
- waves more more slowly through ___ ___ rocks
density
high density
low density
____ detects ground movement and can be useful in calculating the location of an epicenter.
- disadvantage=
seismograph
- need at least 3 seismographs to locate eq
there are 2 ways to measure the damage an eq causes:
magnitude
intensity
define magnitude
the amount of ground motion related to an earthquake
define intensity
effect on humans, and their structures, caused by the energy released by an eq
energy released by eq= related to ___ of rock and ___ of break (length x depth)
strength
area
2 ways of measuring magnitude:
richter magnitude
moment magnitude
richter magnitude=
measures amount of ground displacement/ shaking it produces at epicenter
- scale is logarithmic
- richter scale breaks down above 8.6: use moment magnitude
moment magnitude=
takes into account area of break on the fault surface, the displacement along the fault and the strength of the rock
- ie what happened at eq source rather than how much ground shakes at distant point
Mercalli intensity scale measures _________
impact of an eq on humans/ surface features.
Use roman numerals
- can be used to measure ancient eq based on old texts- useful for eq prediction
eq originate both at shallow depths (at ___ ___) and deep in earth (along ___ ___)
spreading ridges
subduction zones
shallow depth eq associated w/ :
divergent boundaries and transform fault
& on continents due to crustal movements
deep depth eq :
at subduction zones they occur b/c the brittle lithosphere is forced deep into the asthenosphere
true/ false
some eq are not associated with plate margins
- explain
true
- can occur on old faults that were once part of ancient plate boundaries
- some can arise due to isostatic rebound
eq damage can be caused by: (list 4)
- ground motion
- ground failure
- fire
- tsunamis
quality of ___ ___ determines in part the amount of ground shaking/ damage
- ground shaking also depends on movement of ___ and __ waves
bedrock
p and s
examples of ground failure due to eq
- landslides
- cracks
- liquefaction
why is fire a hazard related to eq?
broken gas lines
- esp hazardous w. wood houses
how do tsunamis work?
- motion on fault offsets seafloor and disturbs ocean surface
- wave disturbance moves outward in all directions
- wave height inc dramatically in shallow water near the shore
