Sept 14 Lecture (earthquakes)

Question 1

where do eq usually occur?

Answer 1

• along linear belts

- belts generally parallel plate boundaries (some eq within the plates)

Question 2

eq focal depths are:

Answer 2

deep, intermediate, shallow

Question 3

define eq

Answer 3

= 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

Question 4

3 main types of fault movements that initiate eq:

Answer 4

1. Dip-slip= due to tensile stress
2. Thrust faults= due to compressive stress
3. Strike slip= due to shearing stress

Question 5

focus =

Answer 5

the point on a fault where the first movement/ break occurs during an eq

Question 6

epicenter=

Answer 6

the point on earth’s surface directly above the focus

Question 7

when blocks slip suddenly at the time of the eq, intense vibrations called ___ ___ travel outwards from the focus

Answer 7

seismic waves

Question 8

explain “elastic rebound”

Answer 8

with sudden displacement and release of stress, the rocks snap back elastically to their previous dimensions. The distance of displacement= the “slip”

Question 9

2 types of seismic waves=

Answer 9

1. body wave

2. surface wave

Question 10

define body seismic waves

Answer 10

travels through the interior of the earth

• P-waves= compression waves
• S-waves= shear waves but pass only through solid rock (not magma)

Question 11

define surface seismic waves

Answer 11

travels along the surface

• larger ground displacement than body waves
• results in most eq damage

Question 12

___ of rock will affect the travel time for seismic waves

• waves move fast through ___ ___ rocks
• waves more more slowly through ___ ___ rocks

Answer 12

density

high density

low density

Question 13

____ detects ground movement and can be useful in calculating the location of an epicenter.
- disadvantage=

Answer 13

seismograph

- need at least 3 seismographs to locate eq

Question 14

there are 2 ways to measure the damage an eq causes:

Answer 14

magnitude

intensity

Question 15

define magnitude

Answer 15

the amount of ground motion related to an earthquake

Question 16

define intensity

Answer 16

effect on humans, and their structures, caused by the energy released by an eq

Question 17

energy released by eq= related to ___ of rock and ___ of break (length x depth)

Answer 17

strength

area

Question 18

2 ways of measuring magnitude:

Answer 18

richter magnitude

moment magnitude

Question 19

richter magnitude=

Answer 19

measures amount of ground displacement/ shaking it produces at epicenter

• scale is logarithmic
• richter scale breaks down above 8.6: use moment magnitude

Question 20

moment magnitude=

Answer 20

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

Question 21

Mercalli intensity scale measures _________

Answer 21

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

Question 22

eq originate both at shallow depths (at ___ ___) and deep in earth (along ___ ___)

Answer 22

spreading ridges

subduction zones

Question 23

shallow depth eq associated w/ :

Answer 23

divergent boundaries and transform fault

& on continents due to crustal movements

Question 24

deep depth eq :

Answer 24

at subduction zones they occur b/c the brittle lithosphere is forced deep into the asthenosphere

Question 25

true/ false
some eq are not associated with plate margins
- explain

Answer 25

true

• can occur on old faults that were once part of ancient plate boundaries
• some can arise due to isostatic rebound

Question 26

eq damage can be caused by: (list 4)

Answer 26

• ground motion
• ground failure
• fire
• tsunamis

Question 27

quality of ___ ___ determines in part the amount of ground shaking/ damage
- ground shaking also depends on movement of ___ and __ waves

Answer 27

bedrock

p and s

Question 28

examples of ground failure due to eq

Answer 28

• landslides
• cracks
• liquefaction

Question 29

why is fire a hazard related to eq?

Answer 29

broken gas lines

- esp hazardous w. wood houses

Question 30

how do tsunamis work?

Answer 30

• 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