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EARTHQUAKES

I. Earthquake: general information


A. Definition: a vibration of the Earth usually due to a sudden release of energy

B. Elastic rebound theory: explains most earthquakes


1. Energy (stress) stored and land deforms (strain)

2. Rx can not hold = snaps

3. Land releases energy and returns to normal shape

C. Usually occur on faults


1. Epicenter = on surface





2. Focus a.k.a. hypocenter = a few kilometers to nearly 700 km

D. other causes


1. Folding -- rare

2. Loading & unloading

3. Magma moving or boiling


II. Seismic waves


A. Seismograph = instrument (records waves and time) (i.e. shaking, graph)

B. Types of waves


1. Body waves = travel through the Earth


a) P wave (primary)


(1) fastest -- varies with the material (book gives 6 km/s as typical surface material speed)

(2) weakest

(3) push pull

(4) travel through anything including air

(5) jolt

b) S wave (secondary)


(1) slightly slower

(2) slightly stronger

(3) shear wave

(4) travels through solids only

(5) rolling

2. Surface waves


a) Love wave


(1) slower yet

(2) stronger

(3) sideways shifting

(4) on surface only

(5) heavy shaking


b) Rayleigh wave


(1) slowest but nearly same as Love wave

(2) strongest

(3) backwards flip

(4) on surface only

(5) bucking bronco

C. Locating an earthquake


1. P and S waves travel at same ratio of speeds

2. Time between them varies with distance between observer and focus

3. Need three stations




D. Earth’s layers


1. Compositional = different materials
a) crust -- direct sampling
b) mantle -- indirect sampling via volcanoes but only of the upper most part
c) core -- modeled from meteorites = Fe = up to 10% Ni





2. Strength = behaves differently
a) lithosphere = brittle and generates earthquakes

b) asthenosphere = up to 10% melt
c) mesosphere = ? but becoming more known
d) outer core = liquid
e) inner core = solid

III. Earthquake Measurements


A. Richter magnitude


1. developed in 1930 at Cal Tech, Pasadena

2. based on the deflection of a seismograph with distance figured into calculation

3. log base 10 scale for deflection

4. energy increases about 30 times per number

5. independent of human reaction

B. Modified Mercalli scale of intensity
1. last modified by Richter

2. reports energy and ground response

3. based on damage done to human artifacts

4. given in Roman numerals I to XII

a) I = felt only by seismograph

b) XII = total damage with things thrown up into the air

IV. Earthquake hazards


A. Ground motion: may damage roads, knock things off shelves, etc. -- but usually not a problem

B. Fault movement: may offset things built on fault

C. Fire: may be the biggest hazard

D. Ground failure: mass wasting (landslides)

E. Liquefaction = loss of bearing strength


1. Require saturated silt

2. Shaanxi, China 830,000 people died

F. Tsunami: now known to travel at up to 800 km/hr

G. Isolation

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