Earth Sciences 240A Lecture 13

PART E: Earthquakes

Introduction (from Prerequisite)

Types of faults

Elastic rebound theory

Geometry of a fault

Body waves; surface waves

Scales

Modified Mercalli scale

Richter magnitude scale

Location and magnitude determination

Website

http://www.sciencecourseware.com/VirtualEarthquake/VQuakeExecute.html

Richter Magnitude Scale

“Standard earthquake”

Amplitude 1 mm (log scale)

Magnitude 3 (arithmetic scale)

Distance away 100 km (log scale)

Nomograph

Assumptions

‘Average’ rock type

100 km distance to focus

Scale of released energy uniform

[different for large quakes, so Ms too low]

Other Scales

M_L = Common Richter Scale

m_b where ‘b’ is body wave amplitude (only 1-10 s period used)

M_S where ‘s’ is surface wave (only Raleigh waves 18-22 s period used)

Moment scale or The (Seismic) Moment Scale

Moment = (rock rigidity) x (fault area) x (distance)

Use log scale

Mo = S A d

Distance in cm, force in dynes

Useful for interpretation of mapped faults

Alaska earthquake 1964:

ML = 8.3; Mo = 9.2

Ground Motion

Components:

Vertical

Horizontal

Rate of change of motion= acceleration

Acceleration of 1 g ?

Vertical OK

Horizontal: 0.1g to 0.2g can’t standup; 0.2g structural damage

Big earthquake: ~1.8 g

Period and Resonance

Example of flagpole

Without/with metal cap

With ground wave

Coincidence of resonance (vibration)

Construction design aim

Match building material and foundation material resonance properties

Horizontal Accel. Probability map

Hazard prediction

Recall New Madrid 1811-12

San Francisco Bay

Liquefaction

Transformation from solid to liquid

Water-saturated soils

Loss of cohesiveness

Amplification of ground motion effects

Example: Loma Prieta earthquake

Fatalities vs. Energy Release

(see Table)

Next

Wednesday (Oct.8): San Andreas

Friday (Oct.10): Misc. examples

Monday (Oct. 13): Holiday

Wednesday (Oct.15): Mid-Term

All material (including Prerequisite) to-date.