4. HOW THE GROUND SHAKES What happened in past quakes & will happen in future ones.

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Presentation transcript:

4. HOW THE GROUND SHAKES What happened in past quakes & will happen in future ones

Disturbances as function of space & time that travel through solid earth Like electromagnetic (light) waves, water waves, or sound (ultrasound, sonar, dolphins, bats) Use to study earthquakes and “see” earth structure Davidson 8.11 SEISMIC WAVES

Seismic waves occur because an applied force makes rock deform elastically & then return to original shape Compression - volume changes Shear distortion: shape changes Davidson 5.3

P or compressional waves - volume changes - material compressed or expanded in direction wave propagates S or shear waves - distortion without volume changes - material sheared in direction perpendicular to wave propagation DD 7.2

DD 7.5 April 18, :37 Southern Illinois M component seismograms Earthscope transportable array at Hobbs, NM P wave travel time (min)

Activity 4.1: How fast in miles per hour do seismic waves travel?

Seismic wave amplitudes decrease with distance from an earthquake DD 7.7 Hough et al 2000 University of Nevada, Reno Shaking and damage decrease with distance

Seismic waves travel better in the colder rock of the stable Midwest than in the west An earthquake in the Midwest causes more shaking than a similar sized one in the west S. Hough DD 7.8

Seismic wave amplitudes increase as waves travel from solid (faster) rock to weaker (slower) material, so buildings on soft sediment are more vulnerable than ones on solid rock DD 7.10 San Francisco Marina district landfill 1989 Loma Prieta earthquake

This is analogous to the way water waves grow taller as they go from deep to shallow water DD Sumatra earthquake tsunami

Arrival times of seismic waves used to locate earthquakes Amplitudes used to find magnitudes DD 8.9

Pattern of seismic waves radiated by earthquakes used to find fault geometry & dimensions DD 8.9

At interfaces, seismic waves change direction and amplitude DD 7.11 These changes reveal the physical properties within the earth

Analyzing the times and amplitudes of seismic waves is the primary tool for studying structure of the earth Stein & Wysession, 2003