Geology 101 - Lecture 19

Earthquakes

Great Earthquake links:

USGS up-to-the-minute earthquake information

University of Washington Earthquake Information

Great maps of earthquake hazards

Most destructive forces on Earth. But it is buildings and other human structures that cause injury and death, not the earthquake itself

1988 - Soviet Armenia: magnitude 6.9, 25,000 people died
1985 - Mexico City: magnitude 8.1, 9500 people
1989 - Loma Prieta, CA: magnitude 7.1, 40 people died
1995 - Kobe, Japan: magnitude 7, ~6000 people died

30,000 earthquakes worldwide annually that are strong enough to be felt, but typically only 75 of them are considered to be significant

Earthquakes occur in rigid part of earth - lithosphere (not asthenosphere)

A. Definition

Vibration of earth produced by rapid release of energy (seismic waves) with radiate in all directions from the source (focus)
Like ripples from dropping a stone in a pond, energy dissipates with distance

Earthquakes don't occur randomly. Occur on faults or fractures within the earth
Explained by plate tectonics. Most occur on plate boundaries. Sometimes in plate interiors if enough stress is built up

B. Mechanism

Not known until after 1906 quake in San Francisco
Elastic rebound theory (DEMONSTRATION)
- Forces bend rock on either side of fault, rock strains ever so slowly, then weakest point breaks. Break sends out shock waves, which migrate outwards from he original break, causing shaking. Stress is released
- Aftershocks are adjustments to that change in stress. They are always less strong than the main shock, but they may cause more damage to already weakened structures.

C. Duration of Shaking Varies

1960 San Francisco: 40 seconds
1989 Loma Prieta: 15 seconds
1962 Alaska: 4 minutes!!!!

D. What Causes shaking?

Earthquake waves = seismic waves. Recorded on seismometers on seismographs.

E. Seismic waves are important for 2 things

locating earthquakes - know how to do this using P&S travel time curve
describing the Earth's interior

F. Types of Waves

Surface waves - travel on Earth's surface, away from epicenter.
- Very slow waves. Cause a lot of damage, rolling feeling at end of earthquake

Body Waves - travel through Earth's interior, spread outward from focus
- P waves:
  - compressional (pressure) waves. Vibrate parallel to direction of wave travel. Like slinky (DEMONSTRATION).
  - Fast travel: 4-7 km/sec (15,000 mph)
  - P is primary, or first wave to arrive at recording station
- S waves:
  - shear waves. Vibrates perpendicular to direction of wave travel. Like snapping a rope (DEMONSTRATION)
  - Slower than P wave: 2-5 km/sec
  - So S is secondary, or second wave to arrive at recording station

G. Locating the Epicenter of an Earthquake

P, S and surface waves all start out at earthquake at same time. The further you are away from the quake, the larger the time span between arrival of P and S wave.

Time lag between arrival of P wave and arrival of S wave is used to determine the seismometer's distance from earthquake.
So can tell distance, but not direction.
You did this for the warmup

Earthquake belts - convergent margins, divergent margins, transform margins

Big quakes in convergent margins, small ones in divergent