Geology 101 - Lecture 19
Great Earthquake links:
USGS up-to-the-minute earthquake
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
- 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
- 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
- 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
- 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
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
- 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
- Big quakes in convergent margins, small ones in divergent