Earthquakes

Vibration of earth produced by rapid release of energy (seismic waves) with radiate in all directions from the source (focus) Elastic Rebound Theory Explains how this works.

Like ripples from dropping a stone in a pond, energy dissipates with distance

Earthquakes don't occur randomly 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

Earthquakes 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

Soviet Armenia: 1988

Mexico City - 1985

Kobe, Japan 1995

Alaska 1964

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

Mechanism Elastic rebound theory Duration of Shaking Varies 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 the 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 weakened structures. Duration of Shaking Varies 1960 San Francisco: 40 seconds 1989 Loma Prieta: 15 seconds 1962 Alaska: 4 minutes!!!!

Earth material can either fold or fault when put under stress

If the Earth folds, it is said to be ductile.

If the Earth faults, it is said to be brittle.

Normal Reverse

The following animations are types of faults in motion: Normal, Reverse/thrust, and Strike Slip

There are 2 types of Earthquake Waves: Body Waves and Surface Waves

Body Waves: Because they travel through the body of the earth.

Body Waves P waves: Pressure or compressional waves. Vibrate parallel to direction of wave travel like a slinky. Fast travel: 4-7 km/sec (15,000 mph) P is primary, or first wave to arrive at recording station

Body Waves S waves: Shear waves. Vibrates perpendicular to direction of wave travel. Like snapping a rope Slower than P wave: 2-5 km/sec (11,000 mph) So S is secondary, or second wave to arrive at recording station

Body Waves S waves: Shear waves. Vibrates perpendicular to direction of wave travel. Like snapping a rope Slower than P wave: 2-5 km/sec (11,000 mph) So S is secondary, or second wave to arrive at recording station

Earthquake waves in review

Examples of Body Waves on a Seismogram

Surface Waves: Because they travel along the surface of the Earth Surface Waves: Because they travel along the surface of the Earth. These waves travel slower than Body Waves and are very destructive.

Love Waves: cause particles to move from side to side, perpendicular to direction of travel, similar to S wave.

Rayleigh Waves: travel more slowly than Love Waves and cause particles to move in elliptical patterns

Types of Waves Earthquake waves = seismic waves. Recorded on seismometers on seismographs. 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

Earthquake waves in review

Seismogram Tracings

Locating the Epicenter of an Earthquake P, S and surface waves all start out at same time. The further you are away from the quake, the longer the time span between arrival of P and S wave. The distance of the seismometer to the earthquake can be determined by the time between the arrival of P wave and arrival of S waves. Can tell the distance, but not the direction. Therefore, at least 3 sites must be used to find epicenter.

Studying Earth’s Interior

Travel of P and S Waves Through the Body of the Earth How did geophysicists discover what was inside of our earth?

Travel of P and S Waves Through the Body of the Earth They discovered that P waves will travel through solid and liquid, whereas S Waves will only travel through a solid.