Physical Science Notes

Earthquakes and Earth's Interior

Earthquakes

Over 30,000 eqrthquakes strong enough to be felt occur annually. About 75 significant earthquakes occur each year.

Earthquake: a vibration of Earth produced by a rapid release of energy. Ths energy radiates from all directions from its source, the focus, in the form of waves.

Earthquake waves are also called seismic waves.

Faults: large fractures in the Earth's crust.

Earthquakes occur at faults associated with tectonic plate boundaries.

Elastic Rebound: springing back of deformed rocks.

San Andreas Fault: most studied fault system in the world.

Fault Creep:gradual displacement with little seismic activity.

Stick slip:locked behavior followed by sudden slippage

Foreshock:less intense than the main earthquake event and occurring before

Aftershock:less intense than the main earthquake event and occurring after

Seismology: the study of earthquake waves

Seismograph: an instrument that records erthquake waves

Seismogram: record of ground motion

Types of Earthquake Waves

Surface waves: cause damage

Body waves: Primary (P) waves (compressional)
Secondary (S) waves

Rate of travel: P-waves > S-waves > Surface waves

P waves are transmitted through all states of matter: liquid, solid and gas. S waves are only transmitted through solid material.

Epicenter: the location on the Earth's surface directly above the focus.

Travel-time graphs from three different seismic stations can pinpoint the location of an Earthquake.

Earthquake Scales

Richter scale: based on the amplitude of the largest wave on a seismogram.

The largest recored earthquake measured 8.6 on the Richter scale - the equivalent of one billion tons of TNT

The Richter scale is a logarithmic scale. Each step on the Richter scale is a 10x increase in amplitude and a 30x increase in the amount of energy released.

The moment-magnitude scale is more precise. It is based on the area of the fault's rupture surface multiplied by the distance the earth moves along the fault.

The Mercalli scale is based on actual damage that occurs to structures.

Structural damage depends on:

  1. intensity of vibrations
  2. duration of vibrations
  3. nature of the material upon which the structure rests
  4. design of the structure

Soft sediments amplify vibrations.

Liquefaction: stable soil turns into a fluid

Secondary Effects

Tsunamis: seismic sea waves - tidal waves

Speeds of 300 to 600 mph, heights of 100 feet.

Fires: from damage to gas and electrical lines

Landslides and Ground Subsidence: lowered resistance

No reliable method of short-range prediction has been devised yet.

Earthquake Experiment

Links

Earth's Interior

Knowledge of the Earth's interior comes from the study of seismic waves.

There are four layers of the Earth:

  1. Crust
  2. Mantle
  3. Outer Core
  4. Inner Core

Moho boundary separates crust from mantle

Shadow zone created by deflection of P-waves off of boundary of outer core.

Asthenosphere plastic; capable of flow.

Lithosphere rigid, solid