What are Earthquakes? The shaking or trembling caused

What are Earthquakes? The shaking or trembling caused by the sudden release of energy Usually associated with faulting or breaking of rocks Continuing adjustment of position results in aftershocks What is the Elastic Rebound Theory? Explains how energy is stored in rocks Rocks bend until the

strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to an undeformed shape Energy is released in waves that radiate outward from the fault The Focus and Epicenter of an Earthquake

The point within Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter Seismographs record earthquake events

At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt most of these result from convergent margin activity ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers

more than 150,000 quakes strong enough to be felt are recorded each year The Economics and Societal Impacts of EQs Building collapse Fire Tsunami Ground failure

Damage in Oakland, CA, 1989 What are Seismic Waves? Response of material to the arrival of energy fronts released by rupture Two types: Body waves P and S Surface waves

R and L Body Waves: P and S waves Body waves P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction

as wave movement S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular to wave movement Surface Waves: R and L waves

Surface Waves Travel just below or along the grounds surface Slower than body waves; rolling and side-to-side movement Especially damaging to buildings How is an Earthquakes Epicenter Located? Seismic wave behavior P waves arrive first, then S waves, then L and R Average speeds for all these waves is known After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance

from the seismograph to the epicenter. How is an Earthquakes Epicenter Located? Time-distance graph showing the average travel times for P- and Swaves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves How is an Earthquakes

Epicenter Located? Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn

The intersection of the circles locates the epicenter