Body waves (P and S waves) from distant earthquakes travel a curving path through the earth. As a result, they arrive at distant seismic stations from below.

Seismology is one of several fields which plays a role in monitoring the Comprehensive Test Ban Treaty. Underground nuclear explosions produce seismic waves with unique characteristics which allow the discrimination between explosions and earthquakes

Q: How many earthquakes does Alaska experience in 1 year? A: Over 30,000 earthquakes occur in Alaska every year!

The resulting focal mechanism has perpendicular nodal planes with the same “orange-slice” appearance observed for a normal fault. However, for a thrust fault earthquake, dilatations are on the outside and compressions are in the center.

When an earthquake occurs, seismologists create graphics of focal mechanisms, informally referred to as beach balls, to show the faulting motions that produce the earthquake. The focal mechanisms are based on the direction of the first arriving P wa