(63).Earthquakes.

Does ground shaking cause the greatest damage in an earthquake?

This photo shows the Mission District of San Francisco burning after the 1906 earthquake. The greatest damage in earthquakes is often not from the ground shaking but from the effects of that shaking. In this earthquake, the shaking broke the gas mains and the water pipes so that when the gas caught fire there was no way to put it out. Do you wonder why the people standing in the street are looking toward the fire rather than running in the opposite direction?

Earthquake!

An earthquake is sudden ground movement caused by the sudden release of energy stored in rocks. Earthquakeshappen when so much stress builds up in the rocks that the rocks rupture. The energy is transmitted by seismic waves. Earthquakes can be so small they go completely unnoticed, or so large that it can take years for a region to recover.

Elastic Rebound Theory

The description of how earthquakes occur is called elastic rebound theory(Figure below).

[Figure 1]

Elastic rebound theory. Stresses build on both sides of a fault, causing the rocks to deform plastically (Time 2). When the stresses become too great, the rocks break and end up in a different location (Time 3). This releases the built up energy and creates an earthquake.

Focus and Epicenter

In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus (Figure below).

[Figure 2]

In the vertical cross section of crust, there are two features labeled - the focus and the epicenter, which is directly above the focus.

In about 75% of earthquakes, the focus is in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientists and the media.

Summary

A sudden release of energy stored in rocks causes an earthquake.

The focus is where the rocks rupture. The epicenter is the point on the ground directly above the focus.

Most earthquakes are shallow; these do the most damage.