Earthquakes - Each year many people are killed during the collapse of buildings caused by the shaking associated with earthquakes. The worst earthquake in the 20th century occurred in Tang Shan, China on July 28, 1976 at 3:42 AM. Of the million inhabitants asleep in the city, 240,000 lost their lives in buildings that collapsed. the buildings were made of unreinforced brick walls could not stand the shaking associated with a large earthquake. When walls collapsed, the roofs caved in, and the sleeping inhabitants were crushed. In the United States the 1906 San Francisco earthquake is the most famous.

Elastic rebound - The earth is an elastic body which can be strained during plate tectonic processes. In particular, fault zones at the boundaries of large lithospheric plates lock despite the motion of the plates. Over time large strains can accumulate in rocks next to the fault zone. Finally, stresses become so large at the locked fault zones rupture and slip. This rapid slippage releases the strain energy which was accumulated over a long period of time in a process called elastic rebound. As the fault zone slips seismic waves are released.

Seismic waves - There are two groups of earthquake waves called body waves and surface waves. Body waves travel from the earthquake focus in all directions whereas surface waves travel around the earth rather than through it. Surface waves are slower than body waves. Regardless of its wavelength each type of body waves travels at one velocity.

Compressional waves - body waves characterized by vibrations parallel to the propagation direction of the wave. Because this wave is the fastest of the body waves it is called the primary wave (P-wave). This wave travels at 5 to 7 km per second near the surface of the earth and can pass through the liquid core of the earth.

Shear waves - body waves characterized by a series of sidewise or shearing movements. Because these body waves are slower than P-waves they are called secondary waves (S-waves). It shear waves which can not pass through a liquid such as the outer core of the earth.

Surface waves - By a property called dispersion, surface waves of different wave lengths travel at different velocities. It is the surface waves which cause most property damage because surface waves produce more ground movement and travel more slowly, so they take longer to pass.

Earthquake locations - An earthquake starts at the earthquake focus which is the fault zone along which the earthquake slips. Often the earthquake is located by epicenter or the position on the surface of the earth right above the earthquake focus. Earthquakes are located using a record of the earth vibration called a seismogram. Because earthquake waves travel in at different velocities they arrive at distance seismic stations in a certain order: P-waves before S-waves before surfaces waves. The separation in time of the first arrival of the P- and S-waves correlates directly with the distance of the earthquake from the seismic station and the distance can be graphed on a travel-time curve. For example, an eight minute separation between P- and S-waves on the travel-time curve indicates that the earthquake is 6000 km from the seismic station. Several stations are necessary to pin-point the location of an earthquake. This is done by drawing a circle on the globe with the radius equal to the distance between the epicenter and the seismic station. Having done the same for three stations, the point of intersection of the three circles is the actual epicenter of the earthquake.

Earthquake strength - One method for indicating how large an earthquake was is to measure the earthquake's effect on people and buildings. Earthquake intensity is determined on a scale of Roman numerals between I and XII according to the convention called the modified Mercalli scale. This method has drawbacks because buildings are constructed to different standards in different parts of the world. A more precise measure of earthquake strength is indicated by its magnitude on the Richter scale which has numerical values between 0 and 8.6. The Richter scale is defined by the maximum amplitudes of the P and S waves (the height of the waves on the seismogram) 100 km from an epicenter. Because the Richter magnitude scale is logarithmic, the difference between two consecutive whole numbers on the scale means an increase in earthquake vibrations of 10 times.

Earthquake prediction - Because lithospheric plates slip past each other at a well known rate, it is possible to determine the frequency with which large faults slip must slip in order to keep up the general motion of lithospheric motion. Because most large faults are locked, infrequent earthquake slip will take up slip accumulated over a long period of time. Depending on the size of the earthquake, slip can be released as often as once every dozen years or as seldom as once every 300 - 400 years. The frequency of slip and time of last earthquakes can be determined using a variety of techniques. With such information geophysicists can make predictions about when the next earthquake will occur along a certain fault.

Worldwide distribution of earthquakes - Earthquakes can occur most anywhere in the crust of the earth. However, most of the very large earthquakes occur in several belts which mark the boundary of the large lithospheric plates. On a map of worldwide earthquake distribution, the most concentrated belt is the circum-Pacific belt. Another major concentration of earthquakes is in the Mediterranean-Himalayan belt. Shallow focus earthquakes occur along the summit of the mid-ocean ridges of the world. Deep focus earthquakes are found below ocean trenches in steeply dipping zones called Benioff zones.