Saturday, September 4, 2010

Epicenter of Earthquake

Although displacement in an earthquake is usually along the plane of a fault, seismic waves appear to radiate out in all directions from a single point. The epicenter of a quake is the point on the ground directly above that focus, which is called the hypo center. The focus may be many kilometers deep within the Earth. Quakes on subduction crustal plates can occur at such a depth that the surrounding rock is more molten and therefore too soft to sustain a brittle fracture. These quakes happen when minerals suddenly change into a denser phase as a result of increases in temperature and pressure.
The San Andreas Fault in California is the most famous crack in the world. Since the great earthquake of 1906 in San Francisco, no one has doubted the fault’s destructive power, released as the pacific crustal plat e slides slowly north past the North American plate. In places the fault lines cross urban and industrial areas where earth quake activity could be potentially devastating.

Our knowledge of why earthquakes happen is a major step towards improved prediction. In 1992 a 7.4 magnitude earthquake in California was mapped by combining satellite radar images take before and after the shock. The closest contours around the fault show the zone of maximum ground displacement. Multiple faulting caused the confuse zone near the epicenter. Radar mapping is more accurate than field surveys, which require monitoring equipment to be set up before shock.