Earthquake Seismology
Earthquake Seismology : When a major earthquake occurs, the resultant energy released into the earth will propagate over a wide range of frequencies and velocities. Even though the earth movements discernible to the viewer may be confined to the general region of the earthquake origin, the various seismic wave phases propagating throughout the earth result in small, but measurable, ground displacements which can be detected by a seismometer. A seismograph then provides a visual analog record of the ground displacements at that station. For the purposes of the Tsunami Warning System, consideration is given to three significant seismic wave phases. The first, the P-wave, is a compressional wave travelling through the earth's interior at a velocity varying from approximately 8.0 km/second near the crust-mantle interface to about 13.5 km/second at the mantle-core interface. As such it is the first seismic phase to be recorded at any one seismic station and is the first indication that a distant earthquake has occurred The second seismic phase of importance is the S-wave, or Secondary wave. This phase travels through the earth's interior as a shear wave, following approximately the same travel path as the P-wave but at a reduced velocity varying from approximately 6.7 km/second near the crust-mande interface to about 8.0 km/second near the core. By measuring the time separation between the arrival of the P and S waves, the distance of the epicenter from that seismic station the origin time of the earthquake can be determined. By obtaining the P arrival times at a series of seismic stations appropriately located, the location of the earthquake epicenter can be determined by assuming the "best fit" of a series of intersecting arcs representing distances from the various seismic stations used. These seismic wave phases are classified as body waves due to their propagation through the earth's interior. In addition to providing a location, body waves may be useful in determining the size of an earthquake.The third set of seismic phases to be considered are the surface waves resulting from ground displacements propagating outward along the surface of the earth. These are observed at a seismic station as local or regional surface waves and are the basis for measuring magnitude on the Richter scale. The actual energy released for each increment of the Richter scale is a factor of 32. Thus a magnitude 6.0 earthquake will release 32 times as much energy as a magnitude 5.0, and a magnitude 8.0 is more than 1000 times greater than a 6.0
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