High Precision Estimation of Seismic Wave Arrival Times

When an earthquake occurs, vibrations propagate from the source in all directions. For large earthquakes, vibrations are soon observed even at sites far from the source. Generally, such a vibration is called a seismic wave. The seismic waves which propagate through the interior of the earth are comprised of two types: longitudinal P waves and transverse S waves. Additionally, surface waves (Rayleigh waves, Love waves, etc.) which propagate close to the surface of the earth also exist. These body waves and surface waves propagate with velocities which depend on the physical properties of the earth interior. Generally the velocity of the seismic wave increases with depth in the earth. Furthermore, the velocity of P waves is approximately 1.73 times than that of S waves, while the velocity of surface waves is 0.92 times that of the S waves. Therefore, depending on the distance of the observation point from the hypocenter and the depth of the hypocenter, there are differences in the arrival times of the individual waves. At present a table (travel time table) showing the relation between the distance, depth, and arrival times is available, and may be used to estimate the hypocenter. Observation of the arrival times at many observation points together with the use of the table allows the time and location of the earthquake to be estimated. Also, in contrast, a detailed velocity structures of the earth can be estimated by using many of the observation data of accurate arrival times. This results in the generation of a more accurate travel time table, thus more accurate hypocenter locations.