Earthquake location

Earthquakes are detected by seismographs, instruments that respond to very small ground displacements, veloc ities, or accelerations associated with the passage of seismic waves. Since 1961 there has been an extensive and standardized global network of seismograph stations to monitor earthquake activity. The original World-Wide Standardized Seismograph Network (WWSSN), based on analogue instruments, has gradually been superseded since 1986 by the Global (Digital) Seismograph Network (GSN). By 2004 there were 136 well-distributed GSN stations worldwide, including one on the sea floor between Hawaii and California. It is hoped that this will be the first of several in oceanic areas devoid of oceanic islands for land-based stations. Digital equipment greatly facilitates processing of the data and also has the advantage that it records over a much greater dynamic range and frequency bandwidth than the earlier paper and optical recording. This is achieved by a combination of high frequency, low gain and very broadband seismometers (Butler et al., 2004). Most countries have at least one GSN station and many countries also have national seismometer arrays. Together these stations not only provide the raw data for all global and regional seismological studies but also serve an important function in relation to monitoring the nuclear test ban treaty, and volcano and tsunami warning systems.

Earthquakes occurring at large, or teleseismic, distances from a seismograph are located by the identification of various phases, or seismic arrivals, on the seismograph records. Since, for example, the direct P and S waves travel at different velocities, the time separation between the arrival of the P phase and the S phase becomes progressively longer as the length of the travel path increases. By making use of a standard model for the velocity stratification of the Earth, and employing many seismic phases corresponding to different travel paths along which the seismic waves are refracted or reflected at velocity discontinuities, it is possible to translate the differences in their travel times into the distance of the earthquake from the observatory. Triangulation using distances computed in this way from many observatories then allows the location of the epicenter to be determined.

The focal depths of teleseismic events are determined by measuring the arrival time difference between the direct phase P and the phase pP (Bath, 1979). The pP phase is a short path multiple event which follows a similar path to P after first undergoing a reflection at the surface of the Earth above the focus, and so the P-pP time difference is a measure of focal depth. This method is least accurate for foci at depths of less than

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