Earthquake Location

Abstract

1 Introduction The exact location of a source, radiating seismic energy, is one of most important tasks in practical seismology and from time to time most seismologists have been involved in this task. The intention here is to describe the most common location methods without going into the mathematical details, which have been described in numerous textbooks and scientific papers but to give some practical advice on earthquake location. The earthquake location is defined by the earthquake hypocenter (x 0 , y 0, z 0) and the origin time t 0 . The hypocenter is the physical location, usually given in longitude (x 0), latitude (y 0), and depth below the surface (z 0 [km]). For simplicity, the hypocenter will be labeled x 0 , y 0 , z 0 with the understanding that it can be either measured in geographical or Cartesian coordinates, i.e., in [deg] or [km], respectively. The origin time is the start time of the earthquake rupture. The epicenter is the projection of the earthquake location on the Earth's surface (x 0 , y 0). When the earthquake is large, the physical dimension can be several hundred kilometers and the hypocenter can in principle be located anywhere on the rupture surface. Since the hypocenter and origin time are determined by arrival times of seismic phases initiated by the first rupture, the computed location will correspond to the point where the rupture initiated and the origin time to the time of the initial rupture. This is also true using any P or S phases since the rupture velocity is smaller than the S-wave velocity so that P-or S-wave energy emitted from the end of a long rupture will always arrive later than energy radiated from the beginning of the rupture. Standard earthquake catalogs (such as from the International Seismological Center, ISC) report location based primarily on arrival times of high frequency P waves. This location can be quite different from the centroid time and location obtained by moment-tensor inversion of long-period waves. The centroid location represents the average time and location for the entire energy radiation of the event.