Joint Event Location — The JHD Technique and Applications to Data from Local Seismic Networks

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The joint hypocenter determination (JHD) technique is a relatively simple but efficient way to account for lateral velocity variations not included in the one-dimensional velocity models used to locate seismic events. The basic idea behind the technique is the simultaneous location of a group of events and the determination of a common set of station corrections. Under appropriate conditions the station corrections absorb the unmodeled velocity variations, thus improving the locations of the events. From the experience gained so far it appears that JHD always improves relative locations, while absolute locations may or may not be affected by some amount of error, depending on the nature of the lateral velocity variations. When they are very large, as when sedimentary basins and crystalline rocks are juxtaposed, the JHD locations may be affected by a systematic shift from the true locations. Here we discuss in detail four data sets which show different aspects of the JHD analysis. One is a synthetic data set based on actual data from the flat portion of the Nazca plate recorded in the Andean foreland in Argentina. These data are used to show the effect on location of exact and approximate implementations of the JHD technique. The other data sets come from the Loma Prieta, California, mainshock-aftershock sequence, from the New Madrid, central U.S.A., seismic zone, and from the Campi Flegrei, a volcanic area in Italy. The Loma Prieta and New Madrid data illustrate the improvements in absolute and relative locations that can be achieved when using JHD, while the Campi Flegrei data show the significant effect of complicated velocity variations on earthquake location. Finally, analyses of actual and synthetic data show that the JHD station corrections can be used for a semi-quantitative assessment of the 3-D lateral velocity variations under a local network.