JOINT HYPOCENTRAL LOCATION IN MEDIA WITH LATERAL VELOCITY VARIATIONS AND INTERPRETATION OF THE STATION CORRECTIONS

The application of a new implementation of the joint determination of hypocenters, origin times and station correction terms (JHD) technique to data from the Arkansas swarm (USA), the Campi Flegrei (Italy), and the Nazca plate (Argentina) throws new light onto the problem of earthquake location. In all these cases good-quality P- and S-wave arrival times were available. The main results are as follows. (1) Large lateral velocity variations (LLVV) are generally associated with relatively large station corrections. These corrections, in turn, are usually much larger than the average of the station residuals determined when the same set of events is located using a single-event location (SEL) method. (2) The locations determined by the JHD and SEL methods may be substantially different. (3) Since the effect of LLVV on JHD locations is not well established, simple 3-D velocity models are constructed so that the JHD technique applied to synthetic data generated using these models produce station corrections similar to those observed. (4) The analysis of the synthetic data also shows that the locations determined by the JHD method are much closer to the true locations than the locations determined with the SEL method. The use of the weighted average of the SEL station residuals as 'station corrections' does not give better located events, although there is a large decrease in the r.m.s. residual computed for each event. Therefore, the use of r.m.s. residuals to estimate the quality of event locations is not always warranted. In view of these results it is possible to conclude that the JHD station corrections can be used to detect lateral velocity variations and to produce locations substantially less affected by these variations than the locations determined by the SEL method. Furthermore, under appropriate conditions the JHD station corrections can also be used in a forward modeling scheme to get a semiquantitative velocity model. Such a model may be the only information that can be obtained from a data set when the station and event distributions are not adequate for 3-D velocity inversion.