HR: 1330h
AN: S42E-0204    [PDF]
TI: DETERMINATION OF THE RUPTURE PLANE FOR SMALL EARTHQUAKES RECORDED BY LOCAL NETWORKS.
AU: * Frez, J
EM: jofrez@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Nava, F A
EM: fnava@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Acosta, J
EM: jchang@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AB: A correct determination of the rupture plane of an earthquake is quite important for seismotectonic studies, particularly for regions, such as northern Baja California, where in many places recorded earthquakes are both small and few. For large earthquakes the rupture plane is readily determined from the aftershock spatial distribution or by fitting observed and synthetic seismograms; but when there are not enough well-located aftershocks and the event is small and, therefore, recorded locally so that there is a large influence of the local structure (including lateral heterogeneity) and site effects on the dominant high frequencies in the seismograms, the above mentioned techniques do not work. In these cases, fault plane solutions from first motion polarity and a (double couple) point source model give two nodal planes among which it is not possible to distinguish the fault plane from the auxiliary one. For a dense network operated in northern Baja California, we found that it is possible to identify the rupture plane from the Doppler effect produced by directivity. We first tried this technique on a ML = 3.6 event that occurred near the NW end of the San Miguel fault some 4km from the Vallecitos fault. For this event, directivity clearly shows the rupture propagating in a NE-SW direction roughly perpendicular to the NW-SE orientation of the nearby large faults. The technique was then applied to sample earthquakes of some of the seismicity patches in northern Baja California, where recorded earthquakes are small and epicenters do not conform to documented faults or to regional seismicity patterns. The tectonics implications of the results are discussed in detail.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting