Focal mechanism determination and identification of the fault plane of earthquakes using only one or two near-source seismic recordings

A waveform inversion scheme was developed in order to explore the resolving power of one or two seismic recordings at short epicentral distance for th e determination of focal mechanisms and the identification of the fault pla ne of earthquakes. Two key features are used to constrain the fault paramet ers with a reduced number of stations: (1) a simple finite-dimension source model and (2) the modeling of the complete displacement field, including t he near-field waves. The identification of the fault plane should be possib le, even with a single station, as soon as the seismograms produced by the two nodal planes of a same focal mechanism are significantly different, whi ch is the general case when waveforms are controlled by source finiteness. Seven parameters, including the strike, dip, rake, and dislocation, are exp lored with a grid search, and the minima of the misfit error between the ob served and calculated seismograms are mapped. With such an approach, it is possible to conclude about the uniqueness or nonuniqueness of the solutions . The method is tested with three earthquakes of moderate to large size for which the fault plane is well established and for which strong-motion reco rds are available at maximum distances of a few tens of kilometers. Test ev ents are the 1994 Northridge (M-w = 6.7, California), the 1996 Copala (M-w = 7.3, Mexico), and the 1996 Pinotepa Nacional (M-w = 5.4, Mexico) earthqua kes. In the case of inversions with two stations, we find a unique solution , or a group of similar solutions, with a good estimation of the focal mech anism and the proper selection of the fault plane. Our results also show th at in some cases a single station may be enough to recover the fault parame ters. The inversion scheme presented here may be systematically applied to future earthquakes, especially to those recorded by few stations. It should be particularly useful in the case of blind faults for which the fault pla ne may not be identified with the help of other data.