Source parameters of two moderate Mexican earthquakes estimated from a single-station, near-source recording, and from MT inversion of regional data: A comparison of the results

Two recent, moderate earthquakes (March 27, 1996, Mw=5.4; January 21, 1997, Mw=5.4), which occurred close to the town of Pinotepa Nacional, Oaxaca, Mexico, were recorded at the near-source VBB station of PNIG and by several other stations of the Mexican VBB seismological network located at regional distance. Both events saturated the velocity sensor (STS-2) at PNIG. We first use velocity and displacement records, obtained by direct integration of the accelerograms at PNIG, to obtain location, depth, origin time, focal mechanism, seismic moment, and source time function of the two earthquakes. We then perform a moment tensor (MT) inversion of regional band-pass filtered (20 to 50 sec) displacement seismograms to estimate the source parameters of the two earthquakes. For these events, the MT solutions are reasonably close to those obtained from the near-source data of the single station of PNIG. Our results show that: (a) detailed source parameters can be retrieved from a single, near-source, high-quality three-component recording, and (b) a rough estimation of focal mechanism and seismic moment of Mexican earthquakes may be possible from MT inversion of regional data using presently known crustal structure. But more precise determinations would require a better knowledge of this structure, mainly to estimate the centroid depth