Source mechanism of two 1994 intermediate-depth-focus earthquakes in Guerrero, Mexico
Por:
Quintanar L., Yamamoto J., Jiménez Z.
Publicada:
1 ene 1999
Resumen:
In May and December 1994, two medium-size, intermediate-depth-focus earthquakes occurred in Guerrero, Mexico, eastward of the rupture area of the great Michoacan earthquake of September 19, 1985. Even though these are not major earthquakes (~6.4 M(w)), they were widely felt through central and southern Mexico, with minor damage at Zihuatanejo and Acapulco, located along the Pacific coast, and Mexico City. Both earthquakes, separated by ~100 km, have similar focal depths and magnitudes, however, their focal mechanisms, based upon the polarities of first arrivals, show some differences. The May earthquake shows a clear normal faulting mechanism (? = 307°, ? = 55°, ? = -108°), whereas the December earthquake mechanism solution suggests an initial thrust faulting (? = 313°, ? = 62°, ? = 98°) process. Although previous analysis, including local and teleseismic stations, reported a normal faulting for the December earthquake, we find that modeling using the CMT focal mechanism solution fails to reproduce the first 5 sec of the observed P-wave signal at the nearest broadband station (? = 168 km) and the S-wave polarity at two strong ground-motion local stations (? = 32, 53 km); in fact, the best fit for these stations is obtained using the thrust focal mechanism calculated from the first-motion method. Seismic moment value and rupture duration time deduced from the teleseismic spectral analysis are: 2.0 X 1018 N-m and 6.9 sec for the May event; 2.8 X 1018 N-m and 7.1 sec for the December earthquake. From the inferred seismic moment, an average ?? of ~15 bars for both earthquakes is obtained. Inversion of teleseismic P-wave data indicates a better fit using the CMT focal mechanism solution (normal faulting) than the first-motion mechanism for both earthquakes, although the adjustment's differences are small for the May event; for this earthquake, the rupture consisted of two sources separated by ~7 sec, starting at a depth of ~40 km and then propagating downdip, reaching a depth of ~60 km. The December earthquake however, released, all its energy at a depth of 50 km in two main sources separated by ~10 sec. The non-double-couple components values are -0.004 and -0.01 for the May and December events, respectively, indicating that the December shock has a small contribution of non-d
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