Abstract

The analysis of arrival times indicates that the focus of the 15 June 1999 (20:42 UTC; 6.3 mb; 6.5 Ms; 7.0 Mw) Puebla, Mexico, earthquake is located at 18.15° N and 97.52° W and is 68 km deep in the contact region between the continental American and the Cocos subducting plate. First-motion data and regional and teleseismic waveform modeling indicate that the main rupture occurred as normal faulting along a fault striking N64° W and dipping approximately 42° toward the north-northeast. The aftershock activity, though scarce, is concentrated mainly toward the west of the mainshock. Three successive subevents are necessary to fit observed and synthetic waveforms. The location in space and time of subevents indicates that the rupture started as a small release at 76 km deep and then propagated toward the northwest releasing most of the seismic energy at 73 km deep. Teleseismic spectral analysis indicates that the total seismic moment release was 2.0 × 1019 N-m, and the mainshock fault length is of the order of 18 km with a stress drop of 15 bars. The source-time function consisted of three sources with a total duration of 10 sec. Recorded acceleration amplitudes of the mainshock show a clear enhancement that can be up to eight times higher toward the northwest of the epicenter in the frequency range from 0.06 Hz to 6.0 Hz, compared with the opposite direction. The distribution of reported seismic intensity also shows this feature. The directivity effect cannot be fully explained by the orientation of the fault, neither by the rupture propagation direction; therefore, we investigate the possibility that the directivity could be due to a regional propagation effect by analyzing the Lg coda decay of the mainshock. The average frequency-dependent QLg quality factor for the vertical component in the frequency range from 0.2 Hz to 2 Hz was QLg(f) = 200 ± 12f0.60±0.17 and QLg(f) = 202 ± 7f0.53±0.09 for paths toward the northwest and southeast of the epicenter, respectively. The resulting average factor Qo and η do not show significant azimuthal fluctuations. Therefore, variations of crustal attenuation properties cannot explain the notable extent of felt area and observed damage northwest of the epicenter. Probably the presence to the north of the Transmexican Volcanic Belt has played an important role in the enhancement of low-frequency seismic signals.

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