The November 29, 1978, Oaxaca, Mexico, earthquake: A large simple event

Abstract

The recent Oaxaca, Mexico, earthquake November 29, 1978 (MW = 7.6, Ms = 7.8, seismic moment, M0 = 3.2 × 1027 dyne cm), is of special interest because of its location within a predetermined seismic gap. The event excited long‐period (100–200 s) multiple Rayleigh and Love waves which were well‐recorded by the WWSSN. These data along with P wave first‐motion data and P waveforms were used to constrain the source mechanism. The results indicate an oblique thrust mechanism consistent with subduction of the Cocos plate to the northeast beneath Mexico (dip = 14°, strike = N90°W, rake = +54°); hence this event is indeed of the type anticipated by Ohtake et al. (1977). A local network of stations, installed in a joint University of Mexico‐California Institute of Technology program, began operation 20 days prior to the mainshock (González, 1979; McNally et al., 1979; McNally, 1981; Ponce et al., 1980). The 43 foreshocks of magnitude M ≥2.8 were recorded by the network in a period of 20 days prior to the mainshock. These events show an interesting spatial and temporal pattern, which culminates in the last 1.8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake. This pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity. With supplemental stations, the network continued operation until December 12, 1978 (Singh et al., 1980) and provided good hypocentral control for the more than 169 aftershocks of magnitude M ≥ 2.8. The area of the aftershock zone determined from these events is 5525 km2 (85 km by 65 km). In spite of the large size of the event, P waves for the Oaxaca event indicate an extremely simple source, at the period range of the WWSSN long‐period seismographs. This simplicity suggests that the P waves were generated by a limited portion of the rupture surface, perhaps by the breaking of a fault zone asperity. This result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves. Such simplicity also appears to be the case for the 1965 and 1968 Mexican earthquakes on the adjacent eastern and western ends of the Oaxaca aftershock zone, respectively (Chael et al., 1980). This type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries.