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

Journal Article · Wed Jun 10 04:00:00 EDT 1981 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB086iB06p05053· OSTI ID:6034884

Stewart, G S; Chael, E P; McNally, K C

The recent Oaxaca Mexico, earthquake November 29, 1978 (M/sub W/ = 7.6, M/sub s/ = 7.8, seismic moment, M/sub 0/ = 3.2 x 10/sup 27/ 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/sup 0/, strike = N90 /sup 0/W, rake = +54/sup 0/); 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 (Gonzalez, 1979; McNally et al., 1979; McNally, 1981; Ponce et al., 1980). The 43 foreshocks of magnitude M> or =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> or =2.8. The area of the aftershock zone determined from these events is 5525 km/sup 2/ (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. The 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.

• Research Organization: Seismological Laboratory, California Institute of Technology, Pasadena, California 91125
• OSTI ID: 6034884
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 86:B6; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
AFTERSHOCKS
DEVELOPING COUNTRIES
EARTHQUAKES
FORESHOCKS
LATIN AMERICA
LOVE WAVES
MEXICO
NORTH AMERICA
PLATE TECTONICS
RAYLEIGH WAVES
SEISMIC EVENTS
SEISMIC P WAVES
SEISMIC SURFACE WAVES
SEISMIC WAVES
TECTONICS
WAVE PROPAGATION