A shallow crustal earthquake doublet from the Trans-Mexican volcanic belt (Central Mexico)

The Trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc-parallel faulting. The Mezquital graben is a major intra-arc basin of the central Trans-Mexican volcanic belt. A doublet of moderate shallow shocks occurred in March and October 1976 in the region of this graben. These earthquakes were recorded by the Mexican National Seismological network, in particular, by the Bosch-Omori seismograph (T-0 = 18 sec) at the Tacubaya Observatory in Mexico City. We have carefully relocated the two main shocks and their major aftershocks by reading the original records and using a modified crustal velocity model for this region. A difference of similar to50 km is observed between the locations reported by the Mexican Seismological Service and those obtained in this study, which are additionally supported by the damage distribution of these earthquakes. A first-motion analysis, based on regional and teleseismic records, defines for the March and October shocks normal fault mechanisms, characterized by east-west-striking fault planes, which coincides with the orientation of the master faults of the Mezquital graben. After calculating the instrumental response, the source parameters were obtained from the Bosch-Omori seismograph records by body-wave modeling. For the March earthquake, we estimate a seismic moment of 4.5 X 1023 dyne cm (equivalent to M-w 5.0) and a stress drop of 0.7 MPa, assuming a circular rupture model (radius, 3 km). Given the poor quality of the Bosch-Omori record for the October earthquake, we used comparison with the March event of long-period (T = 20 sec) teleseismic records at two stations to obtain corresponding source parameters for the October event. By assuming a similar stress drop as for the March event, we obtain a M-0 of 5.6 X 10(23) dyne cm and M-w 5.1 with a rupture length of 6.5 km. According to gravity data, the regional east-west faults are longer than reported. In particular, our detailed measurements indicate that the master fault of the Mezquital graben continue westward in the subsurface below the basin fill. In a more regional context, additional magnetic data suggest that the El Mezquital graben and Aljibes half-graben might be connected at their southern limits by part of the same fault system. This is the first earthquake doublet reported from central Mexico and the biggest seismic event in that zone during the past 50 years. From a physical point of view, it is possible that the seismic energy released by the March event was partially transferred toward the east, triggering the October shock further east on the same fault system. The presence of moderate seismicity in this zone should therefore be taken into account when assessing its seismic hazard.