An earthquake doublet in Ometepec, Guerrero, Mexico

Abstract

On June 7, 1982 an earthquake doublet occurred in a gap near Ometepec, Guerrero, Mexico which had been given a high seismic potential. The two earthquakes (first event: M_s = 6.9, m_b = 6.0, 16.3°N, 98.4°W, d = 25 km; second event M_s = 7.0, m_b = 6.3, 16.4°N, 98.5°W, d = 8 km) of the doublet occurred within five hours of each other. We determine the source parameters of these events by inverting surface-wave data at a period of 256 s. The results are for the first event, strike = 116°, dip = 77°, slip = 88° and seismic moment of 2.8 × 10^(26) dyne • cm, and for the second event strike = 116°, dip = 78°, slip = 89° and seismic moment of 2.8 × 10^(26) dyne • cm. Modeling of long-period P waves suggests that the first event has a depth of 20 km and is represented by a single trapezoidal source time function, with an effective duration of 6 s. The second event is best modeled by two sources at depths of 15 and 10 km. The combined effective source duration time for the two sources is about 10 s. The ratio of the seismic moment, obtained from body waves to that from surface waves, is ∼ 0.5 for the first event and 1 for the second. Adding the seismic moment of the two events and considering the first week aftershock area, 3200 km^2, the stress drop is ∼4 bars. These results suggest that the first event, that involved a deeper asperity, caused an incremental stress change large enough to trigger the second event. If the two events of the doublet broke distinct areas of the subduction zone, the coseismic slip is 0.58 m, and accounts for about 25% of the total plate motion between the Cocos Plate and the North America Plate, accumulated since the last large earthquake in the region. Other doublets similar to the Ometepec doublet have occurred along the Middle America Trench during the past 70 years. A regional distribution of comparable-size asperities may be responsible for this relatively frequent occurrence of doublets and for the simplicity of earthquakes in the region. The high convergence rate, which produces rapid strain accumulation and short recurrence intervals for large earthquakes, and the smooth sea-floor subducted along the middle America Trench, may contribute to the homogeneous distribution of comparable size asperities. We found a relation, log T ≈ 1/3 log M_0 (T is the average recurrence time and M_0 is the average seismic moment) for large earthquakes along the subduction zone in the Guerrero-Oaxaca region, where the convergence rate and the properties of the subducted plate are considered relatively uniform. A simple asperity model predicts this relation.

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