Faulting process of the August 8, 1993, Guam earthquake: A thrust event in an otherwise weakly coupled subduction zone

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Journal of Geophysical Research-Solid Earth
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Aug 10
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We study a large M(w) = 7.7 earthquake that occurred on June 8, 1993, slightly offshore and under the island of Guam in the southern Mariana island are. From a complete study of P and SH body waves, a relocation of the aftershocks, and the subevents of the main shock, we propose a relatively simple model of the rupture process of this event. We propose that this earthquake ruptured a shallow-dipping thrust fault that corresponds to the subduction interface under Guam, Like many other earthquakes, this event started with a small foreshock and was followed by two large energy release events located to the northeast of the epicenter along the subduction zone, The rupture process had a relatively short duration of about 32 s, with a weak starting phase that lasted about 8 s, Seismic moments estimated from body waves, surface waves, and Global Positioning System (GPS) are very similar of the order of 4.5 x 10(20) N m. The displacement field produced by our best model was compared to the GPS measurements of coseismic slip obtained by Beavan et al. [1994]. We find an excellent agreement both in displacement direction and magnitude between the predicted and observed GPS displacements. This appears to be then the largest earthquake to have occurred on a shallow-dipping thrust fault in the Mariana subduction zone during this century, Its occurrence requires a reassessment of the concept of seismic coupling in this subduction zone.


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