Abstract

The 2004 Sumatra (Mw 9.1) earthquake was preceded by a seismic quiescence that began 13 years before the mainshock. An earthquake catalog created by the International Seismological Centre is analyzed in the study area, 80°–110° E, 10° S–20° N, between 1964 and 2004, including 1153 earthquakes shallower than 100 km with body‐wave magnitude of 5.0≤mb≤6.7. A detailed analysis of the earthquake catalog using a gridding technique (ZMAP) shows the quiescent area is located between 3° and 6° N, which covers the southeastern part of the focal area, including the rupture initiation point of the 2004 Sumatra earthquake. The observed spatial pattern of quiescence can be explained by stress perturbation due to a long‐term slow slip located on the deeper edge of the mainshock fault, which is predicted by a numerical simulation based on a laboratory‐derived friction law. Although the quiescence is not significant statistically, it may still be considered unlikely that such an anomalous quiescence would occur at almost the same time and location as other long‐term anomalies reported in the previous studies, including decreases in b‐value and tidal triggering.

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