Investigations of Spatial and Temporal Variations in Strain Accumulation at the Sumatra Subduction Zone from Continuous GPS Measurements

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Bock 0001090 The west coast of Sumatra and the Mentawai Islands offer a unique opportunity to study the process of strain accumulation and release along the Sumatran fault and subduction zone. Two giant historical earthquakes (1833 and 1861) suggest that infrequent very-large earthquakes dominate strain relief at the Sumatra subduction zone. The Sumatran fault has also been known to produce several Mw7 earthquakes. GPS measurements collected from 1989 to 1993 revealed the spatial distribution of surface deformation, suggesting that the subduction interface is completely locked along the source region of the 1833 giant earthquake, but is only partially locked along the source region of the 1861 event. The data have also resulted in slip rate estimates for several segments of the Sumatran fault. The investigators plan to conduct campaign-style surveys at approximately fifty of their established GPS monuments on Sumatra and the Mentawai islands. The new measurements from these sites will be analyzed in conjunction with their earlier data to obtain an updated, time-averaged velocity field for the Sumatra forearc and one segment of the Sumatran fault. With this analysis they hope to resolve two important questions about the Sumatran fault and subduction zone. First, they hope to determine whether the strain segmentation of the Sumatran subduction zone observed by the 1989-1994 measurements is a long-term signal, or if it was a transient effect caused by, for example, an aseismic slip event. The second question they hope to resolve is the slip rate for the Sumatran fault segment located about 1 S. Their current estimate of the slip rate there is 235 mm/yr, considerably larger than the 11 mm/yr estimated from stream offsets. This is a collaborative proposal between UCSD's Scripps Institution of Oceanography (SIO) and the Indonesian National Coordination Agency for Surveying and Mapping (BAKOSURTANAL).