Seismic history and seismotectonics of the Sunda Arc

Historic records of the last 300 years reveal two great interplate earthquakes (1833, MW = 8¾ 1861, MW = 8¼–8½), which ruptured major segments of the Sumatra fore arc in western Indonesia; a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically. The ends of these rupture zones are coincident with clusters of large (Ms ≥ 7) and moderate (6 ≤ Ms ≤ 7) shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data. The northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes, a structural arch in the fore arc, and a fracture zone on the subducted plate. The southern portion is coincident with a very prominent cluster and the along-strike extension of a sediment-filled trough on the subducted plate. Prior to this study, Sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century. Java and the Lesser Sunda Islands had major (Ms ≥ 6) earthquakes in the historic record, but none are of the same magnitude as the great events near Sumatra. There is no clear evidence that shocks in the Java fore arc were interplate events, yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the Roo Rise, a prominent bathymetrie high, appears to be interacting with the fore arc. This feature was formed at a ridge crest and is more buoyant, and thus more difficult to subduct, than the surrounding seafloor. There is no increase in seismicity associated with this tectonism, however, that may indicate a lack of contact between the crystalline portions of the plates at this time. We infer that a majority of slip on the plate interface near Java occurs aseismically. The variation in occurrence of great interplate earthquakes along the Sunda Arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere. Great interplate earthquakes occur near Sumatra, where the age of the youngest crust is 46 Ma. The characteristics of the Sunda Arc, and analogies with Pacific arcs, imply that the entire length of Sumatra has the potential to produce great thrust earthquakes; its seismic potential should be considered high (as there has been no recurrence of the great events of 1833 and 1861). In contrast, no such events have been reported off Java and the Lesser Sunda Islands, where the oldest crust is 152 Ma. The plate interface near Java and the Lesser Sunda Islands should be considered to have low seismic potential.

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