The Sumatran Fault is a 1900-km-long trench-parallel strike-slip structure that accommodates much of the right-lateral component of oblique convergence between the Eurasian and Indian/Australian plates between about 10 degrees N and 7 degrees S. Most of the fault crops out subaerially along the mountainous backbone of Sumatra. The basic shape of this 1650-km-long portion of the Sumatran Fault is a 50-km-amplitude sinusoid, which mimics the shape of the Sumatran trench, about 290 km to the SW.
Our detailed map
of the fault, compiled from topographic maps and
stereographic aerial photographs, shows that, unlike many other
great strike-slip faults, the Sumatran Fault is highly segmented.
Cross-strike width of stepovers between the nineteen major subaerial
segments is commonly many kilometers. Most of these principal segment
boundaries are dilatational, but a few are contractional. The
influence of these stepovers on historical seismic source dimensions
suggests that the dimensions of future events will also be constrained
by fault geometry. Geomorphic offsets along the fault range as high
as 17 to 23 km, but only in regions where young volcanism and
dilatational stepovers have not buried or diverted ancient stream
courses. These offsets probably represent the total offset across the
fault. If so, other structures must have accommodated the dextral
component of oblique convergence until about 2 Ma ago. We propose that
the Batee and related faults in the forearc region, as well as the
subduction interface, served this function prior to about 2 Ma ago.
The shape and location of the Sumatran Fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatran volcanic arc have not influenced noticeably the geometry of the active Sumatran Fault. The average center of mass of the active volcanic arc is appreciably northeast of the Sumatran Fault. Mapping necessary to assess the possibility that the Pliocene volcanic arc influenced the location of the Sumatran Fault has not been conducted. Of the nine active volcanic centers within 2 km of the Sumatran Fault, 6 are within dilatational stepovers or on a bounding fault. This indicates that dilatational stepovers in the fault have encouraged volcanic activity locally. Distance of the Sumatran Fault from the subduction deformation front varies by no more than 10% from 290 km. The northwestern 450 km and southeastern 800 km of the fault are nearly parallel to the 110- to 140-km isobaths of the underlying subduction interface. The central 400 km of the fault, however, lies markedly athwart the strike of the subduction interface. This reach of the Sumatran Fault is coincident with a major bend of the subjacent subduction zone, a discontinuity in the volcanic arc and offshore forearc basin, and an 80-km-long crustal zone of reverse and strike-slip faulting. We support previous proposals that the geometry and character of the subducting Investigator fracture zone are affecting the shape and evolution of the Sumatran Fault along this section.
The most regular section of the Sumatran Fault comprises six right-stepping segments between the Equator and 4.3 degrees S. This pattern indicates that the overall trend of the fault deviates 4 degrees clockwise from the slip vector between the two blocks it separates. The regularity of this section and its association with the portion of the subduction zone that generated the giant (Mw 9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike-slip faulting and unusually large subduction earthquakes.
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11/08/99 |