Seismotectonics of the Sunda Subduction Zone

• The Java subduction zone is thought to be slipping mostly aseismically, and thus the occurrence of a large thrust earthquake here in 1994 was unusual.
• The Sumatera subduction zone is dominated by thrust earthquakes, and so the large, principally strike-slip earthquakes that occurred in June 2000 were unexpected.

The 1994 Java Tsunami Earthquake: Slip Over a Subducting Seamount

Journal of Geophysical Research, 106, 6595-6608, 2001

Abstract. On 2 June, 1994, a large subduction thrust earthquake (M_S 7.2) produced a devastating tsunami on the island of Java. This earthquake had a number of unusual characteristics. It was the first recorded large thrust earthquake on the Java subduction zone. All of the aftershock mechanisms exhibit normal faulting; no mechanisms are similar to the mainshock. Also, the large tsunami and the relatively low energy radiated by the mainshock have lead to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the mainshock and the aftershocks. We then invert long period surface waves and broadband body waves to determine the depth and spatial distribution of the mainshock slip. A dip of 12^o, hypocenter depth of 16 km and moment of 3.5x10²⁰ Nm (M_w 7.6) give the best fit to the combined seismic data and are consistent with the plate interface geometry. The source spectrum obtained from both body and surface waves has a single corner frequency (between 10 and 20 mHz) implying a stress drop of ~0.3 MPa. The main energy release was preceded by a small sub-event lasting about 12 s. The main slip occurred at about 20 km depth, down-dip and to the NW of the hypocenter. This area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount (Figure 1) . We interpret the Java earthquake as slip over this subducting seamount, which is a locked patch in an otherwise decoupled subduction zone. We find no evidence for slow, shallow rupture (Figure 2) . No thrust aftershocks are expected if the entire locked zone slipped during the mainshock, but extension of the subducting plate behind the seamount would promote normal faulting as observed. It seems probable that such a source model could also explain the size and timing of the observed tsunami.
 
 

The June 2000, M_w7.9 earthquakes south of Sumatera: Deformation in the India-Australia Plate

Journal of Geophysical Research, submitted, June 2001

Abstract. Two large earthquakes occurred on 4 and 18 June (M_w7.9), south of Sumatera in the Indian Ocean. Both earthquakes were predominantly left-lateral strike-slip on vertical, north-south trending faults that we interpret to be reactivated fossil fracture zones (Figure 3). The 18 June earthquake in the Wharton Basin is one of the largest shallow strike-slip faulting earthquakes recorded. A small, second subevent with reverse slip is required to fit the bodywaves. The orientation of both subevents in our preferred model is consistent with the current stress field in the region. The purely strike-slip source model of the Wharton Basin earthquake obtained by Robinson et al. [2001] is unable to fit the P waves, and does not fit the S waves any better than our preferred model. The occurrence of the 4 June, Enggano, earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. The hypocenter of the Enggano earthquake was at the southeastern end of the rupture area of the 1833 subduction earthquake. The strike-slip subevent of the Enggano earthquake triggered a thrust subevent on the plate interface, which comprised 35% of the total moment and ruptured away from the region of the 1833 earthquake. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India-Australia composite plate. The strike-slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the Enggano earthquake, consistent with previous observations of oceanic and subduction earthquakes.