The 1994 Java tsunami earthquake: Slip over a subducting seamount

On June 2, 1994, a large subduction thrust earthquake (Ms 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 main shock. Also, the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the main shock and the aftershocks. We then invert long-period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip. A dip of 12°, hypocenter depth of 16 km and moment of 3.5×l0²⁰ N m (Mw 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 subevent lasting ∼12 s. The main slip occurred at ∼20 km depth, downdip 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. 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. No thrust aftershocks are expected if the entire locked zone slipped during the main shock, 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.