Stress changes and aftershock distribution of the 1994 and 2006 Java subduction zone earthquake sequences

Abstract

[1] The Mw = 7.8 1994 and the Mw = 7.7 2006 interplate thrust mechanism earthquakes that occurred in the Java subduction zone produced dominantly normal-faulting aftershocks, unusual for large megathrust main shocks. Various models proposed for these earthquake sequences invoke main shock rupture on an isolated portion of a decoupled plate boundary fault, with updip and outer-rise extension leading to the normal faulting. Other models suggest that these aftershocks occurred in a zone of the subduction zone where usually earthquakes cannot propagate or initiate, leading to the occurrence of normal-faulting aftershocks in the outer rise, overriding and subducting plates. Here we examine a simpler possibility, one in which Coulomb stress changes (ΔCFS) imparted by slip during the two large subduction events led to normal-faulting events on favorably oriented planes within the slab and near trench region of the subduction zone. We compute stress changes resulting from both events and subsequent large aftershocks using both uniform and variable slip models for main shock slip, resolved onto both aftershock nodal planes. We find that there is not a clear pattern of aftershock occurrence in areas of stress increase due to main shock slip. This implies that these aftershocks are not simply triggered by the static stress changes from the main shock and additional complexity should be considered to explain these unusual earthquake sequences.