Source Process of the Great 1977 Sumba Earthquake

In order to investigate the applicability of the asperity model to intraplate earthquakes we have studied the rupture process of the great 1977 Sumba normal-faulting earthquake (M_w=8.2-8.3), one of the largest earthquakes since 1963. This event has been variously interpreted as a plate detachment event (i.e., rupture through the entire lithosphere) or a shallow plate-bending event. We have analyzed long-period body waves in order to determine the spatiotemporal rupture characteristics, including the depth extent of the rupture area. Visible depth phases in the initial stage of weak radiation beginning about 15 s before the main pulse suggest that the earthquake nucleated near 29 km depth, which corresponds to the maximum depth of the aftershock sequence. The excitation of a strong tsunami implies that rupture extends to the surface, while deconvolutions of long-period P waves suggest a relatively shallow (≤ 50 km) maximum depth extent. However, the existence of slip below this depth cannot be ruled out, particularly if it preferentially radiates frequencies below the passband of the body waves used (< 0.02 Hz). For undiffracted phases the source time function of the principal rupture, which initiated 30 km west of the epicenter of the precursor, comprises a dominant, initial short-duration (24 s) triangular pulse followed by several smaller pulses. However, slightly diffracted phases indicate a simple basic (underlying) character for the rupture, consisting of the initial 24-s triangle superimposed on a longer-duration (42-44 s) trapezoid. The absence of observable directivity in the deconvolved source time functions suggests a bilateral rupture mode. Although the body waves do not require a fault length greater than 80 km, the aftershock area and surface wave results of Zhang and Kanamori [1988] imply a total length of at least 200 km. The body wave source functions are well modeled by a 200-km-long fault, extending to 30-50 km depth, on which slip nucleates at the center of the fault and has variable displacement along strike. The peak moment release of the deconvolved source functions indicates higher displacement in the hypocentral area than in other areas of the fault. The spatial relationship of the precursor and the main pulse is suggestive of rupture initiating at and propagating along the edge of an asperity prior to the rupture of the asperity itself. The large size of the event may be due to an unusual stress environment arising from strong lateral gradients in seismic interplate coupling along the Java-Timor arc.