Seismological evidence for shallow thrusting north of the Timor trough

Summary . The eastern Sunda arc in Indonesia is the site of subduction of the continental crust of Australia and its shallow seismicity is manifested by infrequent, large ( M s ≥ 7) earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc. In the same region, volcanic activity has ceased and small shallow earthquakes are rare. We examine two of the larger earthquakes: the events of 1962 May 15 ( M s = 7.2) and 1977 August 27 ( M s = 7.0). The 1977 event occurred north of central Timor and the inversion of long‐period P‐ and SH ‐waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea‐level and a seismic moment of 4.1 × 10 26 dyne cm. The P ‐wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone (the Wetar thrust zone) at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone. A broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred. The 1962 earthquake occurred approximately 300 km north‐east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the Banda arc. The similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism. Modelling of P dif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km, for which a seismic moment of 5 × 10 26 dyne cm is estimated. These fault plane solutions indicate that most of the seismic energy released by the collision between Australia and the Sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent. We suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin (the Banda Sea) southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc's polarity.