The tectonics of the eastern Sunda and Banda arcs of eastern Indonesia are inferred from the centroid depths, seismic moments, and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984. Source parameters are estimated by inverting long-period, teleseismic P and SH waves. Earthquakes show that the collision of the Australian continent with the Banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the Timor and Seram troughs. Along the eastern end of the Sunda arc and along the southern Banda arc, earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence, by elongating in the direction perpendicular to convergence, and by thrusting over the back arc basin. Shallow thrust and strike-slip earthquakes beneath the Banda Basin (the back arc basin) demonstrate that deformation within the back arc accomodates some of the northward motion of the Australian continent. The rate of north-south shortening of the entire upper plate at the longitude of Timor calculated from the seismic moments of the earthquakes over the 22-year period is roughly 20% of the predicted convergence rate between Australia and southeast Asia.

Strike-slip faulting within the Banda Basin results also in eastward motion of the Banda arc with respect to western Indonesia and Australia. Eastward extrusion of the Banda arc results in thrusting at the Aru trough. Earthquakes due to thrusting of the back arc basin beneath the volcanic arc are two shallow to corrobrate that the arc's polarity has reversed although the Wetar back arc thrust zone at present has a high seismic slip rate. Earthquakes and their mechanisms provide evidence that the lithosphere of the Weber Basin, a 7500-m-deep forearc basin, rests on the subducting lithosphere, without intervening asthenosphere, so that the Weber Basin subsides in response to sinking of the subducting lithosphere. Closure of the Banda Basin is geometrically incompatible with the Australian plate subducting simultaneously beneath both the Timor and Seram troughs. Instead, the Bird's Head, which subducts beneath Seram, is decoupled from the Australian plate in western New Guinea and probably moves west or southwest with respect to Australia. In the eastern Indonesian example, strike-slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc-continent collision. ¿ American Geophysical Union 1988