Seismic response to recent tectonic processes in the Banda Arc region

Internal structure and recent tectonics of the geologically complex Banda Arc region (latitude 1–9°S, longitude 124–135°E) have been studied by means of the analysis of global seismological data. Relocated hypocentral determinations of the International Seismological Center (EHB data) and fault plane solutions of the Global Centroid Moment Tensor Project have been used together with previously published information on regional geology and dynamics. The analysis considered earthquakes in the lithospheric wedges above subducting slabs and earthquakes in the uppermost portions of the Wadati–Benioff zones (focal depth < 100 km); we avoided intermediate-depth and deep earthquakes that are clearly separated from shallow ones by distinct discontinuities in the Wadati–Benioff zones. Such a selection best reflects present manifestations of tectonic processes. We delimitated 11 domains of earthquakes, each of them spatially distinct and with similar earthquake focal mechanisms. The number of events that did not fit in any domain was negligible. We associated each domain of earthquakes with distinct structural units and/or tectonic processes in the studied region. This correlation revealed that two discrete recent subduction zones act in the region – northward dipping Banda subduction in the south and southward dipping Seram subduction in the north. Though the shallow parts of the WBZs of both these subduction zones reach a similar depth of 100 km, the two WBZs differ substantially by dip angle and relation between shallow and intermediate-depth portions. A depth of 100 km of the shallow WBZs corresponds to – assuming an invariable plate convergence rate – a subduction cycle starting about 1–1.5 Ma ago. The present Banda subduction cycle follows the collisional event of the Timor–Tanimbar segment of the Australian plate; this collision now persists west and east of the Banda subduction zone around Timor and Tanimbar, as revealed by respective domains of earthquakes. No westward dipping WBZ has been observed to interconnect the Seram and Banda subduction zones into a single bent subduction zone. Instead, the area between them is cut by a significant elongated domain of earthquakes corresponding to the westward continuation (up to 129.5°E) of the Tarera–Aiduna fault zone. Another two significant domains of earthquakes characterised by frequent earthquake clustering in space and time are associated with active subduction-related Banda and Ambon volcanic arcs, respectively. A majority of these earthquakes/earthquake sequences is probably induced by magmatic processes beneath calc-alkaline volcanoes. Concentration of such events may reveal the existence of hitherto unknown active submarine volcanism in the Banda region.

► Tectonics of the Banda Arc region was studied by global seismological data.
► Eleven domains of shallow earthquakes were delimited and interpreted.
► Banda and Seram Wadati–Benioff zones point to two active slabs/subductions.
► Banda subduction is bounded by Timor and Tanimbar collisonal environment in the west and east, respectively.
► The left-lateral Tarera–Aiduna fault zone is a key regional tectonic structure.