Timor Collision Front Segmentation Reveals Potential for Great Earthquakes in the Western Outer Banda Arc, Eastern Indonesia

Abstract

In Eastern Indonesia, the western Outer Banda arc accommodates a part of the oblique Australian margin collision with Eurasia along the Timor Trough. Yet, unlike the Wetar and Alor thrusts of the Inner Banda arc in the north and the adjacent Java subduction zone in the west, both recent and historical seismicity along the Timor Trough are extremely low. This long-term seismic quiescence questions whether the Banda Arc collision front along the Timor Trough is actually fully locked or simply aseismic and raises major concerns on the possible occurrence of large magnitude and tsunamigenic earthquakes in this vulnerable and densely populated region. Here, we jointly analyze multibeam bathymetry and 2D seismic reflection data acquired along the Timor Trough to characterize the location, nature, and geometry of active faults. Discontinuous narrow folds forming a young accretionary prism at the base of the Timor wedge and spatially correlated outcropping normal faults on the bending northwest Australian shelf reveal two concurrent contrasting styles of deformation: underthrusting and frontal accretion. We find that those tectonic regimes and their associated seismic behaviors depend on 1) the thickness of the incoming and underthrusting Cenozoic sedimentary sequence, 2) the vergence of inherited normal faults developed within the continental shelf, and 3) the depth of the décollement beneath the Timor wedge. Based on the along-strike, interchanging distinct deformation style, we identify the mechanical and seismic segmentation along the Banda arc collision front and discuss the implications for earthquake and tsunami hazards along the western Outer Banda arc region.

Kota Djogo: The Island that Never Was …

“…The smaller triangular area formed by the Savu Fault and the larger islands of Flores and Sumba is marked to the north by a chain of stratovolcanoes that runs along the southern coast of Flores, which includes Kelimutu, Iya, and other potentially extinct volcanoes in Ende Bay ( Van Suchtelen 1921). The same area is also marked by normal and strike-slip faults, which generate seafloor vertical displacements that could account for sudden sinking (Coudurier-Curveur, Singh, and Deighton 2021;Ely and Sandiford 2010:117). In all, taking geological descriptions into consideration, it may be plausible to propose that Kota Djogo constituted a reef surrounding an ancient undersea ridge, volcano, or mount that lay in close proximity to the Savu Fault.…”