Comparative Seismotectonic Conditions Along the Sunda and Aleutian Arcs from Broadband Analysis of Earthquake Energetics

Abstract:

In the digital era, the Sunda arc has been the site of sixteen large and great (M_W>7.0) earthquakes that generated oftimes devastating tsunamis. In contrast, the largest tsunami earthquakes along the Aleutian arc occurred in the pre-digital era. Given the tectonic and geological commonalities between the two arcs, such as oblique plate convergence and long sediment-rich sections of trench, it is natural to ask whether the numerous and well-recorded Sunda events can serve as analogs to the more sparse Aleutian events for insight into seismic and tsunami potential. The large earthquakes are examined in the context of moderate-sized earthquakes (5.5<M_e <7.0, where M_e is energy magnitude) that occurred from 1987 to 2013. Using broadband hypocenters and radiated energies, we delineate fine details of the subduction zone and assess maturity of the faults. In the Sunda arc, energy release for moderate events was predominantly along the peripheries of the rupture zones of large events. Rather than on the slab interface, they often occurred on splay faults in the overriding plate, on perturbations of the slab interface under the outer-arc high or alongside obliquely subducted ridges and fracture zones. High strength regions (identified by high energy-release faults) resisted rupture during large earthquakes. Other splays seemed tsunami-capable. The greatest tsunamis came from earthquakes with rupture confined to the shallow forearc seaward of the outer-arc high, while greater seismic damage came from earthquakes confined to the deep forearc behind the outer-arc high. For the Aleutians, only two large events occurred during the same time period. But, like Sunda, the numerous moderate-sized events tended to demarcate the periphery of the rupture zone of past large events. Some splays with tsunami potential were observed, but the majority of events occurred at slab bends and around subducting fracture zones and seamounts, rather than about the subdued outer-arc high.