Deep-water Splay Faulting and Trans-oceanic Tsunamis: Exploring the Possibility that a Splay Mechanism Launched the 1957, Mw8.6 Aleutian Tsunami

Abstract:

INTRODUCTION: During high-magnitude (i.e., > Mw8.0) interplate ruptures, splay faults rising from the interplate decollement to surface in deep water (>3000 m) have been considered the likely launching mechanisms for large, trans-oceanic tsunamis. Example tsunamis have been described from the Sumatra-Andaman, Makran, southern Chile, Cascadia, Alaska, Tohoku, Nankai, and Ryukyu margins.

NEW ALASKA OBSERVATIONS: The Alaska Peninsula (155-165 W) was recently found flanked by a zone of deep-water splay faults. In response to a Mw8.6 megathrust earthquake, this margin produced the infamous 1946 trans-Pacific tsunami that devastated Hilo, Hawaii. Reprocessing with modern software of 1970s and 80s legacy seismic data combined with more recent multibeam bathymetry, identified splay fault zones separating the outer forearc’s structural backstop of lithified framework rock from a weakly consolidated frontal wedge of accreted trench sediment. The zone of splay faults runs parallel to the axis of nearby Alaska Trench along the entire margin of the Alaska Peninsula (~900 km).

ALEUTIAN OBSERVATIONS: A Mw8.6 megathrust earthquake beneath the forearc of the Andreanof Aleutian Islands (172-179 W) launched the large 1957 trans-Pacific tsunami. Legacy MCS lines collected by the USGS in the early 1980s suggest that a deep-water splay-fault zone similar to that bordering the Alaska Peninsula also exists off the Andreanof Islands. In the early 1980s, the zone, which runs ~parallel to the Aleutian Trench for several hundred km, had been identified as the right-lateral, Hawley Ridge Shear Zone (HRSZ). Reprocessing of legacy seismic lines is underway to determine if, as surmised, the HRSZs is also a splay-fault zone that roots downward into the interplate decollement and extends along strike as a structural backstop separating the forearc’s Eocene rock framework from a frontal wedge of accreted trench deposits.

CONCERNING STATEMENT: The poorly bathymetrically mapped and seismically imaged Aleutian forearc may well prove to be fronted by lengthy sectors of deep-water splay-fault zones. If present, the likelihood that the Aleutian subduction zone will launch future destructive trans-Pacific tsunamis is increased.