TRACKS OF TRENCH-PARALLELING, OUTER FOREARC HILLS AND DEPRESSIONS ARE POSITED TO BE ZONES OF SEAFLOOR-EMERGING SPLAY FAULTS AND THE LIKELY SOURCE MECHANISMS FOR THE 1946 ALASKA AND 1957 ALEUTIAN TRANS-OCEANIC TSUNAMIS

INTRODUCTION: During high-magnitude (i.e., > Mw8.0) interplate ruptures, splay faults rising from the interplate decollement to surface in the deep water outer forearc have been considered likely launching mechanisms for high 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 recognized to be flanked by a 15-25-km wide, deep water (~5 km) band of irregular hills marking the seafloor emergence of a zone of splay faults rising from the interplate decollement. The zone runs parallel to the trench along the length (~900 km) of the peninsula’s outer forearc. 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 MCS data combined with more recent multibeam bathymetry determined that the splay fault zone separates the outer forearc’s structural backstop of lithified framework rock from a weakly consolidated frontal wedge of accreted trench sediment.

ALEUTIAN OBSERVATIONS: A ~Mw8.6 megathrust earthquake beneath the forearc of the Andreanof Aleutian Islands (172-179 W) launched the 1957 trans-Pacific tsunami. Beneath the Andreanof outer forearc a deep-water (~6 km), ~15-25 km wide and 500-1000-m deep depression, mapped as the right-lateral Hawley Ridge Shear Zone (HRSZ), runs ~parallel to the Aleutian Trench for several hundred km. Legacy MCS lines in combination with GLORIA sidescan imagery suggest that the HRSZ may also be the bathymetric expression of a splay-fault zone. Reprocessing of the legacy seismic lines is underway to more definitively resolve poorly imaged but suspected splay faults flanking and underlying the HRSZ. Along strike, the zone, as true for the Alaska Peninsula, separates the forearc’s Eocene igneous rock framework from a frontal wedge of young accreted trench deposits.

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