Western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington

摘要

We present evidence that the Seattle faultnzone of Washington State extends to the westnedge of the Puget Lowland and is kinematicallynlinked to active faults that border thenOlympic Massif, including the Saddle Mountainndeformation zone. Newly acquired highresolutionnseismic reflection and marinenmagnetic data suggest that the Seattle faultnzone extends west beyond the Seattle Basinnto form a >100-km-long active fault zone. Wenprovide evidence for a strain transfer zone,nexpressed as a broad set of faults and foldsnconnecting the Seattle and Saddle Mountainndeformation zones near Hood Canal. Thisnconnection provides an explanation for thenapparent synchroneity of M7 earthquakesnon the two fault systems ~1100 yr ago. Wenredefi ne the boundary of the Tacoma Basinnto include the previously termed Dewattonbasin and show that the Tacoma fault, thensouthern part of which is a backthrust ofnthe Seattle fault zone, links with a previouslynunidentifi ed fault along the western marginnof the Seattle uplift. We model this northsouthnfault, termed the Dewatto fault, alongnthe western margin of the Seattle uplift asna low-angle thrust that initiated with exhumationnof the Olympic Massif and todaynaccommodates north-directed motion. ThenTacoma and Dewatto faults likely controlnboth the southern and western boundaries ofnthe Seattle uplift. The inferred strain transfernzone linking the Seattle fault zone andnSaddle Mountain deformation zone defi nesnthe northern margin of the Tacoma Basin,nand the Saddle Mountain deformation zonenforms the northwestern boundary of thenTacoma Basin. Our observations and modelnsuggest that the western portions of thenSeattle fault zone and Tacoma fault are complex,nrequire temporal variations in principalnstrain directions, and cannot be modeled as ansimple thrust and/or backthrust system.