Crustal and Subduction Zone Processes in Western Washington
and Background for Episodic Tremor
Dal Stanley (1) and Antonio Villasenor(2)
(1)USGS retired, Enumclaw WA, (2)Institute of Earth Sciences CSIC, Barcelona
Abstract
We present a new synthesis of key geophysical and tectonic data that aid in mapping the Cascadia margin of Western Washington to provide details of subduction, accretion, and recently observed 'episodic tremor'. In particular, aeromagnetic data are combined with previous seismic tomography to provide a detailed view of upper crustal and lower crustal features directly above the downgoing Juan de Fuca plate. The nature of the Seattle fault zone in relation to a northward moving mafic crustal block and interaction of this block with the downgoing Juan de Fuca plate is discussed to explain seismicity patterns and a complex 'transfer zone' as noted by Lamb et al. (2011). A characteristic tremor pattern has often initiated near this transfer zone to expand into a regional tremor event. We interpret that this characteristic tremor pattern reflects the unstable contact of the mafic block south of the Seattle fault with the subducting Juan de Fuca plate and the repeatable tremor pattern could be envisioned as a possible earthquake warning indicator. The tremor pattern from this key area and then surrounding the Olympic Peninsula geologic complex reveals that the tremor follows mapped tectonic details.And although there appears to be some triggering relationship to tidal forces, the very clear feature in the episodes is the role of the transfer zone at the west end of the Seattle fault. In addition to the geophysical details that define the Seattle fault zone block and the gap between this block and features in the deep crust to the north, we have found that that a significant multi-fault junction occurs on the east side of the Puget Lowland. In this junction the east end of the Seattle fault come close to intersecting the SE end of the South Whidbey Island fault, the Darrington-Devils Mountain fault, and the Rattlesnake Mountain fault. In addition the aeromagnetic data are used to define a feature that we call the Yakima Fold Boundary. This feature traverses the Columbia Plateau in a well defined, curving lineament that expresses what we interpret as the accretionary boundary between Cretaceous and older terranes and Tertiary terranes under the main part of the Plateau.
