Near-Surface Geophysical, Geological and Geodetic Constraints on the Seismic Hazard of the Leech River Fault in the Northern Cascadia Forearc

Date: 4/24/2019

Time: 09:00 AM

Room: Puget Sound

The Leech River fault zone on southern Vancouver Island, British Columbia, which initially formed as a terrane-bounding fault during the Eocene accretion of the Crescent-Siletz terrane, has recently been identified as a Holocene-active structure that has produced at least three surface-rupturing earthquakes within the last 9 kyr. The proximity of the Leech River and adjacent faults to populated areas in SW British Columbia and NW Washington necessitates an updated assessment of their seismic (and tsunami) hazard. This requires characterization of potential rupture area, sense and magnitude of slip, and earthquake recurrence. Here we contribute constraints from several methods. Identification of brittle fault segments cutting bedrock and Quaternary deposits has been enabled by lidar and field mapping, along with imaging of the shallow subsurface via electrical resistivity tomography and ground penetrating radar, confirming disturbance along near-vertical zones underlying topographic lineaments. Paleoseismic trenching across some of these lineaments has yielded preliminary estimates of fault slip. Establishment of a new Global Navigation Satellite System network across SE Vancouver Island with an average inter-station spacing of ~4 km will provide additional constraints on the character and rate of slip on the Leech River and adjacent active faults. This network comprises 4 existing continuous stations and 21 campaign sites, 6 of which were previously surveyed in the 1990s and 15 were first occupied in August 2018. Repeated surveys are planned once every 1-2 years, to determine relative crustal motions across the study area, providing constraints on slip rates and on the extent of fault locking. Several years of data collection will be required in order to reduce the influence of megathrust processes (particularly Episodic Tremor and Slip events) on the relative site velocities and enable determination of strain accumulation on the crustal faults.

Presenting Author: Lucinda J. Leonard

Authors