SSA 2021: Estimating fault slip rates in the Cascadia region of North America using joint geologic-geodetic block modeling

This is the presentation I gave at the Seismological Society of America meeting this spring. It highlights ongoing work on incorporating block modeling into seismic hazard analysis.

Authors: Richard Styron, Tiegan Hobbs, Nick Harrichhausen, Murray Journeay

Abstract

The western margin of North America in the Cascadia region is known to be seismically active, but the locations, geometries, kinematics and slip rates of the primary upper plate faults are very incompletely known. This is in large part due to environmental challenges, including rugged terrain, dense vegetation, and rapid geomorphic processes that may quickly modify the signature left by crustal faulting. To improve seismic hazard analysis in the area, we are characterizing hundreds of faults throughout British Columbia, Washington, Oregon and their periphery through the construction of a large-scale, high-resolution tectonic block model. In the model, independently-moving blocks are separated by faults, which slip at the rate of local relative block motion. This technique allows us to incorporate and integrate a range of datasets including geologic mapping, high-resolution topography data, GNSS geodetic data, and Quaternary fault slip rate data. To make the model, fault traces are mapped in GIS. Then, blocks are constructed by connecting fault traces, considering these same data as well as earthquake hypocenters as well as other available information. The block modeling program then solves for the rotation poles of all blocks by inverting GNSS and Quaternary fault slip rate data, and accounting for the effects of locking of faults and the Cascadia megathrust on GNSS velocities, and predicts internally-consistent slip rates for each fault in the model. As this work is in progress, we will present provisional results, and discuss the possibilities and challenges of the work, including the nature of fault connectivity and off-fault deformation.