SCEC Award Number 19126 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Dense, decadal-scale displacement field across the locking-creeping transition zone of the San Andreas fault near Parkfield
Name Organization
Sean Bemis Virginia Polytechnic Institute and State University
Other Participants Michael Vadman, PhD student
SCEC Priorities 1a, 2a, 3e SCEC Groups Geology, Geodesy, SAFS
Report Due Date 04/30/2020 Date Report Submitted 12/02/2020
Project Abstract
The locked-to-creeping transition of the San Andreas fault occurs over an ~30 km length of the fault, roughly centered on Parkfield, CA. This zone exhibits both aseismic creep and multiple historic earthquakes >/= M6. We examine several questions related to near/on-fault processes associated with locked-to-creeping transitions, including whether there is localized off-fault permanent deformation and how the gradient in creep rate changes through time. To examine these questions, we use differential topographic methods on two lidar datasets, the 2005 B4 lidar and 2018 Parkfield lidar, to measure the displacement field across this locked-creeping transition zone. We implement iterative closest point methods using a 50 m window sliding in 25 m increments to provide a very dense quantification of topographic displacements. With this measurement spanning a 13-year time period between lidar datasets, the displacement field consists of a complex and variable combination of tectonic, geomorphic, and anthropogenic signals, as well as several processing artifacts. We have tested a number of manual and logical filters to remove non-tectonic signals from the displacement field, ultimately finding that removing displacement vectors with magnitudes >1 m and/or vector orientations with an azimuth that falls outside 272°-002° (90° range centered on the local strike of the San Andreas fault), results in displacements that are consistent with expected creep rates. We are still in the process of refining our analyses, but an exemplary early finding shows that the steep gradient in creep rate may have shifted to the NW after the 2004 Parkfield earthquake.
Intellectual Merit We seek to contribute new insights into the properties of creeping faults through the application of differential topographic methods and strategies for filtering the complex, non-tectonic signals that complicate displacement fields that span longer time periods of analysis. Characterizing the creep rate gradient in high-resolution will define a key feature of the transition zone, and the position, temporal stability, and the shape of the gradient can be tracked and analyzed to better understand the spatial and temporal behavior of locked-to-creeping fault transition zones.
Broader Impacts We expect that advancing our understanding of the behavior of this transition zone through time will provide new constraints on the earthquake hazards associated with these zones, for both the decadal occurrence of ~M6 earthquakes, and the centennial-scale occurrence of >M7 earthquakes on the adjacent locked portion of the SAF.
Exemplary Figure Figure 2. Creep rate measurements along the San Andreas fault near Parkfield. Our preliminary creep rate data (red triangles) are calculated here as the average of the filtered displacement vectors within 3.5 km sections along the fault. Prior creep rate measurements (blue circles) are from the compilation in Weldon et al. (2013). The steep gradient in creep rate for the time period of 2005-2018 (our data) appears to have shifted to the NW along the fault by several kilometers relative to the gradient defined by prior creep rate measurements.