Developing a Mechanical Explanation for Slip Reduction Near Earth’s Surface on Seismogenic and Creeping Continental Strike-Slip Faults
Session: Effects and Uses of Aseismic Deformation and Fault Creep in Seismic Hazard and Warning
Type: Oral
Date: 4/20/2021
Presentation Time: 06:15 PM Pacific
Description:
Earth’s crust often transitions from fluid-saturated, competent bedrock confined at depth to unsaturated, weakly-consolidated material at its traction-free surface. How the unique combination of factors in the shallow crust (<~100 m depth) affects fault behavior remains unknown, with significant ramifications for earthquake science, engineering, and hazard modeling. Fundamental questions include: Is a shallow slip reduction ubiquitous on strike-slip faults and how does it relate to off-fault plastic strain? Do ruptures systematically break down into echelon segments near Earth’s surface and how does segmentation affect the kinematics and dynamics of the system? Do mechanical changes across the water table impede shallow fault rupture? And importantly, can we understand shallow faulting as a quasi-static process with models that are numerically tractable and by examining creeping faults rather than waiting for the next earthquake?
We begin to address these questions with a synthesis of studies from two earthquakes and a creeping fault. The 2019 M7.1 Ridgecrest earthquake provides a spectacular example of rupture segmentation at Earth’s surface, which we relate to slip variation both along strike and with depth using optical imagery, mobile laser scanning (MLS), and mechanical modeling. For the 2014 M6.0 South Napa earthquake, we combine MLS with fault zone drilling, lab experiments, and mechanical modeling to show that the shallow coseismic slip and afterslip were nearly indistinguishable, both with buried rupture tips seemingly affected by off-fault plastic strain and increased frictional strength in the unsaturated material above the water table. At Mee Ranch along the Creeping Section of the San Andreas fault, elastic modeling of a 50-year-old alignment array indicates that shallow slip distributions can vary in time, with the most recent epoch indicating a reduction in slip at Earth’s surface.
Thus, shallow slip may vary spatially and temporally, raising new questions for how we use static field measurements in long-term hazard models.
Presenting Author: Josie Nevitt
Student Presenter: No
Authors
Josie Nevitt Presenting Author Corresponding Author jnevitt@usgs.gov U.S. Geological Survey |
Benjamin Brooks bbrooks@usgs.gov U.S. Geological Survey |
David Lockner dlockner@usgs.gov U.S. Geological Survey |
Rufus Catchings catching@usgs.gov U.S. Geological Survey |
Todd Ericksen tericksen@usgs.gov U.S. Geological Survey |
Carolyn Morrow cmorrow@usgs.gov U.S. Geological Survey |
Diane Moore dmoore@usgs.gov U.S. Geological Survey |
Mark Goldman goldman@usgs.gov U.S. Geological Survey |
Craig Glennie clglenni@central.uh.edu University of Houston |
Ken Hudnut hudnutken@gmail.com Southern California Edison, Rosemead, California, United States |
Coyn Criley ccriley@usgs.gov U.S. Geological Survey, Moffett Field, California, United States |
Ben Melosh bmelosh@usgs.gov U.S. Geological Survey, Moffett Field, California, United States |
Nima Ekhtari nima.ekhtari@gmail.com University of Houston, Houston, Texas, United States |
Developing a Mechanical Explanation for Slip Reduction Near Earth’s Surface on Seismogenic and Creeping Continental Strike-Slip Faults
Category
Effects and Uses of Aseismic Deformation and Fault Creep in Seismic Hazard and Warning