SCEC Award Number 14073 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Numerical models of stress transfer from plate motion to mature transform faults
Name Organization
Yuri Fialko University of California, San Diego
Other Participants Kang Wang, graduate student
SCEC Priorities 1b, 2d SCEC Groups SDOT, FARM, EFP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
We investigated the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries, survey-mode GPS occupations of a dense array crossing the fault, and numerical models. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1–2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates.
Intellectual Merit New geodetic observations from InSAR and GPS of the rate and pattern of shallow creep along the southern San Andreas Fault reveal a systematic variation in the width of the yielding zone. InSAR observations from multiple viewing geometries allow us to resolve horizontal and vertical motions and provide estimates of the creep rate that are in good agreement with repeated GPS surveys and with creepmeters located along the fault. In areas where the local fault strike results in transpression, creep is localized on a narrow trace. In the intervening transtensional segments, where fault creep had not previously been detected, we find that fault-parallel shear occurs over a zone approximately 1–2 km wide. Using a simple boundary element model, we show that distributed inelastic yielding can occur in areas where the fault geometry causes stresses to exceed the Mohr-Coulomb failure criterion off the fault plane. If distributed yielding in the interseismic period is common, it may explain the shallow slip deficit in strong (M ∼ 7) strike-slip earthquakes and may result in a systematic underestimation of the long-term fault slip rates based on paleoseismic data.
Broader Impacts This project provided training and support for one graduate student (Lindsey). The PI (Fialko) used results of this study in a graduate class taught at SIO.
Exemplary Figure Figure 2.
Change in Mohr-Coulomb failure ratio due to long-term slip on the Southern San Andreas fault (solid black line). The fault has a constant coefficient of friction μ specified in each panel. Material cohesion is (a–c) 20 MPa and (d–f) 40 MPa.