SCEC Award Number 17169 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Development of 4-D models of the earthquake cycle that include spatial variations in crustal rheology
Investigator(s)
Name Organization
Bridget Smith-Konter University of Hawaii at Manoa David Sandwell University of California, San Diego
Other Participants Graduate student
Undergraduate student
SCEC Priorities 1c, 1e, 1d SCEC Groups SDOT, CXM, Geodesy
Report Due Date 06/15/2018 Date Report Submitted 11/13/2018
Project Abstract
 The primary objective of this project was to extend our 4-D earthquake cycle modeling capabilities to incorporate spatial variations in lithosphere rheology, which can then be used to better inform seismic hazard models. To this end, we have developed a semi-analytic approach (and computational code) for rapidly calculating 3D time-dependent deformation and stress caused by screw dislocations imbedded within an elastic layer overlying a Maxwell viscoelastic half-space. The Maxwell model is developed in the Fourier domain to exploit the computational advantages of the convolution theorem, hence substantially reducing the computational burden associated with an arbitrarily complex distribution of force-couples necessary for realistic fault modeling. The new contribution to this code is the ability to model lateral variations in shear modulus. Ten benchmark examples were developed for testing and verification of the algorithms and code. We also developed a preliminary model of interseismic deformation along the San Andreas Fault System where lateral variations in shear modulus are included to simulate lateral variations in lithospheric structure. We find that a decrease in shear modulus in a region surrounding a force-couple results in an increase in deformation. One immediate implication for the Imperial fault is that if the region has relatively low rigidity, then the moment accumulation rate must be smaller than has been estimated using a uniform rigidity model. This implies a lower seismic hazard in the region.
Intellectual Merit A major objective of SCEC5 is to bridge the enduring efforts of several community models through the establishment of the SCEC CRM, a large-scale effort to deliver a provisional rheological description of the lithosphere of southern California based upon a simplified geologic framework. To first order, determining the relationship between strain (or strain rate) and stress requires a fundamental knowledge of material rheology. Geodetic data provided by the Community Geodetic Model (CGM) measure vector surface velocities and strain rates with increasingly high resolution and with broad regional coverage. A physical kinematic model, outfitted with refined fault representations (like those provided by the Community Fault Model, CFM) and governed by informed rheological assumptions, is required to interpret these measurements in a spatially continuous and 3-D manner. Moreover, model estimates of time-dependent earthquake cycle deformation and stress loading rates (contributed to the Community Stress Model, CSM) require a broad understanding of the rheology and structure of the crust and upper mantle. Contributions from a developing Community Thermal Model (CTM), which provides heat flow estimates of the southern California lithosphere, are another essential component. Integrating these community models to better inform the collaborative efforts of the geology, geodesy, seismology, and hazard communities is a critical objective for advancing the science goals of SCEC.
To this end, the primary objective of this project was to extend our 4-D earthquake cycle modeling capabilities to incorporate spatial variations in lithosphere rheology, which can then be used to improve seismic hazard models. The findings of this work promote further investigations into the relationship of seismic moment rate and crustal rigidity, as the new unknowns of this approach to modeling are moment rate rather than slip rate, as in the typical geodetic inversions. One immediate implication for the Imperial fault is that if the region has relatively low rigidity, then the moment accumulation rate must be smaller than has been estimated using a uniform rigidity model. This implies a lower seismic hazard in the region. Moreover, this work contributes to the development of a critical SCEC science question, How are faults loaded across temporal and spatial scales? by conducting numerical studies of deformation and the stress state of the crust and its sensitivity to spatial variations in rheology (Research Priorities P1c-e).
Broader Impacts A component of this SCEC5 funded project emphasized Earth Science education and communication of pertinent and accessible earthquake information to the general public. UH Undergraduate student J. Higa assisted graduate student L. Ward with model benchmarking activities and also participated in several classroom outreach activities that helped set the stage for our supplemental E&O campaign (EAR-1614875). Directly aimed at disseminating geoscience educational material to our local community, we worked closely with the Earth Science on Volcanic Islands summer REU program, hosted by UH’s Department of Geology and Geophysics, to develop visualization content for a visiting REU student. Manao Elementary School and Waialae Public Charter School have benefited greatly from interactive educational products provided by our team, in conjunction with the research activities supported by this award. We have also utilized the UH Hawaiian Institute of Geophysics visualization center many times over the year to display San Andreas visualizations for classroom and public education activities. Coursework lectures and visualization exposure of these datasets were provided to 18 UH undergraduate students enrolled in GG101 Dynamic Earth and GG451 Earthquakes and Crustal Deformation.
Two publications resulted from this project (Sandwell and Smith-Konter, 2018; Xu et al., 2018) and we have one paper in a very mature state of preparation (Ward et al., 2018). Results from this project were also presented at the 2017 SCEC Annual Meeting and the 2017 AGU Fall Meeting. Our latest code distribution of Maxwell is available on GitHub.
Exemplary Figure Figure 1. From Sandwell and Smith-Konter (2018). (top) Three components of interseismic surface deformation (U, fault-perpendicular; V, fault-parallel; W, vertical) in mm/yr driven by slip along segments of the SAFS within a 30-km thick elastic plate. Faults are locked from the surface to 10 km depth and slip from 10 to 30 km. The horizontal scale is kilometers perpendicular to the fault system while the vertical scale is kilometers parallel to the fault system. (bottom) Variable rigidity deformation - uniform rigidity deformation residual. The differences highlight the effects of spatial variations in rigidity. CV = Central Valley; BR = Basin and Range; ST = Salton Trough.