SCEC Award Number 19161 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Assessing the sensitivity of earthquake cycle vertical deformation to spatially variable crustal rigidity
Investigator(s)
Name Organization
Bridget Smith-Konter University of Hawaii at Manoa David Sandwell University of California, San Diego
Other Participants Lauren Ward
SCEC Priorities 1a, 1b, 2a SCEC Groups SDOT, Geodesy, CXM
Report Due Date 04/30/2020 Date Report Submitted 11/13/2020
Project Abstract
 The primary objective of this project was to advance our understanding of vertical deformation and its sensitivity to spatial variations in lithosphere rheology for southern California. Here, we investigate how sensitive earthquake cycle vertical motions are to the rheology of the lithosphere by utilizing both the 10+ year record of background vertical deformation rates as well, as recent vertical geodetic observations from the M7.1 Ridgecrest earthquake. Using a new 4D viscoelastic earthquake cycle model that can incorporate heterogeneous rheological constraints of the southern California lithosphere (i.e., a 40 km thick elastic plate for the Western Basin and Range and Mojave bocks and contributions of a thicker elastic plate from the nearby Sierra Nevada block, consistent with heat flow data and seismically imaged LAB depths) we are able reproduce tectonic deformation and closely match the vertical GNSS solutions in the inter-, co-, and postseismic stages of the earthquake cycle. The coseismic field of the Ridgecrest earthquake sequence reveals alternating quadrants of vertical deformation (+/- 35 mm) that straddle the rupture and span a wide (~200 km) region of the Eastern California Shear Zone. The background interseismic deformation prior to the earthquake sequence is predominantly in the reverse direction and modeled postseismic viscoelastic velocity rates 1 year after the rupture sequence (assuming a viscosity of 5e17 Pa) closely match CGM GNSS solutions. Vertical velocity transients from viscoelastic relaxation are expected to increase for the next few years and eventually decay to near zero within the next decade.
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, and in turn, to provide insight into earthquake cycle vertical velocity variations due to viscoelastic relaxation in the lower crust and upper mantle. The findings of this work promote further investigations into plate-scale variations in rates of crustal deformation and their dependency on elastic plate thickness and influence on seismic moment accumulation rate. For example, faults in regions of relatively low elastic plate thickness (or low crustal rigidity) have both interseismic and postseismic vertical deformation rates that are larger than expected (i.e., if they accompanied a homogeneous elastic plate). In these cases, the moment accumulation rate will be smaller than has been estimated using a uniform rheology model, which 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 vertical surface deformation of the crust and its sensitivity to spatial variations in rheology (Research Priorities 1a, 1b, 2a).
Broader Impacts A component of this SCEC5 funded project emphasized Earth Science education and training, as well as communication of pertinent and accessible earthquake information to the general public. Graduate student L. Ward received partial RA funding and travel support from this award. Manoa and Waialae Elementary Schools, Kamehameha School, Waipahu Intermediate School, and Kailua High School benefited from interactive geoscience educational products provided by our team, in conjunction with the research activities supported by this award. Coursework lectures and visualization exposure of these datasets were provided to over 300 UH undergraduate and graduate students enrolled in ERTH101 Dynamic Earth, ERTH303 Structural Geology, and ERTH631 Solid, Wave, and Fluid Mechanics.
One manuscript supported by activities of this project (Ward et al., 2020) is currently undergoing peer review at JGR and we have one additional paper in preparation (Ward et al., 2021). Results from this project were presented at the 2019 and 2020 SCEC Annual Meetings and 2019 AGU Fall Annual Meeting (Ward et al., 2019a,b; 2020; Smith-Konter et al., 2019a, b) and team member L. Ward also participated in the 2019 SCEC CGM workshop. Our latest code distribution of Maxwell is available on GitHub.
Exemplary Figure Figure 1. (Top Left) CTM/CRM surface heat flow map and depth to the lithosphere-asthenosphere boundary, which was used to create a realistic variable rheology model throughout southern California. (Top Right) Vertical interseismic deformation for southern California when implementing variable rheology using the 4D earthquake cycle deformation code, Maxwell. (Bottom Left) Profiles of vertical velocity across key fault segments (corresponding to red dashed lines in Top Right) for a variable (dashed line) and homogenous (solid line) rheology model. (Ward et al., 2020).