SCEC Award Number 21053 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Validating the SCEC community rheology model with seasonal loading and surface deformation phase shifts
Name Organization
Roland Bürgmann University of California, Berkeley Baptiste Rousset University of California, Berkeley
Other Participants
SCEC Priorities 3b, 1e, 1c SCEC Groups SDOT, CXM, Geodesy
Report Due Date 03/15/2022 Date Report Submitted 03/15/2022
Project Abstract
This project relies on a new approach to assess the rheological parameters of California's crust and upper mantle based on phase shifts between periodic loading and surface deformation recorded by GNSS. The most widespread source of periodic crustal loading in space and time is the seasonal loading due to the annual cycle of the hydrosphere. At the global scale, recent studies have shown that the phase shift between the annual loading and associated surface deformation can provide a lower bound estimate of mantle viscositiy. We applied this approach to the regional scale of the SCEC Community Rheology Model (CRM). To verify and validate our model approach, we employ both analytical and numerical finite element model (FEM) approaches, assessing the amplitude and phase of the response of a viscoelastic lithosphere to period loading by a Gaussian load. We consider synthetic models with both Maxwell and Burger’s rheologies, composed of an elastic upper crust and a lower crust and upper mantle with a range of viscosities spanning those inferred from postseismic and lake loading studies. Next, we plan to model the viscoelastic response due to the annual loading in California that is the sum of soil moisture due to rain, snow and artificial lakes filling and compare the surface deformation response for various rheological structures to the annual deformation observed by GNSS. We aim to provide new estimates of California’s lower crustal and upper mantle rheology, complementing other geological, geophysical and laboratory components of the CRM.
Intellectual Merit The overall merit of this project is the development of a new approach to deducing the mechanical properties of the lower crust and upper mantle, which control large-earthquake cycles along the California fault system. We assess how the transient viscosities that we obtain using our new modeling approach can be reconciled with the geologically derived flow laws of the current SCEC Community Rheology Model (CRM). We will also compare our estimates to the viscosities obtained from postseismic studies after recent Mw > 7 earthquakes in the Western US.
Broader Impacts This project supported the academic career development of a former postdoc Baptiste Rousset at UC Berkeley to become a junior CNRS researcher and faculty member at the University of Strasbourg, during the project period. New postdoc Curtis Baden is now leading the numerical model development of this project. The project also benefits international collaborations through work with junior researcher Kristel Chanard and Prof. Luce Fleitout at IPGP Paris. Results from the project will benefit studies of time-dependent hazard associated with deformation processes controlled by the rheology structure of California.
Exemplary Figure Figure 2. Example of a synthetic model of surface displacement associated with different bulk viscosities. Left: A 2D spatial Gaussian is used as the synthetic loading, with decaying amplitude from 1 to 0. Temporal sinusoidal evolution of the loading amplitude throughout the year. Right: Horizontal and vertical surface displacement at the location (X = 120 km ; Y = 250 km) for two different viscosities in the top 20 km. Credit: Baptiste Rousset.