SCEC Award Number 17028 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title A Community Thermal Model (CTM) of the Southern California Lithosphere
Name Organization
Wayne Thatcher United States Geological Survey David Chapman University of Utah Colin Williams United States Geological Survey
Other Participants
SCEC Priorities 1b, 1e, 2d SCEC Groups CXM, SDOT, Geodesy
Report Due Date 06/15/2018 Date Report Submitted 11/14/2018
Project Abstract
We are developing a 3D Community Thermal Model (CTM) to constrain rheology and so better understand deformation processes in SoCal. Model geotherms within each HFR are constrained by averages and variances of surface heat flow q0 and the 1D depth distribution of thermal conductivity (k) and radiogenic heat production (A), which are strongly dependent on rock type. Crustal lithologies are not always well known and we turn to seismic imaging for help. We interrogate the SCEC Community Velocity Model (CVM) to determine averages and variances of Vp, Vs and Vp/Vs versus depth within each HFR. We bound (A, k) versus depth by relying on empirical relations between seismic wave speed and rock type and laboratory and modeling methods relating (A, k) to rock type.

Many 1D conductive geotherms for each HFR are allowed by the variances in surface heat flow and subsurface (A, k). An additional constraint on the lithosphere temperature field is provided by comparing lithosphere-asthenosphere boundary (LAB) depths identified seismologically with those defined thermally as the depth of onset of partial melting. Receiver function studies in Southern California indicate LAB depths that range from 40 km to 90 km. Shallow LAB depths are correlated with high surface heat flow and deep LAB with low heat flow. The much-restricted families of geotherms that intersect peridotite solidi at the seismological LAB depth in each region require that LAB temperatures lie between 1050 to 1250˚ C, a range that is consistent with a hydrous rather than anhydrous mantle below Southern California.
Intellectual Merit The rheology of the ductile lithosphere and its coupling to the seismogenic upper crust strongly influences the deformation and state of stress of the lithosphere, determines the geodetically-measured surface deformation through the earthquake cycle and can affect the geodetic estimates of fault slip rates
Broader Impacts We contribute to the broad goals of SCEC by generating a SoCal thermal model that provides new constraints on SoCal earthquake processes. We involve graduate students and early career scientists and contribute to their education and conduct cutting edge research
Exemplary Figure Fig. 1