A new way to estimate shear tractions on active faults in southern California

Peter Bird

Published July 22, 2016, SCEC Contribution #6348, 2016 SCEC Annual Meeting Poster #009

Two kinds of thin-shell finite-element (F-E) model of quasi-static neotectonics are available for southern California: (1) kinematic (or “inverse”) F-E models which fit geodetic and geologic data by weighted least-squares but contain limited physics and no stress magnitudes; and (2) dynamic (or “forward”) physics-based F-E models that build on crustal thickness, heat-flow, lab rheologies, fault traces and dips and the momentum equation to predict quasi-static stresses, strain-rates and fault slip-rates (but often fail to match kinematic data). Potentially, fault slip-rates from a good kinematic model can be used as targets for tuning a good dynamic model, in at least two ways: (A) adjusting the effective friction (in a quasi-static approximation) of each fault element; and/or (B) adjusting horizontal shear tractions exerted on the base of the lithosphere by mantle convection. To date, I have adjusted friction (A) to tune a Shells dynamic F-E model of neotectonics in southern California to better match long-term fault slip-rates from the NeoKinema deformation model that was used in UCERF3 and NSHM2014. The Shells dynamic solution is revised 100 times, and in each revision the effective friction of each of the 1000 fault elements is adjusted up or down by a small amount (no more than ±0.01) based on the sense of the current slip-rate error for that fault. Results show that over 60% of the fault elements (including most of the San Andreas and Garlock faults) march straight down to the lower effective-friction limit of 0.01 and stay there. Overall RMS error in fault slip-rates falls from 5.3 to 1.6 mm/a. Fault rakes were not imposed, but have excellent realism. The obvious physical interpretation is that over 60% of active fault area in southern California experiences near-total stress-drop in large earthquakes due to dynamic weakening mechanisms like thermal pressurization or thermal decomposition or fault melting. Thus, the peak pre-seismic shear traction on most of these faults is not much more than the coming stress-drop. Yet, other areas on the fault network retain higher effective friction, concentrate shear tractions, and serve as nucleation sites. However, results are still tentative and preliminary because (a) potential input from the SCEC Community Rheology Model has not yet been used; (b) potential input from the SCEC Community Thermal Model has not yet been used; and (c) method B of adjusting mantle shear tractions has not yet been attempted.

Key Words
fault, friction, finite-element, dynamic weakening

Citation
Bird, P. (2016, 07). A new way to estimate shear tractions on active faults in southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)