The goals of the FSWG are to understand the kinematics and dynamics of the southern California fault system on interseismic and geologic time scales and to apply this understanding to constructing probabilities of earthquake occurrence in southern California, including time-dependent earthquake forecasting. Two broad approaches are encompassed, both rooted in model-based inference: 1) Quantitative comparisons of observations to predictions of models of ongoing crustal deformation and stress evolution, and 2) A systems level approach characterizing and understanding spatial and temporal patterns in regional seismicity, with the ultimate objective of intermediate-term earthquake prediction. FSWG has strong ties to the Unified Structural Representation, Earthquake Source Physics, and RELM Working Groups, and is dependent on observations provided by Earthquake Geology and Tectonic Geodesy.

A list of FSWG grant titles and PI’s illustrates the scope of the effort. Scientific projects using the Systems approach include: Earthquake probabilities based on clustering and stress interactions (A. Helmstetter, Y. Kagan), Implementing and testing earthquake probability models (S. Wiemer, L. Jones, D. Jackson), Analysis & Integration of the Earthquake Stress Cycle Evolution & Pattern Informatics Techniques (K. Tiampo, C. Bowman), Emergent Modes on Earthquake Fault Systems (J. Rundle, W. Klein), Paleoseismic Constraints on Earthquake Simulation Models (S. Ward, L. Grant, T. Rockwell), Integrating Calibrated Triggered Seismicity with Fault Networks (D. Sornette), Structure and Mechanical Significance of Dynamically Generated Off-Fault Damage (C. Sammis), Discrete Element Simulations of Elasto-Plastic Fault Block Controls on Earthquake Distributions (J. Morgan), and Nonlinearity, Phase-Locking, and the Temporal Clustering of Large Earthquakes (C. Sammis). Crucial observations provided by Earthquake Geology include: Holocene and Late Quaternary slip rate of the San Bernardino strand of the SAF (S. McGill, R. Weldon), Constraints on clustering of earthquakes, ECSZ (C. Rubin), Prehistoric Earthquake Chronology of the SJF at Hog Lake (T. Rockwell), Mapping the Vertical Velocity Field in the LA Basin with Aquifers tied to Sea Level Change (K. Mueller), Paleoseismic Characterization of the Calico Fault (G. Seitz, T. Fumal), Timing and Displacement During Paleoearthquakes on the Garlock fault (E. Gath), and Timing of paleoearthquakes on the Blackwater fault (C. Madden).

Development of Community software is a high priority of FSWG: Development of Community Finite Element Models for Fault Systems Studies & Meshing the Community Block Model (C. Gable, B. Hager, M. Simons), Development of a parallelized 3-D finite element code for modeling deformation (C. Williams). Model-related studies include: Driving forces of crustal deformation (E. Humphreys), InSAR investigation of interseismic strain accumulation on faults in the ECSZ (Y. Fialko), Kinematic Model of Fault Slip and Anelastic Strain Rates and Long-Term Seismicity (P. Bird), Southern California Tectonic Deformation Modeling (Z-K Shen, D. Jackson) Inferring Fault Slip and Crustal Motion from Joint Inversion of Geologic and Geodetic Data (B. Hager), Interpreting focal mechanisms in a heterogeneous stress field (T. Heaton), Modeling Geometrically Complex, Intersecting Faults Using the Finite Element Method (S. Kenner), The evolution of the brittle-ductile transition during the earthquake cycle (R. Burgmann), Community Fault Model validation with elastic models (M. Cooke, A. Meigs), Mapping groundwater-related subsidence with InSAR, western Salton trough (R. Mellors), Model of fault-zone properties from postseismic to pre-failure conditions. Application to full-cycle quasi-dynamic model of the Big Bend in the San Andreas Fault (N. Sleep), and Modeling the Mojave Lithosphere: New Approaches (E. Hearn).

The most important FSWG group activity is the annual workshop: “Community Finite Element Models for Fault Systems and Tectonic Studies,” hosted by Los Alamos National Laboratory in August. This locale enables SCEC scientists to benefit from interaction with Lab experts. This year we leveraged SCEC funding with support from NSF EarthScope, NASA, and LANL, allowing us to increase the number of students and senior researchers attending. Part of the group effort is aimed at verifying code accuracy using benchmark problems. Efficient and accurate meshing of complex geologic structures is a very high priority, and hands-on meshing sessions lead by scientists from LANL were extremely useful, with participants installing and learning to use LAGriT.

One of our highest priorities of the is to develop a quasi-static, parallelized finite element code able to represent the deformation and stress fields due to all major faults in southern California, as provided by the Community Block Model, using realistic rheologies and fault behavior. The code should be relatively easy to use and should integrate well with other modeling codes, visualization and meshing packages. Charles Williams (RPI) leveraged SCEC, NSF ITR, and Caltech resources to upgrade Tecton into a SCEC Community code, “Lithomop.” A significant fraction of participants succeeded in setting up and running Lithomop on their computers. The NASA-sponsored Quakesim group also participated in the workshop, and most participants also set up and ran GeoFEST. Thus the focus of the workshop was “learning by doing.”

In order to develop a realistic continuum mechanics model of Southern California, it is crucial to include the fault system geometry and mechanical structure that is the focus of the USR group. The resulting Community Block Model (CBM) is not only an essential product required by Fault Systems, but also provides the natural way of combining the fault surfaces of the CFM and the volumetric properties of the CVM into a Unified Structural Representation. This year Carl Gable of LANL succeeded in meshing the Mojave region of CBM, providing a major step forward for realistic models of the southern California fault system.

Larger imageUSR Community Block Model of the Mojave region, showing two layers of blocks.

Larger imageZoomed view of mesh generated by Carl Gable using LAGriT.

Objectives for the Upcoming Year

• Fault-System Behavior: Assess the ways in which the system-level behavior of faults controls seismic activity and regional deformation; infer rates of change in stress from geodetic and seismic observations; compare and interpret quantitatively short-term geodetic rates of deformation, long-term geologic rates, and rates predicted by seismicity simulators; quantify the space-time behavior of the Southern California fault system in ways targeted to test models of earthquake occurrence and stress evolution; foster collaborations to obtain outside funding to support large, coordinated data-gathering efforts; determine how geologic deformation is partitioned between slip on faults and distributed off fault deformation and how geodetic strain is partitioned between long-term permanent and short-term elastic strain and on-fault slip or permanent distributed strain.
• Deformation Models: Develop, validate, and facilitate use of modular 3D quasi-static codes for simulating crustal motions utilizing realistic, highly resolved geometries and rheological properties (e.g., Burgers body viscoelasticity, rate-state friction, poroelasticity, damage rheology); develop continuum representations of fault system behavior on scales smaller than can be resolved as faulting; develop a closed volume representation of the Community Block Model (CBM) that unifies the geometric representations of CFM and the CVM and that serves as a basis for efficient meshing and remeshing of models; generate finite element meshes of the CBM; assess mechanical compatibility of CFM and how slip is transferred between recognized fault segments; develop a reference model of the time-dependent stress transfer and deformation associated with the 1992 Landers earthquake; extend models of time-dependent stress transfer and deformation of Southern California to cover multiple earthquake cycles addressing geologic slip rates, geodetic motions (including CMM 4.0), and earthquake histories; use these to infer fault slip, rheologic structure, and fault interactions through the transfer of stresses; couple numerical models of the interseismic period to quasi-static full-cycle fault models to better constrain stress transfer and conditions and processes at the start of dynamic rupture, including forcing by realistic coseismic displacements and dynamic stresses (with Source Physics); develop tectonic models that explain the inferred rates of fault slip; develop a plan for post-earthquake geodetic deployments.
• Seismicity Evolution Models: Determine the effects of fault system scale and resolution; develop and validate rapid simulation methods for modeling earthquakes in fault systems over a wide range of magnitudes (with Source Physics); develop, validate, and facilitate use of codes for ensemble models simulating earthquake catalogs using CFM, USR and CBM, as well as effects of faults not included in CFM; incorporate constraints (including data assimilation) from geologic slip rates, geodetic data, realistic boundary conditions, and fault rupture parameterizations, including rate-state friction and normal stress variations; assess the processes that control the space-time-magnitude distribution of regional seismicity; quantify sources of complexity, including geometrical structure, stress transfer, fault zone heterogeneity, and slip dynamics; assess the utility of these models in forecasting earthquakes; quantify signals in the space-time- magnitude distribution of seismicity and understand their physical origin.