SCEC4 Fundamental Problems of Earthquake Physics: Research Priorities and Requirements

The six fundamental problems constitute the basic-research focus of SCEC4 and are listed in the preamble. They are interrelated and require an interdisciplinary, multi-institutional approach. Interdisciplinary research initiatives will focus on special fault study areas, the development of a community geodetic model for Southern California, and a community stress model. The latter will be a new platform where the various constraints on earthquake-producing stresses can begin to be integrated. Improvements will be made to SCEC's unified structural representation and its statewide extensions.

1. Stress transfer from plate motion to crustal faults: long-term fault slip rates.

Priorities and Requirements

1a. Mapping and studying faults in Southern California for which brittle/ductile transitions have been exposed by detachment faulting or erosion.
1b. Focused laboratory, numerical, and geophysical studies of the character of the lower crust, its rheology, stress state, and expression in surface deformation. We will use surface-wave dispersion to improve depth resolution relative to teleseismic studies.
1c. Regional searches for seismic tremor at depth in Southern California to observe if (some) deformation occurs by slip on discrete structures.
1d. Development of a Community Geodetic Model (CGM) for California, in collaboration with the UNAVCO community, to constrain long-term deformation and fault-slip models.
1e. Combined modeling/inversion studies to interpret GPS and InSAR geodetic results on postseismic transient deformation without traditional simplifying assumptions.

2. Stress-mediated fault interactions and earthquake clustering: evaluation of mechanisms

Priorities and Requirements

2a. Improvement of earthquake catalogs, including non-point-source source descriptions, over a range of scales. Traditional aftershock catalogs can be improved through better detection of early aftershocks. Long-term (2000-yr) earthquake chronologies, including slip-per-event data, for the San Andreas Fault system are necessary to constrain long-term clustering behavior.
2b. Improved descriptions of triggered earthquakes. While temporal earthquake clustering behavior (Omori's Law) is well known, the spatial and coupled temporal-spatial behavior of triggered earthquakes, potentially key diagnostics, are not well constrained.
2c. Lowered thresholds for detecting aseismic and infraseismic transients, and improved methods for separating triggering by aseismic transients from triggering by other earthquakes.
2d. Development of a Community Stress Model (CSM) for Southern California, based on merging information from borehole measurements, focal mechanisms, paleo-slip indicators, observations of damage, topographic loading, geodynamic and earthquake-cycle modeling, and induced seismicity. We will use seismicity to constrain CSM and investigate how stress may control earthquake clustering and triggering. We plan to collaborate with other organizations in fault-drilling projects for in situ hypothesis testing of stress levels.
2e. Development of physics-based earthquake simulators that can unify short-term clustering statistics with long-term renewal statistics, including the quasi-static simulators that incorporate laboratory- based nucleation models.
2f. Better understanding of induced seismicity, specifically induced by geothermal power production in the Salton Sea area, which warrant study as potential hazards.

3. Evolution of fault resistance during seismic slip: scale-appropriate laws for rupture modeling

Priorities and Requirements

3a. Laboratory experiments on fault materials under appropriate confining stresses, temperatures, and fluid presence through targeted experiments in collaboration with rock mechanics laboratories.
3b. Search for geological, geochemical, paleo-temperature, and hydrological indicators of specific resistance mechanisms that can be measured in the field. In particular, we will look for evidence of thermal decomposition in exhumed fault zones.
3c. Theoretical and numerical modeling of specific fault resistance mechanisms for seismic radiation and rupture propagation, including interaction with fault roughness and damage-zone properties. At the scale of meters to hundreds of meters, the behavior of the near-fault layer with evolving damage may have to be included in the fault constitutive relations.
3d. Development of parameterized fault rheologies suitable for coarse-grained numerical modeling of rupture dynamics and for simulations of earthquake cycles on interacting fault systems. Currently, the constitutive laws for co-seismic slip are often represented as complex coupled systems of partial differential equations, contain slip scales of the order of microns to millimeters, and hence allow detailed simulations of only small fault stretches.
3e. Dynamic rupture modeling to constrain stress levels along major faults, explain the heat-flow paradox, and understand extreme slip localization, the dynamics of self-healing ruptures, and the potential for repeated slip on the fault during the earthquake. We will collaborate with other organizations in fault-drilling projects to measure temperature on faults before and after earthquakes and thus constrain co-seismic resistance.
3f. Development of earthquake simulators that can incorporate realistic models of fault-resistance evolution during the earthquake cycle.

4. Structure and evolution of fault zones and systems: relation to earthquake physics

Priorities and Requirements

4a. Establishment of special fault study areas for detailed geologic, seismic, geodetic, and hydrologic investigations of fault complexities.
4b. Investigations of along-strike variations in fault roughness and complexity as well as the degree of localization and damage perpendicular to the fault.
4c. Improvements to the CFM using better mapping, including LiDAR, and precise earthquake relocations. We will also extend the CFM to include spatial uncertainties and stochastic descriptions of fault heterogeneity.
4d. Use of special fault study areas to model stress heterogeneities both deterministically and stochastically. We will integrate the results of these special studies into the CSM.
4e. Use of earthquake simulators and other modeling tools, together with the CFM and CSM, to quantify how large-scale fault system complexities govern the probabilities of large earthquakes and rupture sequences.

5. Causes and effects of transient deformations: slow slip events and tectonic tremor

Priorities and Requirements

5a. Improvement of detection and mapping of the distribution of tremor across southern California by applying better instrumentation and signal-processing techniques to data collected in the special study areas, such as those outlined in the proposal.
5b. Application of geodetic detectors to the search for aseismic transients across southern California. We will use the CGM as the time-dependent geodetic reference frame for detecting geodetic anomalies.
5c. Collaboration with rock mechanics laboratories on laboratory experiments to understand the mechanisms of slow slip and tremor.
5d. Development of physics-based models of slow slip and tectonic tremor. We will constrain these models using features of tremor occurrence and its relationship to seismicity, geodetic deformation, and tectonic environment, as well as laboratory data.
5e. Use of physics-based models to understand how slow slip events and tremor activity affect earthquake probabilities in Southern California.

6. Seismic wave generation and scattering: prediction of strong ground motions

Priorities and Requirements

6a. Development of a statewide anelastic Community Velocity Model (CVM) that can be iteratively refined through 3D waveform tomography. We will extend current methods of full-3D tomography to include ambient-noise data and to estimate seismic attenuation, and we will develop methods for estimating and representing CVM uncertainties.
6b. Modeling of ruptures that includes realistic dynamic weakening mechanisms, off-fault plastic deformation, and is constrained by source inversions. The priority is to produce physically consistent rupture models for broadband ground motion simulation. An important issue is how to treat multiscale processes; specifically, does off-fault plasticity regularize the Lorentzian scale collapse associated with strong dynamic weakening? If not, how can adaptive meshing strategies be most effectively used to make full-physics simulations feasible?
6c. Develop stochastic representations of small-scale velocity and attenuation structure in the CVM for use in modeling high-frequency (> 1 Hz) ground motions. We will test the stochastic models with seismic and borehole logging data and evaluate their transportability to regions of comparable geology.
6d. Measure earthquakes with unprecedented station density using emerging sensor technologies (e.g., MEMS). The SCEC Portable Broadband Instrument Center will work with IRIS to make large portable arrays available for aftershock and flexible array studies.
6e. Collaborate with the engineering community in validation of ground motion simulations. We will establish confidence in the simulation-based predictions by continuing to work with engineers in validating the simulations against empirical attenuation models and exploring coherency and other standard engineering measures of ground motion properties.