SCEC Award Number 11048 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Continuing to Evaluate Active 3D Fault Structure and Improve the SCEC Community Fault Model (CFM)
Name Organization
Craig Nicholson University of California, Santa Barbara
Other Participants Andreas Plesch
John Shaw
Egill Hauksson
Peter Shearer
SCEC Priorities A4, C, A3 SCEC Groups USR, Seismology, Geology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
A crucial component for SCEC in terms of modeling earthquake rupture, seismic hazard and geodetic strain is accurately resolving the position, slip and 3D geometry of active faults at seismogenic depths. Thus, our effort was focused on helping to evaluate various seismicity catalogs and on updating and improving the SCEC 3D Community Fault Model (CFM). Working in close cooperation with Andreas Plesch and John Shaw, we initiated major improvements to CFM and its associated fault database. The new CFM incorporates more detailed and complex 3D representations of major active fault systems, a detailed fault surface trace layer, and a new naming and numbering scheme that allows for closer links to the USGS/CGS Quaternary Fault (Qfaults) database. A systematic revision of CFM was triggered by unexpected discrepancies between previous CFM fault representations and the newer Qfaults surface traces, as well as by the availability of extensive relocated earthquake catalogs to better define the complex geometry of active faults at seismogenic depths. More complex 3D models for several major active fault zones were developed, including the San Andreas from San Gorgonio Pass to the Salton Sea, the adjacent Mecca Hills-Hidden Springs, the San Jacinto, the Elsinore-Laguna Salada (including El Mayor-Cucapah), and the San Fernando/Sierra Madre fault systems. These new models allow for more non-planar, multi-stranded 3D fault geometry, including changes in dip and dip direction along strike and down dip, rather than projecting faults to depth assuming a constant (often vertical) dip and dip direction, as is the case for many of the previous fault models in CFM.
Intellectual Merit Many aspects of seismic hazard evaluation, including understanding or modeling earthquake rupture and geodetic strain, developing credible earthquake rupture scenarios, or predicting strong ground motion, are strongly dependent on accurately resolving the position, slip and 3D geometry of active faults at seismogenic depths. This is critical for properly extrapolating near-surface observations to depth, and is particularly important in complex areas along major faults where principal slip surfaces can be multi-stranded, exhibit significant non-planar subsurface fault geometry, and/or intersect other adjacent major faults. Our work to evaluate, update and improve CFM continues to document, detail and model such complex fault behavior, fault interactions and 3D geometry of major active faults. These results include identifying multiple 3D principal slip surfaces and intersecting fault sets through San Gorgonio Pass, modeling the dipping, sub-parallel Mecca Hills-Hidden Springs fault system adjacent to the southern San Andreas fault, modeling the complex 3D rupture of the El Mayor-Cucapah sequence, and characterizing the complex multi-stranded nature of the San Jacinto and Elsinore-Laguna Salada fault systems. The advantage of these new models is that they allow for more variability in dip along strike and with depth, are more consistent with alignments of relocated hypocenters and focal mechanism nodal planes, and have a higher concentration of hypocenters within close proximity (±2 km) of the modeled 3D slip surface than previous CFM fault models. The new 3D fault models also help characterize a more complex pattern of fault interactions at depth between various fault sets and linked fault systems. Thus, the benefit or merit of these revise 3D fault models for CFM, besides providing a more accurate and realistic framework for other SCEC studies and investigations, is that these more complete, detailed, and complex multiple 3D fault models may help explain some of the more enigmatic fault behavior and patterns of deformation that may be otherwise difficult to understand, including the displacement of maximum shear strain 7 km NE of the southern San Andreas fault surface trace and the failure of the 1986 M6 North Palm Springs earthquake to develop into a more full-scale rupture.
Broader Impacts Accurately characterizing the 3D geometry of subsurface faults is particularly important for any number of SCEC activities, including resolving geodetic strain data, evaluating fault stress or stress changes associated with fault slip, and estimating strong ground motions from dynamic rupture propagation. Our continued work to evaluate, upgrade and improve CFM, which has substantially enhanced the collaboration and partnership between UCSB, Harvard and Caltech, is thus fundamental to SCEC’s research objectives across a wide range of scientific disciplines, interdisciplinary focus areas and special projects. In SCEC-IV, several areas of complex fault behavior will be targets of more focused, integrated, multidisciplinary investigations as part of the SCEC Special Fault Study Areas program. All these Special Fault Study Areas will require improved 3D fault models from CFM as a basis for integrating, evaluating, and modeling the results of these investigations. The major upgrade to CFM we have performed and continue to develop, in collaboration with Harvard and Caltech, that includes more detailed, more complex, and more accurate 3D subsurface fault models will thus provide a critical framework and significant impact to the success of these studies, as well as improved understanding of crustal deformation and seismic hazard in southern California.
Exemplary Figure Figure 1. Oblique 3D view of new detailed CFM v.4.0 fault representations for the San Andreas, San Jacinto, Elsinore-Laguna Salada (including El Mayor-Cucapah) and Mecca Hills-Hidden Springs fault systems, plus Qfault surface traces (red lines), and relocated seismicity (dots color-coded by depth) [Nicholson et al., 2011]. The new CFM 3D faults are now registered to the Qfault surface traces and major strike-slip faults are no longer assumed to be vertical, but change dip and dip direction along strike and with depth to better correlate with the relocated hypocenters. Seismicity from Lin et al. [2007] and Hauksson et al. [2011].