SCEC Award Number 16065 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Continuing to Evaluate 3D Fault Geometry in Special Fault Study Areas and to Update & Improve the SCEC Community Fault Model
Name Organization
Craig Nicholson University of California, Santa Barbara
Other Participants Chris Sorlien, Andreas Plesch, John Shaw and Egill Hauksson through related SCEC or USGS-funded projects
SCEC Priorities 4c, 4a, 4b SCEC Groups USR, Seismology, SDOT
Report Due Date 03/15/2017 Date Report Submitted 03/15/2017
Project Abstract
During SCEC4 (2012-2016), I and my colleagues Andreas Plesch, Chris Sorlien, John Shaw and Egill Hauksson continued to make steady and significant improvements to the SCEC Community Fault Model (CFM), culminating in the release of CFM-v5.1 [Plesch et al., 2016]. This systematic update to CFM represents a substantial improvement of 3D fault models for southern California. The CFM-v3 fault set was expanded from 170 faults to over 820 3D fault objects and alternative representations that define over 380 faults organized into 105 complex fault systems. Of the ~650 new fault objects added to CFM, most of these updated 3D fault models (~525) were developed by UCSB or to which UCSB made significant contributions. This includes all the major faults of major fault systems (e.g., San Andreas, San Jacinto, Elsinore-Laguna Salada, Newport-Inglewood, Imperial, Garlock, etc.), and most major faults in the Mojave, Eastern & Western Transverse Ranges, offshore Borderland, and updated faults within the Ventura & San Gorgonio Pass Special Fault Study Areas [e.g., Nicholson et al., 2012, 2013, 2014, 2015, 2016; Sorlien et al, 2012, 2014, 2015, 2016; Sorlien and Nicholson, 2015]. These new models allow for more realistic, curviplanar, complex 3D fault geometry, including changes in dip and dip direction along strike and down dip, based on the changing patterns of earthquake hypocenter and nodal plane alignments and, where possible, subsurface imaging of fault geometry with industry seismic reflection data. In 2015-2016 alone, 120 new or updated fault objects were added to CFM-v5.1 since the initial release of v5.0.
Intellectual Merit Many aspects of seismic hazard evaluation, including understanding earthquake rupture and developing credible earthquake rupture scenarios, modeling geodetic and geologic fault slip, or predicting strong ground motion, are all strongly dependent on accurately resolving the 3D geometry of active faults at seismogenic depths. Having accurate and realistic 3D models of subsurface fault geometry is also important when investigating the likelihood of multi-segment or multi-fault ruptures. The primary purpose and intellectual merit of this on-going, multi-year effort is thus to provide just such improved, more detailed and more realistic 3D fault models for CFM based on the distribution of improved fault surface trace data, relocated earthquake hypocenters, focal mechanisms, seismic reflection and well data. These updated 3D fault surfaces have already proven useful in developing improved dynamic earthquake rupture models along the San Andreas fault, as well as providing a better match to the observed patterns of uplift, topography, geodetic strain data, and fault slip rates along and across these active geologic structures. In addition, recent project results that were incorporated into the CFM on-line database offered compelling alternative interpretations to controversial models regarding the prospects for proposed magnitude ~M8 earthquakes in the Santa Barbara-Ventura area. These results include: 3D observations of mapped South-dipping faults that may be responsible for the uplift at Pitas Point, the identification by Jim Brune of Precariously Balanced Rocks in the foothills above Santa Barbara that largely preclude the strong ground shaking expected from proposed shallow ~M8 earthquakes, and relocated earthquakes that define 3D fault geometry to depths of 18-20 km, well below the ramp-flat fault geometry used to model and explain the possible occurrence of proposed ~M8 earthquakes.
Broader Impacts In addition to providing alternative 3D fault models and interpretations of complex fault geometry, this project has been able to more accurately identify and characterize the degree of spatial interaction between adjacent sub-parallel fault systems, as well as fault splays, secondary faults and detachments. Thus, project results offer significantly different interpretations as to how active deformation is being accommodated, and the broader impacts of how earthquake and tsunami hazards can and should be evaluated. This is particularly true in areas with complex fault geometry and controversial alternative interpretations, including the San Gorgonio Pass and Ventura Special Fault Study Areas, where questions regarding the likelihood of through-going earthquake ruptures (SGP) or possible ~M8 earthquakes (Ventura) continue to persist. Project personnel actively participated in a number of outreach and educational activities related to informing the public, students, colleagues and various stakeholders of the earthquake and tsunami hazards of coastal and Southern California. Besides various CFM products, this project helped develop additional digital classroom and outreach visual aids. This project also helped support and maintain various state-of-the-art interactive 3D visualization, analysis and modeling programs for use by students and researchers at UCSB for the interpretation, mapping, analysis, and modeling of subsurface 3D fault structure, seismicity, and related syntectonic stratigraphy.
Exemplary Figure Figure 3. (left) Example of Precariously Balanced Rocks present in the foothills behind Santa Barbara [Brune, 2009] and directly above ramp-flat fault source model assumed for proposed Holocene ~M8 earthquakes. (right) Oblique 3D view looking East of North Channel-Pitas Point-Red Mountain fault system (red-purple) [Nicholson et al., 2015] and alternative ramp-flat Pitas Point-Red Mountain fault model (dark blue)[Hubbard et al., 2014]. Independent datasets of relocated hypocenters for the 1978 M5.9 Santa Barbara and 2013 M4.8 Isla Vista earthquakes (not included in either original model construction) indicate that the steeply dipping fault model (red-purple) is more consistent with this recent slip than the proposed alternative ramp-flat model (dark blue)[Nicholson et al., 2016].