SCEC Award Number 20117 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Develop Geological Model of Offshore Southern California (California Continental Borderland) for Community Rheology Model - Continuation
Investigator(s)
Name Organization
Mark Legg Legg Geophysical
Other Participants
SCEC Priorities 3a, 1b, 5b SCEC Groups CXM, Geology, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/30/2021
Project Abstract
 Four tectonstratigraphic terranes define crustal blocks of the California Continental Borderland Geologic Framework (GF). Former subduction zone terranes include Patton accretionary wedge, Nicolas forearc, Catalina subduction complex, and Pacific oceanic crust. Using updated fault maps derived from seismic profiles, geophysical investigations, and distributions of well-located seismicity, the Nicolas and Catalina terranes are subdivided into domains separated by major active faults. A Santa Cruz Basin domain is separated from the southern Nicolas domains by the east-trending San Nicolas Island escarpment and NW-dipping reverse fault. Western Catalina Core Complex domains include Santa Monica Basin, Catalina Basin, and San Clemente Basin that represent igneous and metamorphic crust separated by the San Clemente and Catalina fault zones. The eastern Inner Borderland Detachment terrane is separated from the Catalina core complex by northwest-trending San Pedro Basin and San Diego Trough fault zones. At least two stacked low-angle detachment systems lay beneath nearshore domains that are cross-cut by numerous steep faults including the coastal Newport-Inglewood-Rose Canyon-Descanso fault zones. Cross-sections of relocated seismicity show that both vertical strike-slip and low-angle detachment fault systems control earthquake locations across the Inner Borderland. Seismicity clusters align narrowly beneath major strike-slip faults and broadly at complex fault intersections. Well-located hypocenters show activity through the crust to a 16-km seismogenic base and to Moho in places. Fault complexity exists in 3D with detachment links to vertical shallow faults enabling large complex, multi-segment, multi-fault earthquake ruptures. A simple 3-section boundary fault system and landslide analog provides realistic models for transpressional ridge seismotectonics.
Intellectual Merit The tectonic model developed in this project will be used to evaluate earthquake potential offshore southern California and related hazards including strong shaking, induced ground failures, and tsunami. The complex deformation recognized in the California Continental Borderland has been described as an “organized train wreck” (Charlie Hollister, 1998 pers. commun.) and as a tectonic “log jam” (Legg et al. 2015). Accurate geologic framework is needed to produce a realistic Community Rheology Model that may be used for dynamic fault rupture and crustal deformation models for earthquake hazard evaluations. Predictions from deformation models may be tested against the observations used to develop the Geologic Framework for the CRM. Errors or weaknesses in the model may be identified and experiments designed to obtain new data for updating and improving the model. Ultimately, a more accurate and higher-resolution description of the tectonic evolution of the complex Pacific-North America plate boundary will result. More detailed understanding of subduction, oblique-rifting, transpression, and strike-slip faulting may be developed.
Broader Impacts The potential for large complex multi-fault rupture earthquakes must be considered in order to plan effectively for mitigation of associated hazards. Loss-of-function for the ports of Los Angeles/Long Beach and San Diego represents a serious risk to local and national economies. Faults mapped for this project are being used to update the USGS National Seismic Hazard Mapping Program (2023).The project is focused on accurately defining the geology of the major crustal blocks and intervening fault boundaries located offshore southern California. Coastal and offshore faults represent a significant threat to the heavily populated coast due to large earthquakes and potential local tsunamis. The offshore region (Borderland) represents a significant active part of the southern California tectonic plate boundary which must be considered to understand the tectonic evolution of southern California and the regional seismic hazard. Interaction with colleagues in academia, government, and industry represent a crucial component of this research, which would be impossible without such collaboration. Dr. Legg is collaborating with Dr. Mike Oskin at UC Davis to prepare the Geologic Framework in a consistent manner for development of the Community Rheology Model, and with Dr. John Shaw and Andreas Plesch at Harvard for updating the Community Fault Model. In addition, Dr. Legg collaborates with Dr. Chris Sorlien, Dr. Craig Nicholson, Dr. Marie-Helene Cormier, Dr. Marc Kamerling and Dr. Chris Castillo to share knowledge and improve understanding of the complex tectonic evolution of the Borderland. Dr. Vicki Langenheim has provided expertise for this project regarding the geophysical characterization of the Borderland. Interaction with scientists and others involved with the Ocean Exploration Trust in continuing deep sea exploration of the Borderland provides unique opportunities to obtain additional data (seafloor samples, video, etc.) and to engage the broader community (public) with the direct interaction possible through distance learning during the live webcasts of these exploration activities. Presentation of results in scientific meetings as well as in local meetings of geological societies and through interactions at local universities informs the larger audience of both technical and non-technical people interested in the regional geology and natural hazards. Dr. Legg collaborates with Dr. Lynn Dodd (USC) regarding geology of Catalina for her archeology research, and with the Catalina Island Conservancy where he has given seminars and led field trips to educate the public about Catalina Island geology. In 2019, he led a field trip for astronauts, from NASA, ESA, and Japan space agencies training for geologic sampling on the Moon and Mars. The spectacular geology and complex tectonic evolution of this region in SCEC’s backyard, continues to provide exciting and ground-breaking research discoveries for geology on Earth as well as for other planets.
Exemplary Figure Figure 3. Landslide analog for transpressional faulting along the Santa Cruz-Catalina Ridge. Tectonic and landslide margin fault zones are dextral strike-slip with oblique convergence that thrusts crustal (sand) material up and over adjacent crustal (beach) blocks. The seismicity profile (location shown by yellow line) has 1.55x vertical exaggeration. Queried dashed lines are possible sub-seafloor faults that resemble the downward-convergent reverse-separation faults at the landslide edge. The left-stepping echelon pattern of faults at the surface resembles the pattern of faults mapped by Schindler (2010) in East Santa Cruz Basin (dashed red lines). The Santa Cruz-Catalina Ridge fault zone (SCCR) is oblique (convergent) to the Pacific-North America relative plate motion (N40°W) shown by the arrows. The San Clemente (Kidney Bank KB) fault that passes east of Santa Barbara Island (SBI) is parallel to the relative motion. The collision with the Western Transverse Ranges (WTR) complicates the deformation with possible vertical-axis block rotations, under-thrusting, and crustal thickening at Santa Cruz Island (SCZ). Solid red lines are proposed major faults for the Borderland Geologic Framework. CAT=Santa Catalina Island, SCI=San Nicolas Island, ESCB=East Santa Cruz Basin terrane boundary fault, ESCL=East San Clemente fault. (Photos: M. Legg, 1998)