SCEC Award Number 17146 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Develop Geological Model of Offshore Southern California (Borderland) for the Community Rheology Model
Investigator(s)
Name Organization
Mark Legg Legg Geophysical
Other Participants
SCEC Priorities 3a, 1b, 5b SCEC Groups SDOT, Geology, CXM
Report Due Date 06/15/2018 Date Report Submitted 06/15/2018
Project Abstract
 The Geologic Framework (GF) for offshore southern California will consist of mapping the crustal blocks and major fault zone boundaries of these blocks. The three-dimensional geometry of the block boundaries includes the fault trace, dip and depth extent, and thickness of the principal displacement zone where relative movement is focused. The GF is based upon the tectonostratigraphic terranes that describe the major crustal blocks developed during the tectonic evolution of the Pacific-North America transform plate boundary. These blocks, Patton accretionary wedge, Nicolas forearc, Catalina subduction complex, and Pacific oceanic crust, developed during the subduction zone to transform fault system evolution of the North America continental margin offshore southern California. Crustal blocks are described by 1-D crustal columns showing significant parameters, e.g., seismic velocity, density, thickness for the major layers of crustal basement and upper mantle structure. Simple block boundaries consist of major faults included in the SCEC Community Fault Model (CRM) and new faults in the Outer Borderland to be added to the CRM. In this first phase of the project, these boundaries consist of simple vertical strike-slip faults. Updated boundaries may consist of dipping faults including low-angle detachments (or blind thrusts) that are important in many areas. The crustal block columns are derived from published cross-sections of the region based on geophysical and geological investigations. Future versions of the Borderland GF will require refined definitions of the complex block boundaries, and subdivision of the original four blocks may be necessary to describe the crustal character more accurately.
Intellectual Merit The tectonic model being 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 to evaluate earthquake hazards. 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 project is focused on the geology of the major crustal blocks and intervening fault boundaries located offshore southern California, which represents 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 at crucial component of this research, which would be impossible without such collaboration. Dr. Legg is collaborating with Dr. Mike Oskin and student at UC Davis to prepare the Geologic Framework in a consistent manner for development of the Community Rheology Model. In addition, Dr. Legg collaborates with Dr. Chris Sorlien, Dr. Craig Nicholson, Dr. Marie-Helene Cormier and Dr. Marc Kamerling to share knowledge and improve understanding of the complex tectonic evolution 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.
Exemplary Figure Figure 3 - Borderland geologic columns (version 1) represents the first compilation of crustal columns for the Geologic Framework to be used with the Community Rheology Model. The description of the columns is a large part of the text for the project report.