SCEC Award Number 18195 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Develop Geological Model of Offshore Southern California for the Community Rheology Model
Investigator(s)
Name Organization
Mark Legg Legg Geophysical
Other Participants
SCEC Priorities 3a, 1b, 5b SCEC Groups CXM, Geology, SDOT
Report Due Date 03/15/2019 Date Report Submitted 03/15/2019
Project Abstract
 The California Continental Borderland is an active part of the Pacific-North America transform plate boundary. The Geologic Framework (GF) maps the crustal blocks and their major fault zone boundaries. The 3D 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 initial model is based upon the tectono-stratigraphic terranes that describe the major crustal blocks developed during the subduction zone to transform fault evolution of the North America continental margin offshore southern California. These blocks include the Patton accretionary wedge, Nicolas forearc, Catalina core complex, and Pacific oceanic crust. Simple 1D crustal columns showing significant parameters, seismic velocity, density, and thickness for the major layers of crustal basement and upper mantle structure, were derived from published cross-sections of the region based on geophysical and geological investigations. Simplified block boundaries consist of major faults from the SCEC Community Fault Model and new Outer Borderland faults to be added. For the initial model, these boundaries consist of vertical strike-slip faults. Updated boundaries consider dipping faults including the former subduction megathrust along the Patton Escarpment and other low-angle detachments (or blind thrusts) that are important in many areas. Future versions of the Borderland GF will require refined definitions of the complex block boundaries. Major fault zones may be described as “fault wedges” derived from oblique rifting of crustal blocks from the continental margin. These complex fault systems may produce large earthquakes involving multiple faults and fault segments.
Intellectual Merit The project compiles an extensive background of knowledge about the geology and tectonic evolution of the southern California offshore region into a simplified model that will be used in the Community Rheology Model. Applications of these models will enable testing hypotheses regarding seismotectonic character and earthquake potential for offshore fault systems that may produce strong shaking and related ground failures which affect heavily populated and developed coastal areas. Vertical seafloor deformation may also produce local tsunamis which could be destructive to coastal areas. With excellent subsurface imaging of offshore geologic structure, models of fault system behavior can be tested using offshore examples to verify deformation character and history. Developing simple, yet realistic, models of complex geologic structure requires creative thinking to apply a thorough knowledge of the regional geology and tectonic history into a manageable digital Geologic Framework for investigating seismotectonic processes and earthquake potential.
Broader Impacts This project provides reference material - data and model - that will be used by the SCEC community and the broader community of earth scientists interested in understanding the geology, tectonic evolution, and earthquake potential of complex continental margin transform fault systems. Results from this project will be used for publication of a book describing the tectonic evolution of a major transform fault system along a continental margin. Presentations of the data and results has been accomplished at SCEC annual meetings and workshops as well as the 11th National Conference on Earthquake Engineering. It is anticipated that many future presentations will be made to interested groups locally (southern California) and possibly globally where similar complex fault systems exist.
Exemplary Figure Plate 1. Borderland Geologic Framework Terrane Boundary Faults. Major tectono-stratigraphic terranes are labeled. Terrane boundary faults are thick solid red lines, dark red for Patton Escarpment paleosubduction thrust, detachment is hinge where slab bends. Thin red lines are other faults related to the terrane boundaries. Green dashed line marked Eocene is edge of Upper Cretaceous and Paleogene strata (Vedder, 1987). Aeromagnetic anomalies in color are modified from Langenheim et al. (1993). Dot-circles show locations of offshore boreholes. LA = Los Angeles; Ocn = Oceanside; SD = San Diego; Ens = Ensenada. Bathymetry contours in meters. Shaded relief from GeoMapApp- GMRT (Ryan et al., 2009).