SCEC Award Number 17097 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Imaging ductile fault roots with deep crustal anisotropy: Implications for the distribution of lithospheric deformation in Southern California
Name Organization
Vera Schulte-Pelkum University of Colorado Boulder Karl Mueller University of Colorado Boulder
Other Participants
SCEC Priorities 3b, 3e, 1c SCEC Groups SDOT, CXM, Seismology
Report Due Date 06/15/2018 Date Report Submitted 06/18/2018
Project Abstract
This project targeted the subsurface geometry of faults in Southern California, particularly near and below the brittle-ductile transition. Imaging the deep fault structure was via anisotropic contrasts detected using azimuthally varying receiver functions. We analyzed receiver functions for all permanent and temporary stations available from the SCSN and IRIS data repositories. We began collaborating with a separately funded team (Zachary Ross and Yehuda Ben-Zion) working on relocated seismicity in our study area. Results from receiver functions as well as from seismicity show a dominance of NE-dipping structures. Dips from seismicity are intermediate (40-70 deg) to steeper (70-85 deg) throughout the area. Receiver function results are consistent with these estimates. The pervasive NE dip in seismicity and dipping anisotropic contrasts are surprising, as the dominantly strike-slip faults in the area are thought to be near vertical close to the surface. Dipping planes of seismicity appear in listric geometries, some of which connect to surface fault traces, but with many showing a currently active deep portion with intermediate dips that does not connect to shallower seismicity with steeper dips and a surface fault trace. Our findings are consistent with geological studies that relate pre-existing fabrics and faults from previous deformation episodes, such as the development of the San Jacinto and Elsinore faults through the Miocene Western Salton Detachment system. Inherited fabric may thus exert a significant control on the present-day deformation. This hypothesis will be tested in a current SCEC project addressing anisotropic rheology in general and in the study area specifically.
Intellectual Merit This research contributes to understanding of the rheology of the upper and lower crust, including fault behavior across the brittle-ductile transition. The results are relevant to development of the planned Community Rheology model. This project led us to new concepts of the importance of inherited structures in present day deformation and triggered research into anisotropic rheology and connections between tectonic inheritance and present day fault geometry.
Broader Impacts The project triggered close collaboration between geologists and seismologists and between earthquake and structural seismologists. The findings are used in a new SCEC project funding a graduate student. The project leverages parallel NSF funding to PI Schulte-Pelkum. Deep fault geometry and rheology are relevant to seismic hazard.
Exemplary Figure Figure 1
(a) Strikes (bars) of dipping structures/dipping foliation from receiver functions at stations (white triangles), color coded by converter depth (color bar), plotted with CFM5 preferred surface fault traces and topography. Subsurface strikes are parallel to general NW-SE transform fault orientation (A) with rotation in areas such as Transverse Ranges/Santa Monica Mts (B). The dense LARSE1 line shows strike perturbations near a Pelona schist outcrop that are also seen in SKS fast orientations (C). A shallow radial pattern is seen above a proposed magma body in the Coso geothermal field (D). Transform-orthogonal structures near the Salton Sea are also visible in receiver function strikes (E). Seismicity shows matching NE-dipping trends such as on the profile shown by the white line (F), cross section in panel below.
(b) Seismicity from the catalog by Hauksson et al. (2012) on a cross section of the San Jacinto Fault (white line near F in Fig. 1a). Surface trace of the Clark strand of the San Jacinto Fault shown as red dashed line. Seismicity has a listric appearance when nearing the brittle-ductile transition near 20 km depth, with a dip of about 45 deg. Shallower seismicity at 12 km depth and above appears near vertical, but is sparse to nonexistent above 10 km depth below the main strand. Similar NE-dipping structures are seen in seismicity throughout the southern part of the study area along the Elsinore, San Jacinto, and San Andreas faults, with shallower dips away from the coast and steeper to near vertical dips closer to the coast (Elsinore fault). Figure by Ross and Ben-Zion (pers.comm.).