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Initiation of the San Jacinto Fault and Its Interaction with the San Andreas Fault: Insights from Geodynamic Modeling

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Initiation of the San Jacinto Fault and Its Interaction with the San Andreas Fault: Insights from Geodynamic Modeling Ó Birkha¨user Verlag, Basel, 2007 Pure appl. geophys. (2007) DOI 10.1007/s00024-007-0262-z Pure and Applied Geophysics Initiation of the San Jacinto Fault and its Interaction with the San Andreas Fault: Insights from Geodynamic Modeling 1,2 1 QINGSONG LI and MIAN LIU Abstract—The San Andreas Fault (SAF) is the Pacific-North American plate boundary, yet in southern California a significant portion of the relative plate motion is accommodated by the San Jacinto Fault (SJF). Here we investigate the initiation of the SJF and its interaction with the SAF in a three- dimensional visco-elasto-plastic finite-element model. The model results show that the restraining bend of the southern SAF causes strain localization along the SJF, thus may have contributed to its initiation. Slip on the SJF tends to reduce slip rate on the SAF and enhance deformation in the Eastern California Shear Zone. The initiation of the SJF and its interaction with the SAF reflect the evolving plate boundary zone as it continuously seeks the most efficient way to accommodate the relative plate motion. Key words: Strain localization, fault interaction, San Andreas Fault, restraining bend, finite-element model. 1. Introduction The San Andreas Fault (SAF) is the boundary between the Pacific and the North American plates, but in southern California the relative plate motion is distributed over a complex fault system. In particular, the San Jacinto Fault (SJF) slips at 15 Æ 9 mm/yr (BECKER et al., 2005), comparable to southern SAF (SHARP, 1981; ROCKWELL et al., 1990; MORTON and MATTI, 1993; BENNETT et al., 2004; BECKER et al., 2005; MEADE and HAGER, 2005; VAN DER WOERD et al., 2006) (Fig. 1). The initiation of the SJF and other secondary faults in the SAF system and their interactions with the SAF are of fundamental importance for understanding the plate boundary zone dynamics and the associated earthquake hazards. The SJF initiated between 1.5 and 1.0 Ma, based on geological and stratigraphic evidence (MORTON and MATTI, 1993; ALBRIGHT, 1999; DORSEY, 2002). The timing roughly coincided with the formation of a major restraining bend in southern SAF, suggesting a 1 Department of Geological Sciences, University of Missouri, Columbia, MO 65211, U.S.A. E-mail: [email protected] 2 Lunar and Planetary Institute, Houston, TX 77058, U.S.A. Q. Li and M. Liu Pure appl. geophys., 240°E 242°E 244°E Mw 5-6 6-7 7-7.5 7.5- 36°NSAF(Carrizo)34±3 36°N (27 ±8) 30±7 SAF( (16 Moja±12) ve) 24±6 (1±12) SAF(SBM) 2 34°NSAF(Ind5±5 34°N (2 12 3 ±6 SJF ±8 (15±9) io) ) restraining bend 20±5 IMF (39±5) 32°N 32°N 240°E 242°E 244°E Figure 1 Active faults and seismicity in southern California. Numbers on segments of the San Andreas Fault (SAF), the San Jacinto Fault (SJF), and the Imperial Fault (IMF) are fault slip rates estimated from geological data (California Geological Survey, http://www.consrv.ca.gov/CGS/rghm/psha/index.htm) and from geodetic measurements (in parenthesis) (BECKER et al., 2005). Circles show epicenters of earthquakes (M > 5.0) from 1800 to present from NEIC catalog. causative relationship between the SAF and the SJF (MATTI and MORTON, 1993; MORTON and MATTI, 1993). Here we test this relationship and explore the dynamic interaction between the SAF and the SJF in a three-dimensional (3-D) visco-elasto-plastic finite-element model. The model is similar to that in a preceding paper (LI and LIU, 2006), where we simulated the first-order geometrical impact of the entire SAF on regional stress field and strain partitioning. In this study we focus exclusively on southern California, including both the SAF and the SJF in the model. Our results confirm the notion that development of the restraining bend along the San Bernardino Mountain segment of the SAF (Fig. 1) contributed to the initiation of the SJF, and slip on the SJF has broad impact on strain partitioning in southern California. 2. Numerical Model The 3-D finite-element model of lithospheric dynamics used in this study was discussed by LI and LIU (2006). We have modified the model to focus on southern California (Fig. 2). The first-order geometry of the SJF and SAF, including both the Big Bend and the smaller restraining bends, is represented in the model, with both faults dipping at 90 degrees. The model consists of a 20-km thick upper crust (the San Jacinto and San Andreas Faults SAF SAF SAF SJF IMF 49 mm/yr Figure 2 Numerical mesh and boundary conditions of the finite-element model. Abbreviations are explained in the caption of Figure 1. On both ends of the SAF an extra 300-km model domain with a straight fault zone is added to minimize the effects of artificial boundary conditions. schizosphere) with an elasto-plastic (non-associated Drucker-Prager model) rheol- ogy, and a 40-km thick visco-elastic layer (Maxwell model) representing lower crust and uppermost mantle (the plastosphere). The young’s modulus and Poisson’s ratio are 8:75 Â 1010 Pa and 0.25, respectively, for the whole entire region. We explored the viscosity of the plastosphere in the range between 1019 Pa s and 1021 Pa s that has been suggested for southern California (HAGER, 1991; KENNER and SEGALL, 2000; POLLITZ et al., 2001). The schizosphere outside the fault zone has a cohesion of 50 MPa and an internal frictional coefficient of 0.4. The faults in the upper crust are simulated with 4-km thick plastic layers with zero internal frictional coefficients (BIRD and KONG, 1994). The cohesion for the SAF is assumed to be 10 MPa, perhaps the upper bound permitted by the surface heat flux measurements (LACHENBRUCH and SASS, 1980). We used various cohesion values for the SJF to explore the effect of changing fault strength as the SJF evolves. The boundary condition simulates motion of the Pacific plate relative to the fixed North American plate (Fig. 2). The two sides that cross the SAF are free in the direction normal to the boundary plane and fixed in the other two directions. The surface is a free boundary and the bottom is a free-slip boundary. The model calculates plastic deformation both within the fault zones (plastic sliding) and outside fault zones (plastic deformation) when stress reach their respective yield criteria. Allowing plastic deformation outside fault zones prevents pathological stress buildup that would occur in elastic and viscoelastic models when simulating long-term deformation. To reduce the impact of the arbitrary initial conditions (zero initial stress), the model is run more than 20,000 years at 10-year time steps until the system approaches a steady state. Thereafter, the predicted fault Q. Li and M. Liu Pure appl. geophys., slip rates reflect the secular slip rates that depend mainly on the specified tectonic loading rate and fault properties. 3. Model Results 3.1. Initiation of the SJF To test the idea that the restraining bend of the SAF may have caused the initiation of the SJF (MATTI and MORTON, 1993), we started with a model that includes only the main trace of the SAF; both the Big Bend and the restraining bend along the San Bernardino Mountain (SBM) segment are included (Fig. 2). Figure 3a shows the predicted secular slip rates on various segments of the SAF. In general the predicted rates are higher in central (the Carrizo plain segment) and the southern- most segments of the SAF than around the Big Bend. The values depend mainly on the viscosity of the plastosphere: lower viscosity causes higher slip rates on the SAF. Using 2 Â 1020 Pa s produces 35 mm/yr on the Carrizo plain segment, close to the geological rate (Fig. 1). The predicted slip rates on other segments are close to the upper bounds of geological estimates (KELLER et al., 1982; WELDON and SIEH, 1985; HARDEN and MATTI, 1989; POWELL and WELDON, 1992). Using different viscosity and yield strengths affects the absolute values but not the general pattern of the predicted fault slip rates. In essence, the bends of the SAF hamper the relative plate 240° 242° 244° 240° 242° 244° (a) (b) KJ/m2/yr 0.0 600.0 36° 36° 36° ~35mm/a ~22mm/a ~17mm/a 34° 34° 34° ~30mm/a ~0mm/a Fault slip rate 50mm/a Mw ~36mm/a 5-6 6-7 7-7.5 7.5- 32° 32° 32° 240° 242° 244° 240° 242° 244° Figure 3 (a) Predicted slip rates along the SAF. Line thickness is proportional to the slip rates (scale shown in the lower left corner). Lines are major active faults in the region. Only the SAF main trace is included in the model. (b) Predicted rate of plastic strain energy release outside the SAF, vertically integrated through the schizosphere per unit surface area. Note the high-energy band in the location of the SJF. Circles show seismicity explained in Figure 1. San Jacinto and San Andreas Faults motion and force more strain to be partitioned in the surrounding region, similar to the results of the regional scale model (LI and LIU, 2006). The link between the development of the restraining bend and initiation of the SJF can be seen from the resulting strain distribution. Figure 3b shows the predicted release rate of plastic strain energy, defined as the product of plastic strain rates and the deviatoric stress, in the crust outside the fault zone. Such plastic strain is presumably absorbed mainly by secondary faults not included in the model.
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