Crustal Deformation Along the Northern San Andreas Fault System

Mark H. Murray

Introduction

The San Andreas fault system in northern California includes three sub-parallel right-lateral faults: the San Andreas, Ma'acama, and Bartlett Springs. This northernmost segment is the youngest portion of the fault system, forming in the wake of the northwestwardly propagating Mendocino triple junction where the Pacific, North America, and Gorda (southern Juan de Fuca) plates meet. The Pacific plate moves about 35-40 mm/yr relative to central California across a broad $\sim$100-km zone in northern California. Additional deformation in eastern California and the Basin and Range province contribute to the total relative Pacific-North America motion of $\sim$50 mm/yr. The San Andreas fault itself has been essentially aseismic and accumulating strain since it last ruptured in the great 1906 San Francisco earthquake, and no major earthquakes have occurred during the historical record on the more seismically active Ma'acama, and Bartlett Springs faults, which are northern extensions of the Hayward-Rodgers Creek and Calaveras-Concord-Green Valley faults in the San Francisco Bay area.

In Freymueller et al. (1999), we used GPS data collected in 1991-1995 along two fault-crossing profiles near Ukiah and Willits (Figure 22.1). The total deep slip rate on the San Andreas fault system inferred from the GPS data is $39.6^{+1.5}_{-0.6}$ mm/yr (68.6% confidence interval). Although deep slip rates on the individual faults are more poorly resolved due to high correlations between estimated slip rates and locking depths, and between slip rates on adjacent faults, the inferred slip rate on the Ma'acama fault ( $13.9^{+4.1}_{-2.8}$ mm/yr) implies that it has accumulated a slip deficit large enough to generate a magnitude 7 earthquake, posing a significant seismic hazard.

Figure 22.1: GPS sites along the northern San Andreas fault system. Light circles, sites that were observed in early 2003. Dark circles, 2004 surveyed or planned stations. Profile names are capitalized. USGS conduct surveys along the NBAY profile and near Cape Mendocino. Only one continuous GPS station (HOPB) currently operates in this region,with at least 5 planned PBO stations to be installed in Summer and Fall 2004. Rounded boxes, special focus areas along the Bartlett Springs fault where we will establish new profiles in Fall 2004.
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In this ongoing study, we are resurveying the original profiles, adding new profiles to the north (near Covelo) and south (near Healdsburg), and surveying nearly 40 additional stations in the southern portion of the network to provide better monitoring along the Rodgers Creek and Ma'acama faults (Figure 22.1). The survey of the 4 primary profiles was conducted during 2003. Altogether, 43 site positions were measured during 94 session occupations lasting 6.5-8 hours, with the assistance of students and staff of the BSL. We are currently surveying about 40 monuments in the southern portion of network and are planning to install new profiles in special focus areas along the Bartlett Springs fault, whose long-term slip rate, locking depth and surface creep are poorly resolved. Most of the monuments were last observed in 1993-1995, so the new observations significantly improve the velocity estimates and models of average interseismic strain accumulation, including possible spatial variations along the fault system. The 10-station profiles from Pt. Reyes to Cape Mendocino together with planned PBO stations form a primary monitoring network for future observations to detect temporal variations in deformation.

Deformation

Figure 22.2 shows site velocities for the 1994-2004 period relative to stable North America, as defined by a set of 20 fiducial stations. The data are processed using GAMIT/GLOBK software using many of the same techniques used to process the BARD observations described in Chapter 6 that provide a well-defined velocity reference frame with respect to the stable North America. Most of the velocities were derived from data spanning 8-10 years, whereas those with the largest error ellipses include data from only a 4 year span (most of these stations will be reoccupied in 2004). The easternmost stations exhibit motions typically associated with Sierran-Great Valley block (ORLA: 12.5 mm/yr NW). The westernmost sites are moving close to the Pacific plate rate (PTAR: 45.9 mm/yr NW). Fault-normal contraction is observed east of the Ma'acama fault, in the region of the Coast Ranges near the Central Valley where similar contraction has been observed elsewhere (e.g., Murray and Segall, 2001).

We apply angular velocity-fault backslip modeling techniques (e.g., Murray and Segall, 2001) to account for both far-field plate motions and interseismic strain accumulation. We are currently using a set of algorithms provided by Brendan Meade of MIT that extend this simple 2D methods to complex, 3D fault systems (including subduction zones and extensional provinces) by summing backslip on rectangular dislocations. We have successfully used the algorithms in a study of block motions in the Adriatic region and in the the BAVU block modeling efforts for the San Francisco Bay Area, both of which are described in other research chapters.

Preliminary results from our block modeling efforts are shown in Figure 22.2. We assume a simple fault geometry with 2 blocks between the 3 major faults in the San Andreas fault system. We also include the Pacific and North America plates, with sufficient numbers of stations to resolve their relative motions, and a Sierran-Great Valley block to provide better constraints on motions just east of the Bartlett Springs fault. The agreement between observed and predicted velocities is typically less than the 2 mm/yr level. Misfits are larger in a few areas close to faults, such as along the central Ma'acama and near the MTJ, that should be decreased with further refinement of the fault geometry. Total deformation across the San Andreas fault system is 38 mm/yr, in agreement with previous studies, but deep slip is concentrated on the Ma'acama fault (24 mm/yr) and on the Bartlett Springs fault (10 mm/yr), with only 4 mm/yr on the San Andreas. We are currently investigating this result, which is due in part to the high-degree of correlation between the slip rates on the 3 faults, and will test methods for adding geologic and other information using Bayesian techniques, which should reduce the correlations on slip rates and provide better resolution on other parameters such as locking depths. Fault normal contraction of about 3 mm/yr is estimated due to the impingement of the Sierran-Great Valley on the fault system, but more sophisticated error analysis is required to assess whether this result is significant.

Figure 22.2: Velocities of sites in the Coast Ranges relative to North America, with 95% confidence regions assuming white-noise process only. Included are sites from this study plus sites from the BARD continuous network and the USGS North Bay profile. Red, observed velocities. Blue, velocities predicted from angular velocity-backslip block model assuming block boundaries (heavy black lines). We assume Pacific, North America, and Sierran-Great Valley blocks, plus 2 small blocks between the San Andreas, Ma'acama, and Bartlett Springs faults. The most significant misfits, such as near Cape Mendocino, can be reduced by refining the fault geometry.
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Acknowledgements

We appreciate support for this project by the USGS NEHRP through grant numbers 02HQGR0064 and 03HQGR0074. We thank André Basset, Maurizio Battaglia, Dennise Templeton, and especially Todd Williams for assistance conducting the survey.

References

Freymueller, J. T., M. H. Murray, P. Segall, and D. Castillo, Kinematics of the Pacific-North America plate boundary zone, northern California, J. Geophys. Res., 104, 7419-7442, 1999.

Murray, M. H., and P. Segall, Modeling broadscale deformation in northern California and Nevada from plate motions and elastic strain accumulation, Geophys. Res. Lett., 28, 4315-4318, 2001.

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