Research Paper THEMED ISSUE: Seismotectonics of the San Andreas Fault System in the San Gorgonio Pass Region GEOSPHERE New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California GEOSPHERE, v. 17, no. 1 Katherine A. Guns1,†, Richard A. Bennett1, Joshua C. Spinler2, and Sally F. McGill3 1Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA https://doi.org/10.1130/GES02239.1 2Department of Earth Sciences, University of Arkansas at Little Rock, 2801 S. University, Little Rock, Arkansas 72204, USA 3Department of Geological Sciences, California State University San Bernardino, 5500 University Parkway, San Bernardino, California 92407, USA 13 figures; 7 tables; 1 set of supplemental files ABSTRACT Along the SSAF system, there are more than a dozen fault segments com- CORRESPONDENCE: [email protected] pactly arranged within a handful of tectonic provinces along the main San Assessing fault-slip rates in diffuse plate boundary systems such as the Andreas fault (e.g., the continental borderlands, the Eastern Transverse Ranges, CITATION: Guns, K.A., Bennett, R.A., Spinler, J.C., and McGill, S.F., 2021, New geodetic constraints on San Andreas fault in southern California is critical both to characterize seis- the Eastern California shear zone, the Los Angeles Basin, and the Western Trans- southern San Andreas fault-slip rates, San Gorgonio mic hazards and to understand how different fault strands work together to verse Ranges), which together accommodate ~52 mm/yr of plate boundary Pass, California: Geosphere, v. 17, no. 1, p. 39–68, accommodate plate boundary motion. In places such as San Gorgonio Pass, motion (Argus et al., 2010). Yet, these tectonic provinces each have their own https://doi.org/10.1130/GES02239.1. the geometric complexity of numerous fault strands interacting in a small area deformation styles, with varying fault types (right lateral, left lateral, reverse, or adds an extra obstacle to understanding the rupture potential and behavior oblique combinations), map view texture (multiple faults parallel to the plate Science Editor: Andrea Hampel Guest Associate Editor: Michele Cooke of each individual fault. To better understand partitioning of fault-slip rates boundary versus multiple faults striking east-west; straight and clearly defined in this region, we build a new set of elastic fault-block models that test 16 versus sinuous, disconnected traces), geologic level of maturity (e.g., recently Received 31 January 2020 different model fault geometries for the area. These models build on previ- formed in past <2 m.y. versus been accommodating slip since 10 Ma), and level Revision received 26 August 2020 ous studies by incorporating updated campaign GPS measurements from of active seismicity (Fig. 1). Determining which faults play the most important Accepted 20 October 2020 the San Bernardino Mountains and Eastern Transverse Ranges into a newly roles in actively accommodating plate boundary motion requires every tool in calculated GPS velocity field that has been removed of long- and short-term the active tectonics arsenal, including the application of historical geologic off- Published online 10 December 2020 postseismic displacements from 12 past large-magnitude earthquakes to set reconstructions (million-year time scale), tectonic- geomorphologic– based estimate model fault-slip rates. Using this postseismic-reduced GPS velocity geologic slip-rate studies and paleoseismic trenching (hundred- thousand-year to field produces a best- fitting model geometry that resolves the long-standing Holocene time scales), space-based geodetic techniques that allow decadal time- geologic-geodetic slip-rate discrepancy in the Eastern California shear zone scale measurement of the present- day motion of the crust, the pattern of crustal when off-fault deforma tion is taken into account, yielding a summed slip seismicity, and numerical modeling based on crustal deformation dynamics. rate of 7.2 ± 2.8 mm/yr. Our models indicate that two active strands of the One of the most complicated areas of fault interaction in southern California San Andreas system in San Gorgonio Pass are needed to produce sufficiently lies in and around San Gorgonio Pass, at the northern end of the Coachella low geodetic dextral slip rates to match geologic observations. Lastly, results Valley (Fig. 2). As the SSAF stretches north from the Salton Sea toward San Gor- suggest that postseismic deformation may have more of a role to play in gonio Pass, it splits from one main fault strand into three subparallel strands, affecting the loading of faults in southern California than previously thought. including, from southwest to northeast, the Garnet Hill, Banning-Coachella Valley, and Mission–Mill Creek faults. The 14–30 mm/yr of geologically and geodetically measured slip along the Coachella Valley segment of the SSAF ■ INTRODUCTION near or to the south of this juncture (Becker et al., 2005; Meade and Hager, 2005; van der Woerd et al., 2006; Behr et al., 2010; Spinler et al., 2010; Blisniuk Complex plate boundary zones such as that of the southern San Andreas et al., 2013b; Lindsey and Fialko, 2013; Spinler et al., 2015) must somehow be fault (SSAF) system in southern California present an opportunity to investigate partitioned onto these three strands to the north, onto a possible emerging fault how recoverable elastic strain and nonrecoverable fault slip are distributed zone along the “Landers Mojave Earthquake Line” of Nur et al. (1993), or trans- within networks of interlocking and branching fault segments of different and ferred to other faults of the plate boundary zone through other mechanisms sometimes seemingly incompatible azimuths, lengths, ages, and maturities. such as block rotation (e.g., Carter et al., 1987; Powell, 1993). Understanding where this slip is being accommodated in the present day is vital to testing seismic hazard scenarios for this region, particularly in light of the evidence Katherine A. Guns https://orcid.org/0000-0002-2956-1536 This paper is published under the terms of the †Now at Institute of Geophysical and Planetary Physics, Scripps Institution of Oceanography, that the SSAF in the Coachella Valley is overdue for a major earthquake (Fumal CC-BY-NC license. University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA et al., 2002; Fialko, 2006; Field et al., 2015). However, the diffuse nature of the © 2020 The Authors GEOSPHERE | Volume 17 | Number 1 Guns et al. | New GPS-based block modeling accounting for postseismic deformation near San Gorgonio Pass Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/1/39/5219650/39.pdf 39 by guest on 27 September 2021 Research Paper 36˚N 2019, M7.1 Ridgecrest N Fault rlock Ga Eastern California 100 km Shear Zone SA Block Modeling Boundary F Mo Figure 1. Regional fault geometry and historical (≥Mw6.5) jav e S 1999, M7.1 Hector Mine earthquakes in southern California; earthquake surface 1971, M6.6 ec ruptures presented in red and focal mechanism solu- ti on tions plotted from catalog of Wang et al. (2009) and the U.S. Geological Survey Earthquake Catalog. Large- 1992, M7.3 Landers 1994, M6.7 magnitude earthquakes of the past 50 years are labeled Northridge with their location names. Earthquakes that make up the 34˚N Landers-Mojave Earthquake Line of Nur et al. (1993) are San shown with their years and names in black text (1979 S A Homestead; 1975 Galway; 1965 Calico; 1947 Manix; 2008 Gorgonio Palm F C Ludlow). The dashed white box presents the extent of Knot o Springs a our block model boundary in the context of the larger c h e region, and we highlight the location of the San Gorgo- Salton ll Continuous GPS Station a nio Pass knot in a solid white box. Campaign GPS Station Sea (CSU San Bernardino) 1987, M6.6 Campaign GPS Station (UA JOIGN network) 1979, M6.5 Faults (from USGS Quaternary Database) Rupture (from USGS 2010, M7.2 El Mayor Cucapah Historic Faults Database) 0 km 100 km −118˚W −116˚W fault system here hampers our understanding of the role each part plays in there is the possibility of a more complex rupture involving several strands the earthquake cycle. simultaneously, even strands that are unmapped as yet (Yule and Sieh, 2003). An added wrinkle to the challenge that this complex fault geometry poses As the San Jacinto fault has evolved in close proximity to the San Andreas to understanding the distribution of slip rate is the question of how an indi- fault over the past ~1.5 m.y. (e.g., Morton and Matti, 1993; Kendrick et al., 2015; vidual earthquake rupture may or may not propagate along these closely Fattaruso et al., 2016), it has produced and reactivated many smaller connecting spaced faults in and around San Gorgonio Pass. The individual fault strands faults between itself and the San Andreas fault, particularly in the San Ber- together comprise a left- stepping restraining bend in the SSAF zone, which nardino Basin and eastern San Gabriel Mountains (Morton and Matti, 1993). represents a regional-scale structural knot in known fault geometry (e.g., Sykes In such intricate fault geometry, these smaller, less obvious (and potentially and Seeber, 1985; Yule and Sieh, 2003). Geometrical complexities such as this unmapped) faults could play a key role in a complex rupture, adding to the can serve as rupture barriers (e.g., Wesnousky, 2008; Lozos et al., 2015), but challenge of modeling seismic behavior here (Ross et al., 2017). It is critical to evidence from previous large-magnitude earthquakes along the San Andreas understand the overall fault geometry, slip histories, and interconnectedness of fault shows through-going ruptures propagating through large stepovers, such these related faults because these parameters have implications for the seismic as the 1857 Fort Tejon event rupturing through the “Tejon Knot” (Sykes and energy release and intensity of shaking during an earthquake rupture.