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Considering Fault Interaction in Estimates of Absolute Stress Along Faults in the San Gorgonio Pass Region, Southern California GEOSPHERE, V

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Considering Fault Interaction in Estimates of Absolute Stress Along Faults in the San Gorgonio Pass Region, Southern California GEOSPHERE, V Research Paper THEMED ISSUE: Seismotectonics of the San Andreas Fault System in the San Gorgonio Pass Region GEOSPHERE Considering fault interaction in estimates of absolute stress along faults in the San Gorgonio Pass region, southern California GEOSPHERE, v. 16, no. 3 Jennifer L. Hatch1, Michele L. Cooke1, Aviel R. Stern1, Roby Douilly2, and David D. Oglesby2 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA https://doi.org/10.1130/GES02153.1 2Department of Earth Sciences, University of California, Riverside, California 92521, USA 9 figures; 1 table; 1 supplemental file ABSTRACT assess the potential for large and damaging earthquakes through this region CORRESPONDENCE: [email protected] (e.g., Tarnowski, 2017; Douilly et al., 2020). Present-day shear tractions along faults of the San Gorgonio Pass region Dynamic rupture models show that, in general, the size and extent of CITATION: Hatch, J.L., Cooke, M.L., Stern, A.R., Dou- illy, R., and Oglesby, D.D., 2020, Considering fault in- (southern California, USA) can be estimated from stressing rates provided earthquake ruptures can depend highly on the initial conditions of the model teraction in estimates of absolute stress along faults by three-dimensional forward crustal deformation models. Due to fault inter- (e.g., Oglesby, 2005). These conditions include physical aspects, such as fault in the San Gorgonio Pass region, southern Califor- action within the model, dextral shear stressing rates on the San Andreas geometry and location of rupture nucleation (e.g., Lozos et al., 2012; Lozos, nia: Geosphere, v. 16, no. 3, p. 751–764, https://doi.org /10.1130 /GES02153.1. and San Jacinto faults differ from rates resolved from the regional loading. 2016; Tarnowski, 2017), and time-dependent aspects, such as state of stress In particular, fault patches with similar orientations and depths on the two and frictional parameters (e.g., Kame et al., 2003; Aochi and Olsen, 2004; Kase Science Editor: Andrea Hampel faults show different stressing rates. We estimate the present-day, evolved and Day, 2006; Duan and Oglesby, 2007). Dynamic rupture models typically Guest Associate Editor: Doug Yule fault tractions along faults of the San Gorgonio Pass region using the time prescribe initial shear and normal tractions by resolving the remote stress since last earthquake, fault stressing rates (which account for fault interac- tensor, constrained from focal mechanism inversions, onto individual fault Received 6 May 2019 tion), and coseismic models of the impact of recent nearby earthquakes. The elements (e.g., Kame et al., 2003; Oglesby et al., 2003). This approach provides Revision received 21 January 2020 Accepted 11 March 2020 evolved tractions differ significantly from the resolved regional tractions, with spatially variable “resolved” tractions that capture the first-order loading of the largest dextral traction located within the restraining bend comprising the faults, but does not take into account the loading history nor the prior Published online 6 April 2020 the pass, which has not had recent earthquakes, rather than outside of the stress interactions between faults. Not only can individual earthquake events bend, which is more preferentially oriented under tectonic loading. Evolved change tractions along nearby faults, advancing or retarding each fault’s earth- fault tractions can provide more accurate initial conditions for dynamic rupture quake clock (e.g., King et al., 1994; Stein, 1999; Duan and Oglesby, 2005), but models within regions of complex fault geometry, such as the San Gorgonio interaction among neighboring active faults influences their long-term slip Pass region. An analysis of the time needed to accumulate shear tractions rates and stressing rates (Willemse and Pollard, 1998; Maerten et al., 1999; that exceed typical earthquake stress drops shows that present-day tractions Loveless and Meade, 2011). Interseismic stressing rates on faults can be esti- already exceed 3 MPa along portions of the Banning, Garnet Hill, and Mission mated using geodesy (e.g., Smith and Sandwell, 2006). Smith and Sandwell Creek strands of the San Andreas fault. This result highlights areas that may (2006) calculated the Coulomb stress along the southern San Andreas fault be near failure if accumulated tractions equivalent to typical earthquake stress just prior to the A.D. 1812 and 1857 earthquakes, showing elevated Coulomb drops precipitate failure. stress in the predicted epicenter locations for these earthquakes. However, the total accumulated traction along any given fault segment depends on both the accumulated tractions during the interseismic period as well as nearby ■ INTRODUCTION rupture history (e.g., Smith-Konter and Sandwell, 2009; Richards-Dinger and Dieterich, 2012; Tong et al., 2014). For example, Smith-Konter and Sandwell The southern San Andreas fault system (southern California, USA) con- (2009) found that California fault segments with higher rates of interseismic sists of multiple active faults that accommodate the deformation between loading have shorter earthquake recurrence intervals. the North American and Pacific plates. Accurate estimates of the earthquake To account for both loading history and fault interaction and to produce hazard in California require an accurate assessment of the potential for large reasonable estimates of fault stress, we simulate deformation within the San throughgoing earthquakes and the ability of ruptures to propagate through Gorgonio Pass region using three-dimensional forward models that provide fault intersections and complexities (e.g., Field et al., 2014). One region of such both steady-state slip rates over multiple earthquake cycles and stressing rates complexity is the San Gorgonio Pass region (SGPr), the site of a restraining between earthquake events. Because we use steady-state slip rates over multi- This paper is published under the terms of the stepover along the southern San Andreas fault (Fig. 1). Accurate dynamic ple earthquake cycles to drive models that simulate interseismic deformation, CC-BY-NC license. rupture models of the SGPr that simulate potential rupture paths will help us the resulting shear stressing rates incorporate the interactions between faults © 2020 The Authors GEOSPHERE | Volume 16 | Number 3 Hatch et al. | Estimates of absolute stress Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/3/751/5019468/751.pdf 751 by guest on 28 September 2021 Research Paper San Bernardino N 34˚30´ Eastern California Figure 1. Map of the San Gorgonio Pass region, south- Wrightwood 1 Shear Zone ern California (USA). While there has not been a rupture event in the region since ca. A.D. 1400, recent 2 nearby earthquakes may impact the stress within the Mill Creek bend. Fault traces of the San Andreas fault are la- 9 3 beled (GP—Galena Peak; SGPT—San Gorgonio Pass Colton Mission Creek thrust). Rupture traces considered in this study are 4 GP highlighted: A.D. 1992 Landers earthquake (dark red), N 34˚00´ A.D. 1812 Wrightwood earthquake (red), A.D. 1726 Riverside 5 10 SGPT Garnet Hill Coachella Valley earthquake (orange), and A.D. 1400 event (yellow). Paleoseismic sites are numbered as Banning Palm Springs 6 such: 1—Wrightwood (Weldon et al., 2002; 2—Pitman San Francisco Canyon (Seitz et al., 1997); 3—Plunge Creek (McGill Hemet Indio 7 et al., 2002); 4—Burro Flats (Yule and Howland, 2001); San Jacinto Fault Coachella 5—Millard Canyon (Yule et al., 2014; Heermance and Yule, 2017); 6—Thousand Palms (Fumal et al., 2002); 7—Indio (Sieh, 1986); 8—Salt Creek (Williams, 2009); N 33˚30´ 9—Colton (Kendrick and Fumal, 2005; 10—Mystic 8 Lake (Onderdonk et al., 2013). Los Angeles Salton Sea San Diego 0 km 50 km W 118˚00´ W 117˚30´ W 117˚00´ W 116˚30´ W 116˚00´ of the southern San Andreas fault system. The interseismic shear stressing The San Bernardino strand of the San Andreas fault lies at the northwestern rates along with information about time since last earthquake event can be end of the San Gorgonio Pass. Two potential rupture pathways go through used to estimate the shear traction on faults through the SGPr following the the restraining bend connecting the San Bernardino strand to the Coachella approach employed by Tong et al. (2014). The resulting estimates of shear segment of the San Andreas fault. The southern pathway consists of the traction may differ from those resulting from resolving the remote stress San Gorgonio Pass thrust, Garnet Hill strand, and Banning strand of the San tensor onto faults, in that our models explicitly include fault interaction and Andreas fault (Fig. 1). The San Gorgonio Pass thrust dips to the north and fault loading from depth during the interseismic period. Furthermore, we has a corrugated geometry near the Earth’s surface (e.g., Matti et al., 1992). incorporate the effects of recent earthquakes on faults near the SGPr to pro- The eastern end of the San Gorgonio Pass thrust connects to the Garnet Hill duce a more accurate estimate of the current stress state of this system. Using and Banning strands. The north-dipping Garnet Hill and subparallel Banning tractions that incorporate fault interaction and loading history may enhance strands have approximately the same strike. The northern pathway through the accuracy of dynamic rupture models, refining our insight into the nature the SGPr consists of the Mill Creek, Mission Creek, and Galena Peak strands of potential earthquake rupture propagation within the SGPr. of the San Andreas
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