The enigmatic San Gorgonio Pass Doug Yule Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, California 91330-8266, USA The largest discontinuity along the San P 117° 15′ 117° ip 116° 45′ W e s Andreas fault occurs in the San Gorgonio Pass C a t a (SGP) region of Southern California. Here, the n h n 1992 M 7.3 t a r u y w n A s t o a n S San Bernardino and Coachella Valley strands fa u l (from the northwest and southeast, respectively) 1992 Mw 6.2 t disaggregate into a family of irregular and discon- l t tinuous right-lateral, reverse, thrust, and oblique- M i San Gorgonio a u Yucca Valley l l C Mountain f r e a i n e k n t S f a n t o o u normal faults (Fig. 1) (Allen, 1957; Matti et al., an u l t P i M Be rna 1985; Matti and Morton, 1993; Yule and Sieh, rdin . o - .z S f an 2003; Langenheim et al., 2005). Seismological ills An i s s i n H dr M o n C to ea r f s e a f Burro Flats e k studies also show that crustal deformation here Cr au l f a t u l is diffuse and distributed primarily on strike-slip t 34° N 1986 M 5.9 L 1992 M 6.1 and thrust faults at depths of ~5–20 km (Jones w G Desert Hot Spgs S o a n r B C g anning fa o et al., 1986; Seeber and Armbruster, 1995; ult a o c h n e s ll i a s C a Magistrale and Sanders, 1996; Hauksson, 2000; o P f m oac V a l t s y e he a Banning u s t G a lla 1948 Mw 6.0 lle S r Va y Carena et al., 2004). Deformation along the San a n e t lle - n y - S J H B a a i an n l l ni A Andreas fault zone in the SGP region is therefore c ng nd S a i f a r n u fa ea C n ul s t l t f broadly distributed, in contrast to regions to the o o t au s J a c i n t o lt f a a north and south where deformation is restricted L u o l m t C o a Palm Springs a c to a much narrower zone. h e l l a V a l l e y f ot a H Sp N r V a l l e y Interpretations of the surface and subsur- u in San Jacinto Peak. l t g s face data have produced competing three- fa u l 33° 45′ dimensional (3-D) kinematic models for the 0 10 km t SGP region. All models include a combination of thrust and strike-slip faulting. Based on their Figure 1. Shaded-relief topographic map of San Gorgonio Pass (SGP) region. Traces of active faults shown in red, inactive faults shown in black. Mapping from Allen (1957), analysis of aligned nodal planes and slip vectors, Matti et al. (1985), and Yule and Sieh (2003). Beach-balls show epicenter and oblique slip dur- Seeber and Armbruster (1995) proposed that ing earthquakes on the north-dipping San Gorgonio Pass–Garnet Hill fault system at depth. a throughgoing, near-vertical strike-slip fault Stars show epicenters of other earthquakes on faults outside of SGP. exists at depth beneath the active SGP thrust. In this model, the thrust has “beheaded” the active San Andreas fault in the upper 5 km of the crust. Cooke (2009, p. 119 in this issue of Geology) 2005; Dair and Cooke, 2009; W. Behr, 2008, A second model envisions the San Andreas fault have taken advantage of an ever-increasing personal commun.). Even Dair and Cooke’s zone as a complex system of strike-slip and computing capacity and the relatively new vertical fault model reveals this trend. As they north-dipping thrust faults that transfer dextral- Southern California Earthquake Center Com- discuss, this decrease in slip can be explained oblique slip through the SGP region (Matti and munity Fault Model (CFM) (Plesch et al., 2007) by slip transfer to the west onto the San Jacinto Morton, 1993; Magistrale and Sanders, 1996; to compare vertical versus dipping fault mod- fault zone and to the east onto the Eastern Cali- Yule and Sieh, 2003; Carena et al., 2004). A els in the SGP. By applying a 3-D numerical fornia Shear Zone. However, the geologic slip third model is based on the analysis of gravity model to vertical and north-dipping scenarios, rates reported for SGP come from single fault and aeromagnetic data that identify a buried Dair and Cooke found that the dipping model strands, and therefore provide a minimum esti- magnetic source beneath the SGP. Langenheim provides a better match to the available strike- mate. The broad zone of deformation defi ned by et al. (2005) interpreted this to be a block of San slip and uplift data for the San Andreas fault and the active faults in Figure 1 suggests that an as Gabriel and/or Mojave/San Bernardino Moun- southern San Bernardino Mountains. This inno- yet undetermined amount of dextral slip is parti- tains basement. In this model, a thrust-bounded vative result implies that, at least in the case of tioned onto a number of structures in the SGP. It wedge of Peninsular Ranges basement has been SGP, an oversimplifi ed fault model introduces is conceivable that an additional 50% of dextral inserted beneath the Banning and SGP thrust systemic errors and yields fl awed results. Given slip (3–5 mm/yr) could be carried by secondary faults and above the buried magnetic block. this result, it is intriguing to consider how model structures (folds and faults) in SGP, increas- Several researchers have used a relatively results may change in other regions where over- ing the total slip rate to 8–15 mm/yr. If correct, simple, vertical fault geometry to model the simplifi ed fault geometries have been used. this overlaps with slip rates obtained from both kinematics of the San Andreas fault in the SGP All of the available data indicate that San the vertical and the dipping model of Dair and (Meade and Hager, 2005; Olsen et al., 2006; Andreas fault slip decreases to a minimum of Cooke (2009). This point does not diminish their Smith and Sandwell, 2006). The choice to over- 5–10 mm/yr at the SGP, from 24 ± 3.5 mm/yr at fi ndings, but illustrates the need to continue this simplify the crustal structure at SGP relates to Cajon Pass on the northwest and 12–22 mm/yr work as well as geologic slip-rate studies. For the limits of computing capacity and the dif- at Indio on the southeast (Weldon and Sieh, example, it would be interesting to take Dair fi culty in using dipping fault grids. Dair and 1985; Yule and Sieh, 2003; Meade and Hager, and Cooke’s study a step further and analyze a © 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2009; 2009 v. 37; no. 2; p. 191–192; doi: 10.1130/focus022009.1. 191 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/37/2/191/3941948/i0091-7613-37-2-191.pdf by guest on 29 September 2021 dipping fault model that incorporates all of the to consider using this method to evaluate sce- Matti, J.C., Morton, D.M., and Cox, B.F., 1985, active faults in the SGP region (Fig. 1). nario earthquakes. For example, by studying Distribution and geologic relations of fault sys- tems in the vicinity of the central Transverse One overarching objective of the research the available active fault maps (e.g., CFM), Ranges, Southern California: U.S. Geological in the SGP region is to answer a long-standing one could test whether or not scenario ruptures Survey Open-File Report 85–365, 23 p. question: is a throughgoing San Andreas rupture violate the 3-4 km limit set by historical data, Meade, B.J., and Hager, B.H., 2005, Block models possible? A defi nitive answer to this question either by rupturing across a >3-4 km step or by of crustal motion in Southern California con- has fundamental implications for forecast- stopping at a < 3-4 km step. At SGP, though dis- strained by GPS measurements: Journal of Geophysical Research, v. 110, B03403, doi: ing the earthquake hazard in the Los Angeles connected, the active faults shown in Figure 1 10.1029/2004JB003209. region. If “yes,” the region can expect relatively reveal no step larger than 3 km. Assuming that Olsen, K.B., Day, S.M., Minster, J.B., Cui, Y., Chourasia , infrequent but very large earthquakes (~Mw 7.8) slip occurs on all active structures in the SGP, A., Faerman, M., Moore, R., Maechling, P., and that rupture through SGP and simultaneously the Wesnousky (2008) model therefore supports Jordan, T., 2006, Strong shaking in Los Angeles expected from southern San Andreas earth- involve the Coachella Valley, San Bernardino, the Mw 7.8 ShakeOut scenario of a San Andreas quake: Geophysical Research Letters, v. 33, and the Mojave segments. If “no,” the region fault earthquake rupturing through the SGP. L07305, doi: 10.1029/2005GL025472.