Sensitivity of uplift patterns to dip of the San Andreas fault in the Coachella Valley, California

Abstract

Three-dimensional mechanical simulations of the San Andreas fault system within the Coachella Valley in southern California produce deformation that matches geologic observations and demonstrate the firstorder impact of fault geometry on uplift patterns. To date, most models that include the Coachella Valley segment of the San Andreas fault have assumed a vertical orientation for this fault, but recent studies of seismicity and geodetically observed strain suggest that this segment of the fault may dip 60°-70° to the northeast. We compare models with varied geometry along this segment of the fault and evaluate how well they reproduce observed uplift patterns in the Mecca Hills and Coachella Valley. Incorporating wellconstrained fault geometry in regional models will provide a more accurate understanding of active faulting in southern California, which is critical for rupture and hazard modeling that is used to identify regions most susceptible to earthquake damage.We have tested three boundary-element method models for the active geometry of the Coachella Valley segment of the San Andreas fault: one contains a vertical Coachella segment, the second contains a northeast ~65° dipping Coachella segment, and the fi nal alternative contains a vertical Coachella segment plus a subparallel northeast-dipping fault at depth. This fi nal model honors the geometric interpretation of seismicity from the Southern California Earthquake Center Community Fault Model version 4.0. The models containing vertical Coachella Valley segments both produce uplift between the San Andreas and San Jacinto faults that is more uniformly distributed than geologic observations suggest, and these models fail to produce uplift in the Mecca Hills. The dipping model produces tilting of the Coachella Valley consistent with geologic observations of tilting between the San Jacinto and San Andreas faults. The dipping model also produces relative subsidence southwest of the fault and localized uplift in the Mecca Hills that better match the geologic observations. These results suggest that the active Coachella Valley segment of the San Andreas fault dips 60°-70° to the northeast.

Crustal-scale tilting of the central Salton block, southern California

“…Other studies suggest that the active Coachella segment of the SAF dips 60°-70° northeast based on seismicity patterns, aeromagnetic data, and modern strain documented with GPS and interferometric synthetic aperture radar (Lin et al, 2007;Fuis et al, 2012a;Lin, 2013;Lindsey and Fialko, 2013). The hypothesis for a steeply northeast-dipping SAF in this area is supported by a recent 3-D boundary-element modeling study that tested alternative fault geometries using comparison of model results to geologic evidence for vertical surface displacements around the Coachella Valley (Fattaruso et al, 2014). Debate over the dip on the southern SAF highlights the need for improved understanding of vertical motions in strike-slip fault Dorsey and Langenheim | Tilting of the Salton block, southern California GEOSPHERE | Volume 11 | Number 5 zones over geologic time scales (~1-2 m.y.…”

“…1, Line 7; Kell et al, 2012;Fuis et al, 2012bFuis et al, , 2012c; see following discussion). A steep northeast dip on the SAF is suggested by seismic, magnetic, geodetic, and modeling studies (Lin et al, 2007;Fuis et al, 2012a;Langenheim et al, 2012;Lindsey and Fialko, 2013;Fattaruso et al, 2014) and is used in our cross section (Fig. 10).…”

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

“…1). To reproduce observed patterns of vertical deformation and fault strain rates in the Mecca Hills and Indio Hills, on the northeast side of the Coachella Valley, and in the Santa Rosa block of the Peninsular Ranges on the southwest side of the Coachella Valley, Fattaruso et al (2014) used boundary-element modeling to test various geometries for the SAF and San Jacinto fault (Figs. 1 and 2a).…”

“…Fuis et al (2012) modeled a northeast-dipping SAF from the Salton Sea to the Mojave Desert. Lindsey and Fialko (2013) modeled a northeast-dipping SAF from Indio to the Salton Sea, and Fattaruso et al (2014) modeled a northeast-dipping SAF from San Gorgonio Pass to the Salton Sea. Model dips in all cases are in the 60°-70°range.…”

“…12b). As Fattaruso et al (2014) point out, however, a slight mismatch in the strike of the SAF and plate-motion directions in the Coachella Valley gives rise to the compression that produces the Mecca and Indio Hills.…”