Character and Implications of a Newly Identified Creeping Strand of the San Andreas fault NE of Salton Sea, Southern California
Susanne U. Janecke, & Daniel MarkowskiAccepted 2015, SCEC Contribution #7083
Detailed mapping and structural analysis at the southern SAF tip provide insight into the Coachella section of the San Andreas fault (SAF), the site of the model earthquake used in the ShakeOut exercise in southern California, and the conflict between cross-fault and pull-apart models involving the Extra fault array and shortening in Durmid Hill. Geologic mapping, LiDAR, seismic reflection, magnetic and gravity datasets, and NIR aerial photography reveal the presence of an additional, previously unknown East Shoreline strand (ESS) of the SAF, that is ~0.5 to ~2.5 km southwest of the main trace. Groups of strike-slip cross-faults connect these two master dextral faults of the SAF. There is no evidence of the sinistral-normal faults of the Extra fault array near the main strand of the SAF. The ESS cuts and folds upper Holocene beds and appears to creep, based on discovery of ~150 m long NW-striking cracks in modern beach deposits. We mapped ~15 km of the ESS in a band on the northeast side of the Salton Sea between Bombay Beach and Salt Creek. Other data indicate that the ESS continues N to the latitude of the Mecca Hills, and is >35 km long. The 1-km wide ESS contains short, discontinuous traces of NW-striking dextral-oblique faults. These en-echelon faults bound steeply dipping Pleistocene beds, cut out section, parallel NW-trending folds, and produced growth strata. Beds dip toward the ESS on both sides and reflect the component of NE-SW shortening across the ESS. The dispersed fault-fold style of the ESS is due to decollements in mud-rich sediment and ramps and flats o the strike-slip faults. A sheared ladder-like geometric model of the two master dextral strands of the SAF and their intervening cross-faults best explains all the datasets. Contraction across >40 km2 of the southernmost SAF zone in Durmid Hill suggest that contractional interactions among active structures in the SAF zone may inhibit the nucleation of large earthquakes in this region. This conclusion conflicts with the cross-fault and pull-apart geometric models. The ESS may cross Coachella Valley to join the blind Palm Spring dextral fault. The ESS may also continue north along the northeast margin of the Salton Trough or have both a NW and NE branch. The risk of a future large earthquake directly beneath the greater Palm Springs metropolitan area may be larger if the ESS and the Palm Spring fault are a single active structure. Further work will explore this possibility.
Citation
Janecke, S. U., & Markowski, D. (2015). Character and Implications of a Newly Identified Creeping Strand of the San Andreas fault NE of Salton Sea, Southern California. Poster Presentation at 2015 Southern California Earthquake Center Annual Meeting. http://scecinfo.usc.edu/core/cis/2015am/view_abstract.php
Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)
