The East Shoreline strand of the San Andreas Fault and its implications for the next Big One in southern California

Susanne U. Janecke, Daniel Markowski, Roger Bilham, James P. Evans, Michael Bunds, Jack Wells, Jeremy Andreini, & Robert Quinn

Published August 24, 2016, SCEC Contribution #6629, 2016 SCEC Annual Meeting Poster #116

The southern San Andreas Fault last ruptured about 1670 AD, and its southern tip could nucleate the next Big One (Shakeout summary). We use new geologic data along a 15-km long swath at the southernmost tip of the San Andreas Fault zone (SAFZ) to determine the presence of a new important fault strand and to evaluate its earthquake potential. Geologic mapping, structural and stratigraphic studies, interpretation of LiDAR, false and natural color aerial photography, published InSAR interferograms from several time intervals, measurements from creepmeters, published reflection and refraction seismic, and gravity data, and preliminary data from high-resolution aerial imagery and derivative digital elevation models acquired by unmanned aerial vehicle, all document the presence of a second significant strand of the San Andreas Fault zone. We named this strand of the SAFZ the East Shoreline strand of the SAF (ESSSAF) for its position along the northeast shore of the Salton Sea, 1 to 3 km southwest of the main trace of the SAF.

Seven key data sets provide evidence for the existence of the East Shoreline strand of the San Andreas Fault zone. These are: Mapping of many tens of steeply to moderately dipping, northwest-striking dextral faults within the East Shoreline fault zone; Miocene (?) to Pleistocene stratigraphic units that are cut out by these dextral faults; Monoclines, synclines and anticlines that parallel dextral strike-slip faults in the ESSSAF for significant distances; The dozens of left- and right -lateral cross faults that connect between the ESSSAF and the main strand of the SAF and form a coordinated ladder-like fault mesh; Many of the cross faults change their strike toward parallelism near the two master dextral faults and display a symmetric sigmoidal geometry; There are east-striking left-lateral strike-slip faults between the ESSSAF and the main strand of the SAF, that rotated about 45° clockwise in a bookshelf manner between two dextral strands of the SAF since their inception in the Pleistocene, and a close correspondence between the mapped East Shoreline zone and deformation on InSAR scenes.

The ESSSAF is an active dextral-reverse structure that probably creeps at shallow crustal levels. Analysis of several sets of InSAR interferograms show that ~7 ± 2 mm/yr of right-lateral creep and a few mm/yr of southwest-down vertical motion accumulated across the ESSSAF and the inactive faults of the northern Salton Sea between 2003 and 2010. Most of this deformation likely accumulated across the ESSSAF because published reflection seismic studies identified mostly homoclinally dipping beds in the northern Salton Sea. Pleistocene, Holocene, and modern sediment are folded and faulted within the fault zones, and older units are more deformed than younger ones. There are linear creep-related fractures, drought-enhanced fractures and aligned and elongate desert sinks (FDEFS) along strike of one another in modern beach deposits and on the floors of washes continuously along a 7 km long stretch of the ESSSAF. Spot checking and aerial imagery reveal more discontinuous linear fractures along ~ 18 km of the fault zone. The main strand of the San Andreas Fault and its immediate damage zone also has FDEFS, they closely resemble those within the ESSSAF, and formed in well known as well as many newly identified faults in the southern half of Durmid Hill. Fault-related structures within the ESSSAF formed progressively and show less deformation in the upper Brawley Formation than in older Brawley Formation across angular unconformities. Together the main and East Shoreline strands of the San Andreas Fault form a coordinated sheared ladder-like fault zone in map view, and bound dozens of left- and right-lateral cross faults and folds between them. This active fault mesh resembles the Brawley seismic zone immediately to the south, except that the Durmid ladder structure of the SAFZ is a transpressional dextral-reverse fault zone with internal uplift, whereas the Brawley seismic zone is transtensional belt with internal subsidence. From the Salton Sea northward, a part of the ESSSAF may cross the Coachella Valley to Palm Springs and San Gorgonio Pass, as we proposed in 2013. A NW branch may become the blind (?) Palm Springs dextral-reverse fault beneath the greater Palm Springs metropolitan area.

Durmid Hill was severely shaken by the El Major Earthquake in April 2010, and this shaking changed the prevailing slip regime, creep rates and locally even the slip sense at the Ferrum, Salt Creek and Durmid creepmeters. For three years another strand of the SAFZ became active, producing highly unusual creep meter records along its main strand. Analysis of the data is ongoing, but initial analysis suggests that creep along the East Shoreline strand may partly explain these changes.

Contraction across strike-slip fault zones tends to inhibit the nucleation of large earthquakes in favor of more neutral or extensional stretches. There is much contraction in the Durmid Hill ladder structure, and neutral to mildly extensional zones farther north along the SAF may be more likely to nucleate future large earthquakes than the southern tip of the fault zone.

The northeast branch of the ESSSAF spans at least 48 km between Bombay Beach and the latitude of Thermal Canyon, CA. This is constrained by published seismic data of Fuis et al. (2012), bathymetry, published InSAR data, fault scarps, and a 14 km long belt of uplifted Pleistocene basin fill on the southwest side of the main strand of the San Andreas fault in the Mecca Hills. From Salt Creek to the latitude of Coachella the ESSSAF accommodated as much as 3-7 mm/yr of right lateral creep between 2006 and 2010 according to the InSAR data of Lindsey et al (2014), as long as the ESSSAF is the only active fault zone on the floor of Coachella Valley.

Key Words
Damage zone, creep, fault structure,

Citation
Janecke, S. U., Markowski, D., Bilham, R., Evans, J. P., Bunds, M., Wells, J., Andreini, J., & Quinn, R. (2016, 08). The East Shoreline strand of the San Andreas Fault and its implications for the next Big One in southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)