SCEC Award Number 12137 View PDF
Proposal Category Individual Proposal (Special Fault Study Area)
Proposal Title Testing a step-over model of the southern San Andreas fault at Durmid Hill
Investigator(s)
Name Organization
Susanne Janecke Utah State University
Other Participants Graduate student, one
SCEC Priorities 4a, 4b, 4c SCEC Groups Geology, USR, SoSAFE
Report Due Date 03/15/2014 Date Report Submitted 11/15/2017
Project Abstract
Abstract
Three parts: Original proposal (2011), provisional report (2013), and a final summary in the form of the MS thesis of Daniel Markowski (2016). The abstract is the abstract of Janecke's preliminary report except that I updated the name of the fault to East Shoreline strand of the SAF. A different "Shoreline fault" is already in the Quaternary fault and fold database. See Markowski (2016) and Janecke et al. 2016 for additional updates.

Abstract (2013):

Final Report for 2012 SCEC Grant to Susanne Ursula Janecke

Testing a step-over model of the southern San Andreas fault at Durmid Hill
Susanne U. Janecke, Utah State University

Main findings:

1. The southernmost San Andreas fault is experiencing contraction and strike slip strains and is therefore unlikely to be a future nucleation point for the next “BIG ONE”.

2. The southern San Andreas fault has an additional, active strand that is near the shoreline of the Salton Sea. The San Andreas fault zone is thus at least 3 km wide in this area.

3. This EAST SHORELINE strand of the San Andreas fault (ESSSAF) was active in the Holocene, and may be continuous with an ~30 km-long northwest-striking deformation boundary detected in many INSAR datasets in the urban centers of Palm Springs, Palm Desert, Cathedral City, Rancho Mirage, and La Quinta (Martin, 2011; Tong et al., 2011).

4. There is little evidence for interactions with the NNE-striking left-oblique Extra fault zone on the northeast shore of the Salton Sea.
Intellectual Merit Phase I of our integrated field and structural project tested the sawtooth (Bilham and Williams, 1985), cross-fault (Hudnut et al. 1989; Brothers et al., 2009, 2011), and contractional-stepover models of the southern tip of the SAF (Janecke, 2012 SCEC proposal). The stepover model is the only model that successfully explains the complex sigmoidal geometry of contractional strains in the Durmid Hill area and the Holocene faulting and folding along the northeast shore of the Salton Sea (Fig. 3). The focus of the next phase of our proposed research will shift to the SAF itself, in order to capitalize on exciting discoveries of phase one.
Results of SCEC funded research in phase I:
Mapping and structural analysis by Utah State University MS student Daniel Markowski and Janecke at Durmid Hill confirmed our working hypothesis of the presence of a contractional stepover between the main SAF and an active but previously unmapped dextral fault zone near the northeast shore of the Salton Sea (Janecke, 2012 SCEC proposal; Janecke and Markowski, 2013). We focused on the southwest side of the main strand of the SAF and the deformation near the shore of the Salton Sea (Fig. 3). We enumerate key results below:
Abundant evidence exists for a second active dextral strand of the San Andreas fault system ~3 km SW of the main trace of the SAF (Fig. 3). The central part of this newly identified Shoreline fault strand of the San Andreas fault system is probably offshore under the Salton Sea. It parallels the edge of the Salton Sea and likely produced a small step in the landscape along which the shoreline is localized.
Onshore deposits on the NE side of the Salton Sea provide field evidence for a wide and expansive damage zone of the Shoreline fault strand. Exposures include a 0.5-1.5 km wide band of highly deformed late Pleistocene Brawley and Ocotillo formations and unconformably overlying Holocene sediment striking NW, parallel to the fault zone. One major fault zone cuts Holocene sediment containing pumice clasts and is therefore likely to be a few thousand years old or younger (Schmitt et al. 2013a and b). Folded and faulted angular unconformities in the damage zone record active deformation during deposition of late Pleistocene to Holocene sediment.
Open to tight folds in the Brawley and Ocotillo Formations within the damage zone of the Shoreline and SAF zones have fold axes parallel to the fault zone and the shore (Babcock, 1974; Burgmann, 1991; Phase I of this research). These axes trend 30-50° clockwise of fold axes that lie between the two damage zones within the contractional stepover in the Durmid Hill area. Field tests detected few NE to NNE-striking sinistral faults in the Durmid Hill area (Fig. 3). Thus, the cross-fault models of Hudnut et al. (1989) and Brother et al. (2009, 2011) are unlikely to apply to the southern tip of the SAF. Contractional strains across the two strands of the SAF in the Durmid Hill area likely inhibit the nucleation of large ruptures there. Dan Markowski will explore the implications of the Shoreline strand of the SAF in his MS thesis and his defense is scheduled for the summer of 2014.
Some aspects of the “contractional-stepover model” of Janecke (SCEC proposal 2012) were incorrect or incomplete and will be investigated in Phase II of this work:
Instead of branching from the SAF north of Salt Creek, as predicted, the Shoreline fault diverges from the main SAF to the NW, and disappears northward beneath the Salton Sea. From that point, it may parallel the main trace of the SAF and lie a few kilometers SW of the main SAF in Coachella Valley. It is likely to correlate with fault strands detected geophysically by Fuis et al. (2013) and Langenheim et al. (2013) ~2.5 km SW of the SAF in the Mecca Hills. The fault might continue northward parallel to the SAF.
Our preferred interpretation, however, is that north of Mecca Hills, the ~10-15 km long Shoreline strand of the SAF strikes obliquely across the northern Coachella Valley with a N57°W strike and is the same structure as the 45 km long unnamed fault of Smeed and Brandt (2007), Tong et al., (2013), Wisely (2013), and Martin (2011) under Palm Desert, Rancho Mirage, Cathedral City, and Palm Springs along the front of the San Jacinto Mountains (Janecke and Markowski, 2013). InSAR subsidence patterns, barriers to groundwater flow, direct measurement of subsidence, and gravity data all document the unnamed fault under Palm Desert. The unnamed fault and the Shoreline fault strike toward each other across ~30 km of the valley floor in latest Holocene lakebeds, sand dunes, and fields.
Broader Impacts Societal relevance: This study shows that the San Andreas fault is significantly wider than mapped and that a major but unmapped strand probably cuts through the urban centers of Palm Springs, Palm Desert, and Cathedral City. We will publish the new structural map of the Durmid Hill area and show the hundreds of active Holocene faults in this complexly faulted part of the San Andreas fault zone.
Our work tests and disproves high profile hypotheses of Hudnut et al (1989) and Brothers et al. (2009 and 2011). We show that the likelihood of the next large earthquake on the southern San Andreas fault nucleating in the Durmid Hill area is much less than thought by those workers. Follow-on research is needed to validate our hypotheses.

Teaching:
The research completed to date on this project funded the MS thesis of Dan Markowski. Dan is completing his degree and expects to finish in the summer of 2014. The female PI, Janecke, advanced her research and broadened the scope of her work into a new study area and onto the San Andreas fault zone. Her prior research had focused on the slightly slower San Jacinto fault zone in the western Salton Trough.

Methodology: This project and a prior SCEC-funded study of the Extra fault zone provided a full on test of a new digital field mapping method and the Panasonic Toughbook. Digital field mapping proved to be challenging and we are exploring other ways to combine precise location data of the Toughbook with the ability to map on high resolution and geo-referenced imagery. The current approach is too cumbersome, difficult, and produced many computer crashes and resulted in frequent losses of data. A variety of additional mapping techniques were applied and refined in this effort.
Exemplary Figure Figure 3 Geologic and structural map of Durmid Hill on a basemap of processed false-color NAIP aerial orthophotography. Related structures are coded with the same color. En echelon faults of the Eastern California Shear zone are blue. The Shoreline strand of the San Andreas fault is yellow, and the main strand of the San Andreas fault is red. Its strongly sheared damage zone is almost entirely on its southwest side (also red). Notice how the false color images allow marker units and fault zones to be identified. The Salton Sea is black in the west and south.