Assessing stratigraphic correlations and fault zone extent at the 18th Ave trench site, Banning strand of the San Andreas Fault, North Palm Springs, California

James C. McNeil, Doug Yule, Katherine M. Scharer, Sally F. McGill, Devin McPhillips, Bryan A. Castillo, & Alan Pace

Submitted August 15, 2017, SCEC Contribution #7761, 2017 SCEC Annual Meeting Poster #160

A trench opened in March, 2017 by Petra Geoscience, Inc., provided a valuable opportunity to examine the previously unconstrained earthquake record of Banning strand of the San Andreas Fault (also see Castillo et al., this meeting). The initial trench exposures at the 18th Avenue site (33.9172°, -116.538°) revealed a 10-m-wide fault zone with a northward-thickening stratigraphic section consistent with deposition in a small transtensional stepover basin along the fault. However, the initial trench exposures did not uncover the full width of the of the fault zone. Further, it was difficult to make unambiguous stratigraphic correlations across the 20-m-wide trench and thus we had uncertainties in the relative stratigraphic position of paleoearthquakes on each wall. We excavated additional small trenches in June 2017 to try and improve confidence in the stratigraphic and paleoearthquake correlations. We extended the walls on both sides of the trench to the north by ~10 m and dug an east-west connector trench across the north end. These new exposures reveal additional fault splays and northward-thinning stratigraphic units, thus constraining the northern limit of the stepover basin. The formation of this basin provides the key to identifying paleoearthquakes at this site because the basin contains numerous fault splays that displace a ~6,000 yrs BP and ~10-m-thick sedimentary section with growth strata. Gravel-sand packages, capped by muddy, charcoal-rich layers, likely record flood events that filled the stepover after earthquakes lowered the basin floor in the stepover. On the flanks of the basin, the same strata are thinner and comprise <2 m of section. Knowing the extent of the stepover provides greater confidence that we have uncovered the full width of the fault zone and likely have a complete paleoseismic record at this site. The new exposures reveal clear evidence for three paleoearthquakes and possible evidence for an additional paleoearthquake. These four events may correlate with four of the eight events (five likely and three possible) detailed by Castillo et al. (this meeting). The stratigraphic correlations and evidence for paleoearthquakes documented in the June trench exposures therefore supplement and strengthen the event record at the 18th Avenue paleoseismic site.

Key Words
Banning Strand San Andreas Fault, Paleoseismology

McNeil, J. C., Yule, D., Scharer, K. M., McGill, S. F., McPhillips, D., Castillo, B. A., & Pace, A. (2017, 08). Assessing stratigraphic correlations and fault zone extent at the 18th Ave trench site, Banning strand of the San Andreas Fault, North Palm Springs, California. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)