SCEC4: New Slip Rate estimates from Wallace Creek and Phelan Creek Paleoseismic Sites Re-sampling Re-dating and Re-Synthesizing

SCEC4: New Slip Rate estimates from Wallace Creek and Phelan Creek Paleoseismic Sites Re-sampling Re-dating and Re-Synthesizing New Slip Rate estimates from Wallace Creek and Phelan Creek Paleoseismic Sites. Re-sampling, Re-dating and Re-Synthesizing In 2012 we propose to bring this work to completion and establish a better constrained geological slip rate record for the Carrizo section of the SAF. (1) Continue to synthesize data collected by Sims and colleagues, and write up the study for publication as a USGS open file report or BSSA manuscript (led by ASU). (2) Excavate a new trench between Sieh and Jahns original trenches, to collect new organic samples from channel deposits and confirm original stratigraphic correlations which underpin the slip rate at Wallace Creek (UCI& ASU). The trench will be sited NW of the intersection of WC-10 and WC-2 (see Fig 1) in a critical area for correlation of channel stratigraphy. The exact location of the trench may be limited by burrows of threatened/endangered species at the site. We will work with BLM to gain as much access as possible. (3) Write up results from Wallace Creek for publication (led by UCI) Examining the cause of significant ground deformation between 780 and 1031 A.D. at the Dry Lake Valley Paleoseismic Site: Do large earthquakes rupture In our 2012 SCEC proposal, we asked the question: Does the central creeping section of the San Andreas Fault (SAF) rupture with the northern or southern SAF or even the northern SAF? To address that question, we conducted paleoseismic field work along the heart of the creeping section, establishing the Dry Lake Valley paleoseismic site (DLV site; Fig. 1; Toke et al., 2012). This site is a fault bounded sag depression within an alluvial fan complex that slopes SW across the SAF. The sag is ~40 m wide and ~250 m long. It is steep sided with a 6-9 m high scarp on the NE side and a 2-3 m high scarp on the SW side. In 2012 wWe excavated cut a 45.546 m long fault-perpendicular trench across the sag. Our excavations revealed an intriguing stratigraphic relationship in which a poorly sorted gravel package (circa 600 A.D.; Fig. 2), sourced from a nearby paleochannel (Fig. 1), is arranged sub-vertically within the westernmost fault zone. The fault zone gravel package appears to be overlain by nearly horizontal stratigraphy (Fig. 2). This arrangement of up to boulder-sized clasts within the fault zone is difficult to explain with only fault creep. However, despite the fact that this evidence would certainly be interpreted as a paleoearthquake along a non-creeping fault, it remains challenging to discount aseismic creep as the responsible mechanism because it overprints all paleoseismic evidence along this portion of the fault. Refining the south-central San Andreas Fault slip rate at the 6 ka timescale: Phelan Creeks Technical Description Introduction The south-central San Andreas Fault (SAFthe portion between Parkfield and the Big Bend) is relatively simple geometrically and has the highest slip rate in California (>3 cm/yr over the millennial timescale). The fault slip rates are an essential input for seismic hazard (e.g., UCERF3 Plan, 2012) and their comparison with the velocity gradient across the SAF from decadal timescale geodetic measurements (30-37 mm/yr; Schmalzle, et al., 2006) suggest the apparent steadiness of earthquake-modulated strain accumulation and release. However, the geologic rate is defined by just a few published slip rate investigations in the Carrizo Plain (Wallace Creek at 33.92.9 mm/yr measured by Sieh and Jahns, 1984; and VMR at 29.3-35.6 mm/yr measured by Noriega, et al. 2006). Our own recently SCEC-funded study of the Wallace Creek site is yielding a preliminary slip rate of approximately 3.7 cm/yr (Figures 1 and 2). Sieh and Jahns (1984) landmark study at Wallace Creek determined two slip rates over different time scales (Figure 2). The longer one examined a 475 m offset of young alluvial fan material derived from across the SAF scarp as it began to rise at 13,2501,650 yrs ago. This offset-age pair yielded a rate of 35.8 +5.4/-4.1 mm/yr. The smaller 128 m offset of the active Wallace Creek channel over the last 3780 yrs yielded a slip rate of 33.9 2.9 mm/yr. With SCEC support over the last two years, we revisited the age of the smaller offset by excavating portions of the channel and collecting/analyzing many more 14C dates than the original study. Our results are similar to Sieh and Jahns (1984) conclusion that channel offset was initiated sometime after about 3500 BP (see below). Our confidence in the 3500-3700 year slip rate for the south-central SAF is now strengthened, but how steady is this rate at the millennial time scale? Given the similarity with the geodetically implied rate, we hypothesize that the simplicity of the SAF geometry between Parkfield and the Big Bend promotes a long term rate that is constant and not sensitive to along fault rate fluctuations or interactions with nearby structures. Recent preliminary work along the Mojave segment of the SAF (Compton, et al., 2012) has shown that the millennial scale slip rate there may vary by more than a factor of 2. This surprising result derived from a compilation of various dated offsets suggests that a further constraint for the SAF slip rate in the Carrizo Plain is warranted. We propose to determine the age of a beheaded channel which has been offset about 230 m at the Phelan Creeks site thus filling in a gap in the age-offset constraints of millennial scale fault slip along the south central SAF in the Carrizo Plain.