Ayush Joshi / Earth & Environmental Sciences / Faculty Mentor: Nathan Brown
The San Andreas Fault (SAF) in southern California exhibits a multistranded configuration, with slip partitioned between northern and southern pathways, each consisting of a network of fault strands. Determining which pathway accommodates slip during large earthquakes is critical for seismic hazard assessment and requires constraints on the late Pleistocene to Holocene activity of individual faults. The Mission Creek Fault (MsCF), a key strand of the northern pathway, has been proposed as a major slip carrier, but its Holocene activity northwest of Pushawalla Canyon remains debated due to conflicting geomorphic evidence and a lack of geochronological constraints on its rupture history. This study presents new depositional ages from two sites near Mission Creek Stonehouse, northwest of Desert Hot Springs, to assess the Holocene activity of the MsCF. At Mission Creek Valley, single-grain post-infrared infrared-stimulated luminescence (pIR-IRSL) dating of alluvium overlying the fault trace yields a mean depositional age of 0.80 ± 0.04 ka, with a maximum depositional age of ~2.5 ka, contradicting previously inferred ages of >3–18 ka. A comparison of these luminescence ages with cosmogenic 10Be abandonment ages from alluvial surfaces across the Mission Creek catchment reveals widespread sediment reworking, including a potential major reworking event at ~30 ka affecting surfaces Q1b and Q2b. The young alluvium ages and evidence for catchment-wide sediment recycling indicate that the valley is undergoing rapid resurfacing, which would erase rupture signatures unless they are a few hundred years old. To evaluate slip along the MsCF, we analysed two offset streams crossing the fault’s southern splay atop the San Gorgonio Overlook fan. Alluvium was sampled upstream and downstream of the fault trace to estimate the timing of deposition before offset accumulation, while additional samples from the host surface were used to constrain the maximum possible age of offset initiation. However, steep downstream gradients and terrace degradation prevented the preservation of alluvium along the pre-earthquake streamflow direction, precluding a slip rate determination. While our findings do not provide definitive evidence of Holocene rupture along the MsCF, they also do not support its inactivity. The absence of rupture signatures may be attributed to rapid sediment reworking rather than a lack of fault movement. These results highlight the need for further geochronological and geomorphic investigations along regional fault strands to fully assess the role of the MsCF and the northern pathway in accommodating slip.
Keywords: Precision Agroecology, Environmental Policy Optimization, Data-Driven Agricultural Analytics.
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