Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

270 PATA Days 2024 Soil pits were excavated to a depth of 2 m on the footwall surfaces of two displaced intermediate age (Qfi) alluvial fans (Figs. 3 and 4). Characteristics of individual soil horizons exposed in the pits were described including color, structure, consistence, and texture. The degree of carbonate accumulation (CaCo 3 stage) was assessed to evaluate the relative age of surface abandonment. Samples were collected at 25 cm intervals from each pit for 36 Cl depth profile analysis (in progress) to assess the absolute age of the soil/surface. The soil exposed in Soil Pit 1 is characterized by a 20- cm-thick sandy clay loam textural Bt horizon and an 80-cm-thick loamy sand Bk horizon with stage II+ to III carbonate development. The soil exposed in Soil Pit 2 is characterized by a 20-cm-thick Bt horizon with a silty clay loam texture and a 45-cm-thick silty clay to clay Btk horizon with Stage II+ carbonate development. Both pits have relatively thick textural Bt and Bk horizons suggesting relative antiquity, however the soil in Soil Pit 2 has slightly less carbonate development. Based on the degree of carbonate development and comparison to regional datasets (e.g. Machette et al., 1985) we infer that the surfaces were abandoned and soil began to form about 100- 200 ka. 36 Cl results from the collected soil samples are anticipated in summer 2024 and should place tighter constrains on the age of the surfaces. Topographic profiles were extracted from lidar point cloud data orthogonal to fault scarps that displace Qfo and Qfi surfaces and used to measure vertical separation across the scarps (Figs. 3 and 5). Profile 1 across the scarp in the vicinity of Soil Pit 1 indicates a vertical separation of 4 m of the Qfi surface. Profiles 2 and 3 extend across multiple sub parallel scarps that displace a Qfo surface and indicate a cumulative vertical separation of 21 m. Profiles 4 and 5 extend across two scarps in a Qfi surface near Soil Pit 2 and indicate a cumulative vertical separation of 6.3 m. The cumulative displacements are considered a minimum because an unknown amount of displacement may be accommodated along the range front trace and subtle scarps outside the limits of the profiles may exist. We calculate a broadly constrained late Pleistocene slip rate for the Buffalo Valley fault by dividing the cumulative displacement of the Qfi surface at Soil Pit 2 (6.3 m) by the relative age of the faulted surface (100-200 ka). The resulting vertical slip rate is 0.03-0.06 mm/yr which is consistent with other faults with similar geomorphic expression in the region (Koehler and Wesnousky, 2011). Given the Stage II+ carbonate development in Soil Pit 2 and regional assessments of fan deposition and soil development (Koehler and Wesnousky, 2011) it is possible that the Qfi surface at Soil Pit 2 was deposited somewhat later than the carbonate development suggests, possibly during Marine Isotope Stage IV ~71 ka. If true, the slip rate could be faster around 0.09 mm/ yr approaching the value of 0.1 mm/yr used in the US NSHM (Hatem et al, 2022a). Our slip rate estimate will be refined once the 36 Cl depth profile results are obtained.