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

Fig. 2: Photographs of trench cross-sections. (a) North side of the trench_A. Multiple layers of fluvial sediments indicative of a steam- dominant alluvial fan are present, with no evidence of paleo earthquakes observed. (b) South side of trench_B (in Fig. 1c). Unconsolidated sediment layers have been deformed by faulting. ERT survey performed perpendicular to the estimated paleo surface rupture trace revealed a zone of sharp resistivity contrasts in the subsurface. However, seismic reflection surveys did not detect any clear anomalies. Prior to trenching, three boreholes were drilled east-west direction. While sedimentary basement rock was encountered at about 15 m in the outer holes, the central borehole reached bedrock at about 10 m. Based on drilling, physical surveys, and topography, an initial excavation (Fig. 2a; Trench_A) on the T4 surface revealed repetitive layers of fluvial gravel and sandy sediments. However, no evidence of paleoseismic surface ruptures was found. Approximately 680 m north along the lineament, a second trench (Fig. 2b; Trench_B) was excavated. This trench clearly revealed evidence of surface ruptures. West of the rupture plane, a cobble layer with an erosional boundary above a soil wedge developed sandy and clayey sediments. These layers are all tilted approximately 30-40º degrees to the west. On the east, well-sorted gravel layers capped by sand and clay are present, all showing deformation. Several minor faults, colluvial wedge (CW), and fissure-filling features related to earthquake events were observed within the sedimentary layers on the eastern side. D I S C U S S I O N A N D F U R T H E R S T U D Y Event numbers of paleo earthquakes At Trench_B, the observation of minor normal faults, CW, and fissure-filling features within the layers, allows for the estimation of the number of earthquake events. The interpretation of these features as colluvial wedges (CW) is based on the presence of thick, coarse particles near the fault, which become finer and exhibit a pinch-out geometry further from the fault. If the sediment layer is identified as CW, there was one earthquake event that caused the formation of the CW, followed by a second earthquake evidenced by fissures developing in the gravel layer deposited after