INTEGRATING TECTONIC GEOMORPHOLOGY WITH SEISMIC HAZARDS ON THE MAE CHAN FAULT, NORTHERN THAILAND: DECIPHERING LAND SURFACE CHANGE FROM TECTONICS AND HUMAN ACTIVITY

Characterizing active faults in Southeast Asia is critical to determine their contributions to regional fault kinematics and associated seismic hazards in a densely populated region of the world. Here we quantify slip rates and earthquake potential of the Mae Chan fault, Northern Thailand. The >150 km-long Mae Chan fault is part of a group of EW-NE-striking left-lateral strike-slip faults at the southern edge of the Shan Plateau. Moderate earthquakes (<Mw 6.3) have occurred on the Mae Chan fault but with no evidence of surface faulting. In contrast, historical records suggest a large earthquake occurred between 454-554 AD that devastated the former Lan Na Kingdom in Northern Thailand. To constrain the seismotectonics of the Mae Chan fault and evaluate its potential source for large earthquakes, we combine new datasets from high-resolution topographic data, fault mapping, geochronology, slip rates, and paleoseismic data. Offset Pleistocene-Holocene alluvial fans and channel deposits at four sites constrain late Holocene faulting using ¹⁰Be surface exposures, OSL, and ¹⁴C dating techniques. Pre-1980s aerial photographs, ALOS data, drone surveys, and DEMs document major anthropogenic land surface changes corrected for surface elevation and slip measurements. Best-fit slip rate estimates average between 1-1.5 mm/yr. Geologic slip rates coincide with average geodetic modelled velocities of ~1 mm/yr on individual faults across the Shan Plateau, but is lower than inferred long-term slip rates of up to 4 mm/yr if the fault initiated ~10 Ma. Paleoseismic studies on the Mae Chan fault document at least 2 events in the last 3-4 kyrs (Weerachat et al., this meeting). Fault scaling relationships, geomorphic and paleoseismic trench relations suggest the Mae Chan fault is capable of generating an earthquake of ~Mw 7 with 4-5 m slip-per event at the millennial timescale recurrence interval. The paucity of surface earthquakes in the last ~1500 yrs and a crustal loading of 1-1.5 mm/yr imply that the Mae Chan fault is primed to relieve its interseismic strain accumulation via a similar large earthquake. Lan Na Kingdom’s past destruction is a cautionary tale for modern day Chiang Mai and Chiang Rai large populations with major infrastructures at risk of an earthquake similar in size to those recorded in the geologic and historic records.