EVIDENCE OF LARGE PALEOEARTHQUAKES GENERATED BY THE MAE CHAN FAULT, CHIANG RAI, NORTHERN THAILAND

We applied remote sensing, aerial photographic and paleoseismologic techniques to study the NE-SW trending Mae Chan Fault, located in Chiang Rai, northern Thailand. The main morphotectonic landforms associated with the Mae Chan Fault are fault scarps, offset streams, linear valleys, offset ridge crests, beheaded streams, hot springs, and linear mountain fronts. Data from trench sites along the Kio Sa Tai segment of the Mae Chan Fault, at Ban Pong Pa Kham (this study), Bang Pong Khom (Weldon et al., 2016), those of Kosuwan et al. (2002) and Department of Mineral Resources (2009) were used to analyse the history of fault movement in the area. This study revealed young Quaternary sediments containing evidence for at least eight ground-rupturing earthquakes along the fault. Based on C14 AMS, OSL and TL ages, preliminary approximate ages of the paleoearthquake events are: (1) 20,000 yr BP; (2) 10,000 yr BP; (3) 8,000 yr BP; (4) 7,000 yr BP; (5) 4,000 yr BP; (6) 3,000 yr BP; (7) 1,500 yr BP and (8) 900 yr BP. It is possible that the recurrence interval of seismic events on the Mae Chan Fault is 1,000-3,000 years. Terraces and debris flows offset from a few meters to a kilometer, dated largely by cosmogenic isotopic techniques, yield a slip rate of ~1.4 mm/yr. We also estimate paleoearthquake magnitude (or Maximum Credible Earthquake: MCE, using method of Wells and Coppersmith (1994)) generated by the Mae Chan Fault, to be Mw 6.9. Thus, we concluded that the Mae Chan Fault is active; these data suggest that this fault is the most active fault in Thailand, and that the Mae Chan area and vicinity will be subject to future moderate to large earthquakes.