Mapping offshore portions of the Khlong Marui and Ranong faults in Thailand: Implications for seismic hazards in the Thai peninsula

Thailand experiences Mw < 6.5 earthquakes, but the frequency of these earthquakes is considerably less within Thailand than at plate boundaries. Faults in Thailand that are potentially active, but have not historically hosted a large earthquake pose an unknown seismic hazard. Two such faults are the Khlong Marui and Ranong faults, which are left lateral strike-slip faults that strike northeast across the Thai peninsula and have been assumed to continue into the Andaman Sea. The Ranong and Khlong Marui fault zones have clear surface expression onshore, but their offshore extent is unknown. An estimated 100 km of sinistral displacement has occurred in the last 52 million years on the Ranong fault zone and the Khlong Marui fault zone is assumed to be similar (Watkinson et al., 2008; Kornsawan and Morley, 2002). Five Mw < 4.5 earthquakes have occurred near the inferred offshore extension of the Ranong and Khlong Marui faults since 2005. However, the maximum earthquake magnitude possible and recurrence interval of events on these faults is unconstrained, leaving southern Thailand unprepared for a Mw < 6 earthquake. To constrain the location of offshore portion of these two faults we performed a marine seismic reflection survey in the Andaman Sea, and construct an offshore fault map. Additionally, we are working to resolve the depth extent of displacement associated with faulting in the seismic data to constrain the timing of fault motion. Using empirical scaling between fault area and earthquake size we will be able to estimate a maximum earthquake magnitude for the Ranong and Khlong Marui faults. This will provide additional information to help southern Thailand prepare for potential seismic events. Kornsawan, A., & Morley, C. K. (2002). The origin and evolution of complex transfer zones (graben shifts) in conjugate fault systems around the Funan Field, Pattani Basin, Gulf of Thailand. Journal of Structural Geology, 24(3), 435-449. http://doi.org/10.1016/S0191- 8141(01)00080-3 Watkinson, I., Elders, C., & Hall, R. (2008). The kinematic history of the Khlong Marui and Ranong Faults, southern Thailand. Journal of Structural Geology, 30, 1554-1571. http://doi.org/10.1016/j.jsg.2008.09.001