Paleoearthquake investigation along the Ranong Fault Zone, Souhtern Thailand

Abstract

Investigation along the Ranong Fault Zone (RNF) in southern Thailand is aimed at identifying its characteristics, locating active faults, identifying numbers of earthquake faulting, and determining paleoearthquake magnitudes and slip rates using remote sensing and field survey, enhanced seismic data, dating data, and focal mechanism. The result shows that the RNF strikes in the northeast- southwest direction and dips eastward at steep angles. It is also discovered that the RNF extends into the sea to the Gulf of Thailand and the Andaman sea. The RNF on land is estimated to have the length of 300 km from Prachuab Khirikhan to Ranong province. The RNF consists of 19 fault segments and its extended segments to the sea have the length between 45 and 100 km. Seismic reflection data reveal that the RNF cuts through the seafloor. On land the RNF is the strike-slip fault with the left lateral sense of movement, whereas in the sea the RNF is the oblique-slip fault with the major normal sense of movement. The result confirms very well with the focal mechanism data and field relation regarding several morphotectonic features including trigular facet, offset stream linear valley, pressure ridge and fault scarp. The stratigraphic – log results from two paleoseismic trenches across the RNF along with the previous and recent geochronological data and structural section from seismic interpretation lead to the conclusion that there are at least 6 earthquake events and the latest movement occurred at about 2,000 yrs. The RNF used to trigger the largest earthquake with the magnitude of 7.4 Mw as evidenced by the Ranong segment and the fault has the maximum slip rate 0.7 mm/yr from the Nong Ki segment. It is therefore concluded that the RNF is active fault with the sinistral sense of movement and the recurrences interval of 2,000 yr.