PALEOEARTHQUAKE ALONG XAIGNABOULI FAULT ZONE IN WESTERN LAO PDR

Abstract

Paleoseismological investigation has been carried out to determine the fault evidence in the Xaignabouli (XYB) province, western Lao PDR, prompted by two recent earthquakes (5.4 and 3.9 ML in 2011) with similar sense of movement as the focal mechanism determined from recent events in the area. The main objectives are to determine the age of the most recent movement, the sense of movement and estimate the paleoearthquake recurrence interval. Major techniques include remote-sensing, field investigation, and paleoseismic excavation together with optical stimulated luminescence (OSL) dating. Several morphotectonic features, particularly offset streams, linear valleys, facet spurs, sag pond, and scarplets, have been recognized along the fault within the two basins, Phiang and Xaignabouli, in the XYB province. Two major strike-slip fault zones have been recognized with an approximate trend in NNE-SSW direction, displaying not only sinistral but also normal movement faults. The sinistral movement along the studied fault, for the first zone passes the western edge of the Xaignabouli basin with a total length of 250 km, consisting of 20 segments ranging from 10 to 110 km. The second zone, showing better tectonic geomorphology, consists of 3 segments varying from 18 to 35 km and passes the eastern edge of the Phiang basin. The surface rupture length from the investigated faults indicate maximum credible earthquake magnitudes between 5.6 and 7.3 M. Results from OSL dating show 3 earthquake events, ca. 3,000, 2,000 and 1,000 years ago, suggesting a recurrence interval of 1,000 years. Results also indicate that vertical and horizontal slip-rate along two studied faults control the development of the Phiang and Xaignabouli basins, and that they extend further and connect northward to the Dien Bien Phu Faults in northern Vietnam and southward to the Uttaradit Fault in north Thailand.