Earthquake Geology of the Lembang Fault, West Java, Indonesia

Abstract

The Lembang Fault is a major fault in western Java that skirts the northern edge of Bandung, one of Indonesia's largest cities, just south of the active Tangkuban Perahu volcano. Although it has no recorded or historical large earthquakes, the Lembang Fault shows obvious geomorphic evidence of recent activity and has long been thought to be active. In this study, we use geomorphic analysis to unequivocally establish that the fault has a dominantly sinistral sense of movement with a slip rate of 1.95–3.45 mm/yr. This proves that the fault is accommodating trench parallel slip resulted from a slight obliquity in plate convergence at the JavaTrench. With a length of 29 km, this suggests that the Lembang Fault could produce a Mw 6.5–7.0 earthquake with a recurrence time of 170–670 years. We also conducted paleoseismological trenching of the Lembang Fault and found evidence for at least 3 earthquakes in the 15th century, 2300–60 BCE and 19,620–19,140 BP. The 2300–60 BCE earthquake had a measurable vertical displacement of 40 cm, which is consistence with a Mw 6.5 earthquake. This is the first mapping of a source of crustal earthquakes in Java, Indonesia, the world's most densely populated island in one of its most tectonically active areas. The Lembang Fault and other faults in Java are likely to pose substantial risk to not only Bandung but many of Java's major urban agglomerations.