Evidence for Multiple Ground-Rupturing Earthquakes in the Past 4,000 Years Along the Pasuruan Fault, East Java, Indonesia: Documentation of Active Normal Faulting in the Javan Backarc

Instrumental and historical records of earthquakes supplemented by paleoeseismic records reveal the earthquake potential of an area. The back-arc region of the Javan subduction system has been considered to be inactive seismically and is identified as a region of low seismic hazard on the seismic hazard map of Indonesia. However, our study documents recent geological activity along Pasuruan Fault, a high-angle normal fault located within this zone. The fault is expressed in the landscape as prominent ~13-km-long, 2- to 50-m-high scarps cutting Pleistocene sediments on the north coast of East Java. We excavated two fault-perpendicular trenches across a relay ramp structure and documented evidence for past earthquakes in both trenches. Our preliminary age model, constrained by 23 radiocarbon dates on detrital charcoal, indicates that since 2006 ± 134 cal BCE, there have been at least six ground-rupturing earthquakes along the fault at this location. The most recent is attributed to the 1852 Grati earthquake. Our modeled 95th percentile ranges for the next four earlier earthquakes (and their mean, in years CE) are 1786–1846(1816), 1666–1777(1721), 1086–1648(1367), and 726–1093(909). Using these events, we calculate a recurrence interval of 236 ± 46 years. The Pasuruan Fault is located near the western end of the Bali-Flores back-arc basin. The late Holocene activity along the fault shows that extensional faulting is ongoing though the mechanism causing this structure remains unclear. Although the fault is considered as a minor structure, its potential for producing large earthquakes may pose significant hazard to the densely populated region.