Fault systems of the eastern Indonesian triple junction: evaluation of Quaternary activity and implications for seismic hazards

Eastern Indonesia is the site of intense deformation related to convergence between Australia, Eurasia, the Pacific and the Philippine Sea Plate. Analysis of tectonic geomorphology, drainage patterns, exhumed faults and historical seismicity highlights faults that have been active during the Quaternary (Pleistocene to present day), even if instrumental records suggest some are presently inactive. Of twenty-seven largely onshore fault systems studied, eleven show evidence of a maximal tectonic rate, a further five show evidence of rapid tectonic activity. Three faults indicating slow to minimal tectonic rate nonetheless show indications of Quaternary activity, and may simply have long interseismic periods. Although most studied fault systems are highly segmented, many are linked by narrow (<3 km) step-overs to form one or more long, quasi-continuous segments that are capable of producing M >7.5 earthquakes. Sinistral shear across the soft-linked Yapen and Tarera-Aiduna faults and their continuation into the transpressive Seram fold-thrust belt represents perhaps the most active belt of deformation and hence greatest seismic hazard in the region. However, the Palu-Koro Fault, being long, straight and capable of generating supershear ruptures, is considered to represent the greatest seismic risk of all the faults evaluated in this region in view of important strike-slip strands that appear to traverse the thick Quaternary basin fill below Palu city.