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Mw 6.5 Earthquake Strikes Northeast Aceh, Sumatra, Indonesia
On the morning of 7 December 2016, a damaging earthquake struck the Indonesian province of northeast Aceh, Sumatra. Strong ground-shaking collapsed many buildings and caused significant casualties (over 50 deaths reported at the time of publication). Aftershocks are likely to continue to affect the region.
Focal mechanisms determined from seismic recordings show that the earthquake originated from the rupture of a vertical strike-slip fault located near the coastline (5.281°N 96.108°E). This particular type of fault is common in Sumatra; the 1,600 kilometre-long Sumatran Fault Zone is a system of interconnected, active strike-slip faults that run along the mountainous spine of the whole island. While this great strike-slip fault system is well known and has caused many damaging earthquakes, today’s earthquake was a “surprise” as it occurred outside of the region where active fault strands have been mapped. This is reminiscent of the 2 July 2013 earthquake that struck Takengon, also in Aceh, due to the rupture of a previously unmapped strike-slip fault.
Scientists can assess seismic hazard using geological mapping and geophysical measurements, and most of the active and potentially hazardous faults in Sumatra have already been identified. However, previously unknown structures can rupture, and communities in tectonically-active areas like Sumatra should prepare for earthquakes even if active faults have not been discovered nearby.
In December 2004, coastal areas of the Aceh province were swept by a great tsunami that originated from the rupture of a subduction thrust fault offshore. This morning’s earthquake was much smaller in magnitude (Mw 6.5) than the 2004 event (Mw 9.1), and the strike-slip motion did not cause any sea surface uplift capable of producing tsunami waves. However, many residents of the affected region made the correct decision to move toward high ground after they felt the earthquake in order to avoid a potential tsunami.