A Mw 6.4 earthquake struck eastern Taiwan on 6 February 2018 at 11.50 pm (Singapore time). According to Taiwan’s Central Weather Bureau (CWB), the magnitude-6.4 earthquake had originated offshore approximately 15 kilometres (km) north of Hualien city at a depth of about 10 km.

Experts at the Earth Observatory of Singapore (EOS) and National Taiwan University (NTU) believe that this earthquake was triggered by the earthquake swarm that shook eastern Taiwan on 4 February 2018. This earlier earthquake swarm took place on an offshore thrust fault system. In comparison, yesterday’s Mw 6.4 earthquake originated on a strike-slip fault system that runs in the approximate N-S direction. It is likely to have occurred on the northern extension of the Milun fault system that stretches from Hualien city to the north.

Our preliminary analysis of the seismological data provided by the Taiwan Earthquake Research Center (TEC), as well as photographs taken in Hualien, suggest that the earthquake originated from a 15-20 km-long left-lateral strike-slip fault just north of Hualien city. The fault rupture is likely to have propagated southward into the city, resulting in the destruction of buildings in the city’s eastern parts. Preliminary post-earthquake surveys done in the morning of 7February confirm that part of the Milun Fault had ruptured during the Mw 6.4 mainshock, with surface offsets ranging from a few centimetres to several tens of centimetres.

The Milun Fault, one of the most active faults in Taiwan, has sparked concern and keen interest in scientists. A recently updated version of the Taiwan Earthquake Model (TEM) projected a relatively high chance (~30%) of the Milun Fault producing a significant earthquake in the next three decades. The last destructive earthquake event produced by the Milun Fault was in 1951. The 1951 earthquake sequence consisted of several quakes measuring approximately 7 in magnitude. They had propagated in a north-to-south direction along the longitudinal valley in eastern Taiwan, causing significant damage that stretched from Hualien to Taitung.