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Mw 6.4 earthquake strikes southern Taiwan

07 Feb 2016

In the early morning of Febuary 6, 2016, a moderate earthquake struck Tainan, southern Taiwan, right before the Chinese New Year holidays. The quake damaged several buildings in the city, one of the oldest settlements at Taiwan. The Central Weather Bureau at Taiwan and U.S. Geological Survey determined the magnitude of the Tainan earthquake to be 6.4, with the focal depth of about 20 km beneath the surface. The energy released from the Tainan earthquake is about four times greater than the Sabah earthquake in June 2015.

This earthquake not only caused many residential and office buildings to collapse and others to tilt, resulting in more than 70 casualties. The Taiwan High Speed Rail was also damaged by the temblor, forcing its operator to call off the high-speed train service until the damage could be repaired. The cancellation of the high-speed rail service hampered travel plans for thousands of Taiwanese planning to travel back to their hometowns for the Chinese New Year holidays.

Based on the focal mechanism solution, the ground motion distribution of the mainshock, and the aftershock epicentres, the Tainan earthquake appears to be sourced from the rupture of a blind thrust fault beneath the foothills of southern Taiwan. The recent event shares many similar characteristics to the Mw 6.3 Jiasian earthquake in March 2010, which occurred approximately 10 kilometres east of the present mainshock. Both of these events seem to be generated by the structures differ from the faults mapped on the surface, underneath the orogenic system.

Although the epicentre of the Tainan earthquake is in the foothills, the city suffered considerable damage from this earthquake because it is built on the soft sediment layers that were deposited several thousand years ago. When the seismic wave propagates from the hard rocks to the soft sediments, its amplitude is magnified, resulting in stronger shaking in the plains. This seismic site effect may be one of the reasons that caused damages to buildings in southern Taiwan. Scientists in Taiwan are now collecting more data on this earthquake to understand its real cause.

More aftershocks are expected in the next few days near the Tainan area. Within just three hours from the mainshock, more than 10 aftershocks occurred near Tainan, with magnitudes ranging between 3.7 and 4.9. While these aftershocks are not likely to cause more damages than the mainshock, there is a small chance that the Tainan earthquake could trigger another fault rupture nearby, causing another major earthquake in this region.

The risk of tsunami in southern Taiwan would be elevated from this earthquake is not expected as this earthquake is far away from the Manila Trench; the ground motion offshore is not strong enough to trigger the submarine landslide. However, a tsunami warning is likely to be given to the southern Taiwan if a strong earthquake event occurred offshore Southern Taiwan, much like the 2006 Hengchun earthquake that resulted in submarine landslide that broke a submarine cable.

Cover image credit: US Geological Survey