Halloween 2013 was potentially very scary for residents of Taiwan after an earthquake of magnitude 6.3 (M6.3) struck the country late in the evening. Early information from the United States Geological Survey (USGS) indicates that the epicentre of the tremor was onshore around 45km from the town of Hualian, at a depth of 12 km.
At the time of writing there were no reports of any damage or injuries and no tsunami warning was issued, although Reuters news agency reported that buildings shook in the island’s capital, Taipei.
Tectonic Setting of Taiwan
The island of Taiwan is seismically vulnerable, caught as it is in the complex tectonic setting of the western Pacific. Forming part of the boundary between the Eurasian and the Philippine plates by extensive deformation zones which broadly follow the axis of the island characterize this area.
The main plate boundary is a subduction zone to the south, with the Philippine plate descending below the Eurasian, but the existence of a second subduction zone, the Ryuku trench, to the east and a zone on non-subduction convergence to the north and west complicates the pattern.
Between them, these plate motions contribute to a variety of different fault mechanisms: Taiwan exhibits extensional and lateral movements as well as thrust faulting associated with subduction. Against this background, and without detailed information, it’s difficult to be clear about the causal mechanism for the October 31 event, but the depth and location relative to the boundaries suggest that it may be associated with reverse faulting in the upper crust.
Taiwan’s Tectonic History
Seismologist R. Yeats notes that earthquake records for Taiwan are by no means as extensive as elsewhere in the western Pacific (Active Faults of the World) and seismicity maps of the country reflect both this and the diffuse and complex nature of the plate margins around the island. Despite this, existing records are revealing: the USGS historic earthquakes list for the past century or so includes 16 tremors greater than M6.5, 11 of them with a magnitude of at least 7.0.
Over the years these events have proved costly in terms of human life, several of them having death tolls in the thousands. Although the largest earthquake on the list (an M8.0 in 1920) killed just five people, other tremors have been more costly.
Most recently, an estimated 2,400 lives were lost as a result of the M7.6 which struck in 1999. The high death toll may be accounted for by the inland location, which the USGS notes is unusual as the majority of the tremors are, like the most recent, towards the eastern side of the island.
Taiwan Quakes: Not So Scary Recently
More recently, however, the island’s earthquakes have been less of a threat to life: records show that since the beginning of the century the country has experienced nine earthquakes of between M6.2 and M7.1, with just eight lives lost.
Reuters. Earthquake of 6.7 magnitude strikes Taiwan: USGS. Accessed October 31, 2013.
USGS. Historic Earthquakes Magnitude 7.6 Taiwan. Accessed October 31, 2013.
USGS. M6.3 – 45km SSW of Hualian, Taiwan. Accessed October 31, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.