M 5.6 Earthquake Strikes Northeastern Taiwan

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M 5.6 Earthquake Strikes Northeastern Taiwan

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A magnitude-5.6 earthquake occurred offshore Ilan in northeastern Taiwan on 12 May 2016. According to the United States Geological Survey, this is the second significant quake to have struck Taiwan this year.

Approximately an hour after the M 5.6 mainshock took place, a M 5.5 aftershock occurred east of the mainshock’s point of origin. This series of earthquakes is commonly referred to as a “doublet,” where two quakes of similar magnitudes occur in succession within a relatively short period of time.

The real-time fault-plane solutions1 show that both of these earthquakes were a result of normal faulting.2 Even though it was reported that tremors could be felt as far as northern Taiwan, including Taipei, the degree of ground motion in Taipei was moderate and therefore the resulting damage to properties in the capital city is unlikely to be extensive.

The epicentre of the 2016 Ilan earthquakes and their focal mechanisms are shown here. The colour maps show the seismic intensity of these two earthquakes (the tremors were reported to reach northern Taiwan). The mainshock produced stronger ground motion on the Ilan plain as a result of its higher earthquake magnitude and its closer distance to landmass. These earthquake maps are taken from the Central Weather Bureau of Taiwan and the focal mechanisms data are from the Real-Time Moment Tensor Monitoring System of Taiwan.
 
Although earthquakes of M 5.0 to 6.0 are not a rare occurrence in Taiwan, the recent Ilan earthquakes bear unique characteristics in comparison to other earthquake events, most of which are a result of the collision and subduction processes between the Eurasian and Philippine Sea Plates. The recent Ilan earthquakes’ normal faulting focal mechanisms indicate that they are caused by the active extension of the Okinawa trough from southwestern Japan to eastern Taiwan. This active extension is the primary tectonic force that created the triangular-shaped Ilan plain in northeastern Taiwan and the submarine volcanoes off the shores of northeastern Taiwan. 
The regional tectonic setting of the western end of the Okinawa Trough modified from Konstantinou, 2014. The black arrow shows the tectonic motion direction. Near northern Taiwan, the north-south extension of the Okinawa trough created the Ilan plain and the submarine active volcanoes north of the Ryukyu islands. The blue dot denotes the location of the 2002 earthquake, and the orange colour indicates the 2005 earthquake epicentres. The white star shows the epicentre of the 2016 earthquake. 
The current data shows that the Ilan earthquakes may not be a simple faulting event as it shares a similar focal mechanism with the magnitude-5.7 doublet earthquakes that struck Taiwan in March 2005. The M 5.6 Ilan earthquake is also similar to the M 6.8 earthquake that struck Taipei in April 2002, which caused a construction crane to topple from the Taipei 101 Mall. The focal mechanism solution shows that these events are not just simple normal faulting events as they have expansion components in the shallow part of the crust.

This expansion component is reminiscent of the aforementioned March 2005 earthquakes that had occurred to the west of the epicentre of today’s earthquake. The seismic distribution and ground motion of the 2005 earthquakes imply an association with dike, or fluid intrusions, associated with volcanic activity in which magma, or the fluid from magma migrating from the deep crust to the shallow earth, and triggered the earthquake activity.

The preferred model used to explain the occurrence of the 2005 earthquake sequence, which was associated with a fluid dike intrusion event. The 2016 earthquakes may be associated with the same tectonic process along the southern boundary of Ilan Plain. Figure modified from Lai et al., 2009

The May 2016 event is possibly the result of the same volcanic-related process. Such volcanic-related phenomenon is not rare in Ilan area. The Turtle Island which, similar to a large sea turtle swimming along the Ilan’s coastline, is an active volcano island with a relatively “young” eruption record (< 6000 yrs). The Qingshui geothermal field, along the southern edge of the Ilan basin, may also be related to the igneous heat source. Further analysis of these earthquake sequences will help us to better understand the source of these earthquakes, and the tectonic kinematics behind them.

Further reading: Taiwan Earthquake Research Center

Thumbnail image from US Geological Survey

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1Real-time fault-plane solutions are data collected by seismologists immediately after an earthquake event. It provides information on the orientation of the fault, as well as the slip on the fault. 

2Normal faulting is a process by which two blocks of rock are pulled apart by tension forces. The rock above the fault plane moves downwards relative to the rock mass beneath the fault plane.

 

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