A devastating magnitude 6.4 earthquakeA devastating earthquake of magnitude 6.4 occurred on the eve of the Chinese New Year around Tainan area in southern Taiwan. The event occured at 3:57 am, local time, on February 6, 2016. Its epicenter is located near the town of Meinong, with a focal depth of 15-25 km. The earthquake caused damages on numbers of houses, some fallen down, particularly in the eastern part of Tainan City, roughly 20-30 km west of the epicenter. It is somewhat surprising that this earthquake, with a moderate magnitude and a relatively deep mainshock, has a coseismic slip propagation apparently ceased a few kilometers deep without any fault rupturing on the surface level. On the other hand, the PGA intensity measurements revealed a strong concentration of the coseismic shaking and acceleration in the eastern part of Tainan, which suffered the most damages of human constructions as well as earthquake shaking site effects, such as liquefactions, landslides, and mud volcanoes eruptions.
Causative fault(s) ?From the focal mechanism of the main shock and preliminary results of the aftershocks' distribution, the causative fault appears to be a geological unknown blind structure, probably located below the décollement of the regional fold-thrust belt, which is estimated at 8-10 km of depth. The earthquake started at about 16-18 km (or 22-23 km?) deep on a fault trending WNW-ESE (az:280), with a very gentle shallow dip angle of 15 degrees to the north. The coseismic slip then propagated toward the west and finally went upward in the western uppermost part of the fault, below eastern Tainan area.
Most of the aftershocks are not located on the above suspected earthquake causative fault. Instead, a large part of them lies on a cluster situated 20-30 km depth and 20 km west of main shock, below the causative fault. From the observations of the wave forms graphs, there is a strong speculation that it was a doublet earthquake with the 2nd mainshock occurring near this cluster of aftershocks, about 4 second following the 1st mainshock (see discussion on the Facebook page of TEC : Link).
Focal mechanism and seismotectonics in SW TaiwanAs for the focal mechanisms, the main shock indicates a slightly oblique thrust with left-lateral slip component, which was characterized by a co-seismic stress of NE-SW compression and oblique NW-SE extension. It is intriguing to observe that the NE-SW compression shows a significantly different direction with a regional WNW-ESE main compression derived from GPS interseismic strain and geological fault-slip data. On the other hand, this SW-compression is similar with the 2010 M=6.1 Jiahsian earthquake that occurred just east of the Meinong earthquake also at a crustal level of about 10-30 km of depth.
However, the studies of the Jiahsian earthquake indicated that the co-seismic slip on the causative fault might propagate upward to cut through the regional décollement into shallow accetionary wedge. Nevertheless, combining these two recent events, it appears that there might be some significant strain decoupling between the shallow accretionary wedge of fold-thrust belt and the deep crust below the décollement. In addition, the SW-compression from the two magnitude 6 earthquakes (Jiahisan and Meinong) is consistent with the regional SW-direction escape in the Southwestern Taiwan. In other words, such SW-compression earthquake events should not be considered as unusual events in this area.