Aftershocks are expected, after Taiwan was struck by its strongest earthquake so far this year. Last Sunday, a magnitude 6.5 earthquake rattled Yilan County, and was soon followed by a temblor of magnitude 5.4. The earthquakes occurred over a subduction zone, which is where one tectonic plate sinks beneath another. Experts warn that in the coming days, the area may see aftershocks of magnitude 4 or higher.

On Sunday, a magnitude 6.5 earthquake struck Yilan’s Nan’ao Township. It caused rockslides that blocked a road leading to Qingshui’s geothermal park, which will now be closed for three days. The temblor had occurred along a subduction zone. Experts say that in the current phase of the seismic cycle, Taiwan is more prone to subduction zone earthquakes.

Kuo Kai-wen
Former Seismology Center director
In the 100-year cycle, the first 50 years is a period of high activity. The order of the activity is basically this: major earthquakes along the western half, for instance the 1992 Jiji earthquake. That’s followed by greater subduction zone activity. Once that activity recedes, it goes back to the western half, and that completes the cycle.

The epicenter of the magnitude 6.5 temblor was deep, and so it caused relatively little damage. But experts say that subduction zones can produce powerful and devastating earthquakes.

Chen Wen-shan
NTU Department of Geosciences
Pressure builds up where the tectonic plates meet. If a lot of energy is built up, that can cause “megaquakes,” with magnitudes above 8 or 8.5. The 2011 Tohoku earthquake in Japan was also a subduction zone earthquake.

Experts say Nan’ao Township is a hotspot for earthquakes due to its proximity to a subduction zone and a fracture zone. That makes it a very seismically active area.

Kuo Kai-wen
Former Seismology Center director
Nan’ao sees a lot of shallow earthquakes. There isn’t a fault zone there, it’s a fracture zone. Fracture zone earthquakes don’t really reach magnitudes of 6.5.

The Central Weather Bureau says aftershocks of magnitude 4 and up may follow in the next three days, so be on the alert in case more shaking comes.