Wed, Jan 24, 2018 - Page 8 News List

Taipei quake serves as reminder

By Ma Kuo-Fong and Pan Chang-Chih 馬國鳳,潘昌志

A magnitude 5.7 earthquake with its hypocenter 140km underground happened in Taipei on Wednesday last week. As the earthquake was detected in Taipei’s Beitou District (北投), and there had been reports of large amounts of white smoke billowing from the nearby Qixingshan(七星山), it led to public and media concern that it might have been caused by volcanic activity in the Datun Volcanic Group (大屯火山群).

However, there is no causal relationship between the quake and volcanic activity and the main reason for that is the hypocenter’s depth.

The quake occurred 140km beneath the Earth’s surface, a distance that, when converted horizontally, is similar to the distance between Taipei and Taichung.

According to recent research, the magma chamber of the volcanic group is about 20km to 30km underground. The depth differences between the Datun magma chamber and the epicenter of the earthquake is equal to the distance between New Taipei City and Taichung.

Furthermore, a quake of magnitude 5.7 will only cause geological fractures ranging within a few kilometers, which is to say, a 140km-deep quake will not affect volcanic activities more than 100km away.

The reason the quake took place more than 100km below Beitou District is that the area is situated in the subduction zone where two tectonic plates converge.

Taiwan was formed by a collision between the Philippine Sea Plate and the Eurasian Plate, and the convergence of the two plates has given shape to the steeple-like mountain ranges in central Taiwan.

In the northern part of Taiwan, the Philippine Sea Plate keeps moving northward and sinking into the Earth’s mantle at the subduction zone. Earthquakes occur both on the subducting plate and at the convergent boundaries.

The Beitou quake exemplifies the characteristics of the subduction zone deep under northern Taiwan and is therefore not directly related to the volcanic activities higher up.

The volcanic area located right above the hypocenter of the earthquake is formed by a geological process called partial melting, which is a result of the dehydration process when a slab submerges deep into a high geothermal regime.

The formation of volcanoes is directly related to the movement of subducted plates, but earthquakes occurring along the boundaries of subducted plates have nothing to do with the activities of volcanoes higher up.

Volcano-related earthquakes usually take place in shallow volcanic subsurface structures or around a magma chamber. The quake that happened in the subduction zone this time, on the other hand, should be attributed to long-term tectonic plate movement.

However, from a disaster-risk perspective, the quake does not demand a closer understanding of the Datun Volcanic Group, but rather of what the effects would be if there was a greater quake.

Historically, the catastrophic quakes that have occurred at deep-lying subduction zones that have brought the greatest damage to Taipei and Keelung were the 1909 Taipei quake, which had a magnitude of 7.3 on the Richter scale with a hypocenter that was about 80km deep.

It is often thought that because the hypocenter is so far beneath the Earth’s surface, there will not be disastrous damage.

However, because deeper-lying quakes create a longer vibration cycle, they could have a significant impact in northern Taiwan with all the tall buildings and skyscrapers in the region.

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