Tue, Feb 06, 2018 - Page 3 News List

Bureau denies it tampered with quake measurement

RESERVATIONS:What happened was not an ‘earthquake swarm’ nor does it signal the start of a cycle of powerful earthquakes, the Seismological Center acting director said

By Shelley Shan  /  Staff reporter

Seismological Center Acting Director Chen Kuo-chang on Sunday in Taipei briefs reporters on a flurry of earthquakes that have struck off the coast of Hualien.

Photo: EPA / David Chang

The Central Weather Bureau yesterday denied that it had changed its measurement for an earthquake that occurred off the coast of Hualien on Sunday from magnitude 5.8 to magnitude 6.1.

“The United States Geological Survey uses the moment magnitude scale and said that the quake reached magnitude 6.1. We use the Richter scale. The two have different ways of measuring an earthquake’s magnitude,” Seismological Center Acting Director Chen Kuo-chang (陳國昌) said, explaining the discrepancy between the bureau’s measurement and that of the US agency.

As of 3pm yesterday, 61 aftershocks had been recorded since the main earthquake happened at 9:56pm on Sunday, center data showed.

Two of the aftershocks reached magnitude 5, and 11 of them were between magnitude 4 and 5. The rest of the aftershocks were below magnitude 4.

People need to be careful about possible damage caused by the aftershocks, which are likely to continue for two weeks, Chen said, adding that some of the aftershocks could reach magnitude 4.

The main earthquake was triggered by the Philippine Sea Plate in the southeast subducting beneath the Eurasian Continental Plate in the northwest, Chen said.

It happened on a geological rupture zone, which is at the front end of the subduction zone, Chen said.

The rupture zone is in the coastal area between Yilan’s Nanao (南澳) and Hualien, which is why earthquakes happen frequently in this part of the nation, he said.

Chen said that what happened was not an “earthquake swarm,” adding that the term is being misused.

“When an earthquake swarm occurs, a group of earthquakes with very similar magnitudes happen successively within a short period around the same area. There might be a couple of relatively bigger earthquakes among them, but one can hardly distinguish between the main earthquake and aftershocks,” he said.

The main earthquake and its aftershocks can be called a seismic sequence, he said.

The bureau also has reservations about a claim by National Central University professor Lee Chyi-tyi (李錫堤), who said that the earthquake is a sign that the nation has entered a 100-year earthquake cycle, Chen said.

Two recorded magnitude 8 earthquakes happened in Taiwan in the 20th century, one in 1910 off the coast of Yilan and the other in 1920 off the coast of Hualien, Lee said.

As we are approaching the 100-year mark since the last magnitude 8 earthquake, there is a very high probability that another earthquake of similar magnitude might happen within the next decade, Lee said.

“We respect the opinions of different researchers. However, the nation started collecting earthquake data only about 100 years ago and we have reservations about calling this a ‘100-year earthquake cycle,’” Chen said.

Most seismologists believe that magnitude 8 earthquakes are likely to happen around the Ryukyu Trench, which is about 500km to 600km from Hualien, within 10 years, Chen added.

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