Two earthquakes of stronger than magnitude 5 struck off the east coast early yesterday, the Central Weather Bureau said, adding that they were aftershocks from a magnitude 6.1 earthquake that struck early on Sunday.
Sunday’s earthquake was the largest so far this year.
The bureau had warned the public to be vigilant from Monday to today for aftershocks of magnitude 4 or greater.
The earthquakes could be precursors to larger earthquakes, because Taiwan has had eight earthquakes of magnitude 5 or greater over the past month, the bureau said.
Data from the bureau’s Seismology Center showed that a magnitude 5.5 temblor occurred at 12:56am yesterday.
The epicenter was 73km east-southeast of Yilan County Hall, at a depth of 69.3km, and Yilan County recorded the nation’s highest intensity, Level 3.
At 12:58am, a magnitude 5.7 earthquake was detected 47.9km southeast of Yilan County Hall, at a depth of 23.9km.
The observation station in Yilan again recorded the nation’s highest intensity, Level 4.
The quake was the nation’s second-largest to affect Taiwan this year.
Despite having different epicenters, yesterday’s and Sunday’s earthquakes occurred in a subduction zone formed by the Eurasian Plate and the Philippine Sea plate, Seismological Center Director Chen Kuo-chang (陳國昌) said.
Because of this, the bureau categorized yesterday’s two earthquakes as aftershocks of the magnitude 6.1 earthquake, he added.
Most of the earthquakes of magnitude 5 or greater this year have occurred in the subduction zone, particularly around the Heping Sea Basin, Chen said, adding that seismic activity became more frequent around the sea basin last year.
In a year, Taiwan typically records about 18 earthquakes magnitude 5 to magnitude 6, and about two earthquakes greater than magnitude 6.
So far this year, Taiwan has already had eight earthquakes of magnitude 5 or greater, he said, adding that the bureau offers two possible explanations.
“The frequent release of seismic energy helps relieve the stress accumulated in the mantle, which prevents larger earthquakes,” he said.
Also, large earthquakes are more likely to occur in subduction zones or around sea trenches, he said, adding that many earthquakes of magnitude 8.5 or greater have had one or more foreshocks.
It is possible that these earthquakes of magnitude 5 or greater are foreshocks, he said.
A week before Japan’s northeast region was devastated by a magnitude 9 earthquake and a subsequent tsunami in 2011, a magnitude 7.7 earthquake occurred in the region, which seismologists at one point considered the main shock, Chen said.
As scientists are still unable to predict the occurrence of earthquakes, it is necessary to continue monitoring earthquakes off the east coast, he said.
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