Two magnitude 5.7 earthquakes that struck on Saturday night and yesterday morning were aftershocks triggered by a magnitude 6.4 quake on Tuesday last week, a seismologist said, adding that the epicenters of the aftershocks are moving westward.
Saturday and yesterday’s earthquakes occurred as people were preparing for the Lunar New Year holiday this week.
As of 10am yesterday, the Central Weather Administration (CWA) recorded 110 aftershocks from last week’s main earthquake, including six magnitude 5 to 6 quakes and 32 magnitude 4 to 5 tremors. Seventy-one of the earthquakes were smaller than magnitude 4.
Photo courtesy of the National Science and Technology Center for Disaster Reduction
Thirty-one of the aftershocks were felt nationwide, while 79 were regional.
The operations of Taiwan Railway Corp (台鐵) and the high-speed rail were slightly affected due to the quakes.
The hypocenter of the magnitude 5.7 earthquake on 7:49pm on Saturday was in Tainan’s Dongshan District (東山) at a depth of 7.7km, while the second one, which struck at 7:38am yesterday, was in Tainan’s Nansi District (楠西) at a depth of 8.1km.
The CWA categorized both as “very shallow earthquakes.”
Most of the aftershocks were recorded in Nansi, CWA data showed.
That the epicenter of Saturday’s quake was in Dongshan showed that aftershocks have begun to move west, National Chung Cheng University earth and environmental science professor Wen Yi-ying (?怡瑛) said.
“The quake was near Zengwen Reservoir (曾文水庫), and its epicenter was surrounded by faults in the south, north and west. Subsequent aftershocks in that area could damage the reservoir,” Wen said, adding that residents near the reservoir should be cautious.
Former Seismological Center director Kuo Kai-wen (郭鎧紋) said the magnitude 5.7 earthquake occurred because of movement on the Lunhou Fault (崙後斷層), a 48km-long fault that stretches from east of Chiayi County’s Chukou (觸口) to Tainan’s Zuojhen District (左鎮).
Of the 15 earthquakes recorded on Saturday, nine hit from Dongshan and four from Nansi, Kuo said.
The Nansi quakes were aftershocks of the earthquake that struck on Tuesday last week, while those from Dongshan were more related to the Lunhou Fault, he said
“It is not an active fault, and records show that it did not have a significant change in seismic activities over the past few years. However, it has a high strain rate,” he said. “The quake shows that the fault is in a state of aseismic slip, which is detectable without notable earthquakes. It also implies that crustal strain accumulates rapidly in the fault area, which has a very high potential for large earthquakes and should be closely monitored and studied.”
The CWA said that the reservoir was built on hard rock using the highest earthquake resistance coefficient and should withstand earthquakes, adding that the Water Resource Agency has also inspected the reservoir and found no problems.
Even though the largest intensity generated by the first 5.7 earthquake was Level 4, it lasted one to two seconds, the CWA said.
The farther away from the epicenter, the lower the intensity of the quake, it said.
The CWA urged people to evacuate the area immediately and protect themselves when they receive an earthquake alert on their cellphones from the Public Warning System.
That alert means the earthquake is serious, it said.
“The principle of evacuation is to lower the center of gravity, seek cover, and protect the head and neck,” it said. “When indoors, use items and hands to protect your head and neck, and avoid taking refuge under hanging objects or near movable furniture. When outdoors, stay away from dangerous areas such as buildings, trees and bridges, and avoid moving at high speeds to avoid accidents caused by the shaking.”
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