The Central Weather Administration (CWA) said a magnitude 4.9 earthquake that struck off the coast of eastern Taiwan yesterday was an independent event and part of a stress-adjustment process.
The earthquake occurred at 4:47pm, with its epicenter at sea about 45.4km south of Yilan County Hall at a depth of 5.9km, the CWA said.
Photo courtesy of the Central Weather Administration
The quake's intensity, which gauges the actual effects of a temblor, was highest in several townships in Yilan and neighboring Hualien County, where it measured 4 on Taiwan's seven-tier intensity scale, the CWA said.
Lin Po-yu (林柏佑), a division chief at the CWA's Seismological Center, told a news conference that the quake occurred in the Heping Basin, an offshore basin in the waters between Yilan and Hualien.
It was caused by the Philippine Sea Plate subducting beneath the Eurasian Plate, with fracturing occurring during the process, Lin said.
Center Director Wu Chien-fu (吳健富) said the quake was not an aftershock of the magnitude 7 earthquake on Dec. 27 last year, adding that its more southerly location and shallow depth led the agency to preliminarily classify it as an independent event.
Lin said that a magnitude 4.6 earthquake struck the same area on Friday, followed by another magnitude 4.7 tremor at about 1am yesterday.
Since 1973, there have been 20 earthquakes of magnitude 5.5 or higher within a 20km radius of the epicenter of yesterday's magnitude 4.9 quake, including a magnitude 6 event on May 10, 2024, he added.
Wu said earthquakes occurring in basins are typically shallow, citing the Friday and early Sunday quakes, which had depths of about 10km or less.
The recent string of earthquakes is likely part of a stress-adjustment process, Wu said, adding that as energy is released in one area, stress can build up in surrounding areas, increasing strain on rock layers and making additional quakes more likely as the system seeks a dynamic balance.
Wu said that most earthquakes of about magnitude 4.5 occur along fractured zones and release relatively limited energy, after which the area enters a new phase of energy accumulation.
With about 70 percent of Taiwan's earthquakes occurring in the main island's eastern half and its offshore waters, Wu said that magnitude 4 to 5 quakes are common in the region and there is little point in further classifying magnitude 3 tremors as aftershocks.
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