Magnitude 4.9 quake off eastern Taiwan independent event: CWA

Taipei, Jan. 18 (CNA) The Central Weather Administration (CWA) said a magnitude 4.9 earthquake that struck off the coast of eastern Taiwan on Sunday was an independent event and part of a stress-adjustment process.

The earthquake occurred at 4:47 p.m., with its epicenter located at sea about 45.4 kilometers south of Yilan County Hall at a depth of 5.9 kilometers, according to the CWA.

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, said at a news conference that the quake occurred in the Heping Basin, an offshore basin in the waters between Yilan and Hualien, and was caused by the Philippine Sea Plate subducting beneath the Eurasian Plate, with fracturing occurring during the process.

Center director Wu Chien-fu (吳健富) said the quake was not an aftershock of the magnitude 7.0 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 around 1 a.m. Sunday.

From 1973 to the present, there have been 20 earthquakes of magnitude 5.5 or higher within a 20-kilometer radius of the epicenter of Sunday'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 around 10 kilometers or less.

The recent string of earthquakes is likely part of a stress-adjustment process, Wu said, explaining 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 noted that most earthquakes of around magnitude 4.5 occur along fractured zones and release relatively limited energy, after which the area enters a new phase of energy accumulation.

With around 70 percent of Taiwan's earthquakes occurring in the 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.