Subduction-zone quake off Yilan produces few aftershocks: CWA

Taipei, Dec. 28 (CNA) A magnitude 7.0 earthquake struck off Taiwan's northeastern coast late Saturday, producing only two aftershocks as of Sunday noon, according to the Central Weather Administration (CWA).

The limited aftershocks contrast with last year's major Hualien quake, as Saturday's earthquake occurred at a greater depth in a subduction zone.

The earthquake struck at 11:05 p.m. Saturday, with its epicenter about 32.3 kilometers east of Yilan County Hall offshore, at a depth of 72.8 kilometers.

Shaking was felt in 17 cities and counties north of Tainan and in eastern Taiwan, reaching intensity level 4 on Taiwan's seven-tier seismic scale, according to the CWA.

Data on the CWA website showed that one aftershock struck at 00:45 a.m., with a magnitude of 4.7 and a depth of about 60.8 kilometers.

The other occurred at 10:55 a.m. with a magnitude of 3.8 and a depth of about 66.5 kilometers.

Wu Chien-fu (吳建富), head of the CWA's Seismological Center, said the difference from the April 3, 2024 Hualien quake, which was followed by frequent aftershocks, is due to both depth and tectonic setting.

The Hualien quake was a shallow fault-zone event, occurring at just 15.5 kilometers deep, while Saturday's Yilan quake was a subduction-zone earthquake caused by the Philippine Sea Plate subducting northward beneath the Eurasian Plate, Wu said.

Such deep earthquakes generally generate fewer aftershocks and weaker surface shaking, he added.

Wu cautioned residents to remain alert, as aftershocks in the magnitude 5.5-6.0 range are still possible over the coming week.

The CWA said the quake was among the strongest to hit Taiwan in recent decades, following the magnitude 7.3 earthquake in Nantou in 1999 and last year's Hualien quake.

Because it occurred offshore and at significant depth, it was widely felt and shook for a relatively long duration but was less likely to cause severe damage, according to the agency.

Meanwhile, as some residents complained that the national earthquake alert arrived after the shaking had already begun, Chen Da-yi (陳達毅), a section chief at the Seismology Center, said the alert was issued about 15.2 seconds after the quake.

Chen explained that the offshore location of the epicenter and the quake's depth meant seismic waves took longer to reach ground stations, while the limited number of sea-based monitoring stations further delayed calculations before the nationwide alert could be sent.