CWA explains erroneous islandwide earthquake alert after offshore quake

Taipei, Aug. 25 (CNA) The Central Weather Administration (CWA) issued national-level earthquake alerts across Taiwan on Tuesday after an offshore earthquake was initially overestimated at magnitude 6.6 because of limited seismic data, CWA officials said.

The quake, which was later determined to have a magnitude of 5.8, registered an intensity of 4 on Taiwan's 7-tier intensity scale in Taitung County and 3 in Hualien, Chiayi and Yunlin counties, as well as Tainan and Chiayi City. Most other areas experienced little or no shaking.

Despite the relatively limited impact, national-level earthquake alerts were issued across Taiwan except for Taoyuan and Keelung.

Wu Chien-fu (吳建富), director of the CWA's Seismological Center, explained at a press briefing that the quake's depth was initially estimated at more than 30 kilometers, about 17 kilometers deeper than its actual depth.

The mistake caused the system to overestimate the quake's magnitude at magnitude 6.6, triggering the nationwide alerts, Wu said.

The CWA initially relied solely on data from its Green Island seismic station, while the limited number of seismic stations in the area made it difficult to accurately determine the earthquake's location and depth, he said.

Wu further said that the earthquake was very shallow, meaning its strongest effects were limited to areas near the epicenter. Green Island experienced seismic intensity Level 4, with shaking lasting about nine seconds.

Under the CWA's earthquake alert criteria, alerts are issued to areas expected to experience seismic intensity Level 3 or above when a detected earthquake is estimated at magnitude 6.5 or greater, and to areas expected to experience Level 4 or above when the estimated magnitude is 5.0 to 5.5.

Wu noted that the earthquake was caused by the northwestward movement of the Philippine Sea Plate against the Eurasian Plate. The zone where the two plates collide is one of Taiwan's most seismically active areas.

He said aftershocks of magnitude 5.0 or above could occur over the next week.

CWA data showed the magnitude 5.8 earthquake struck off southeastern Taiwan at 3 p.m. The epicenter of the temblor was located at sea, about 48 kilometers east-southeast of Taitung County Hall, at a depth of 13.1 km.