Aftershocks of up to magnitude 5 could occur within a week of the earthquake that struck northern Taiwan last night, although they are unlikely to be strongly felt due to their depth, the Central Weather Administration (CWA) said today.

The magnitude 5.6 earthquake struck at 8:36pm, with its epicenter in New Taipei City’s Shuangsi District (雙溪) at a depth of 95.7km.

Photo courtesy of the Central Weather Administration

The quake registered an intensity of 3 on Taiwan’s seven-tier intensity scale in Yilan County, New Taipei City, Taipei, Taoyuan, Hsinchu County, Hualien County, Taichung and Miaoli County.

The earthquake was caused by the subduction of the Philippine Sea Plate, CWA Seismological Center Director Wu Chien-fu (吳健富) said.

As the plate extends northward from waters off Hualien and descends at an angle to a depth of about 90km to 100km, earthquakes can occur beneath a broad area stretching from waters off Yilan to New Taipei City and Taipei, Wu said.

He said aftershocks measuring around magnitude 4.5 to 5 cannot be ruled out within the next week.

However, as they would occur at considerable depths, much of the seismic energy would be absorbed before reaching the surface, resulting in a surface intensity of only 1 to 2, likely making them barely noticeable to the public, he added.

The earthquake last night prompted the CWA to issue Public Warning System alerts for Yilan County, Taoyuan, Keelung, Taipei and New Taipei City.

Wu said that because the quake occurred deep underground, surface monitoring stations needed to receive sufficient signals before processing the data, meaning the alerts were issued about 17 seconds after the quake and the timing of the warnings and the tremors felt by the public varied slightly among areas.

Meanwhile, a magnitude 4.8 earthquake struck Tainan in southern Taiwan at 10:14am today, the CWA said.

There were no immediate reports of damage or injuries.

The epicenter was about 36.7km east-northeast of Tainan City Hall at a depth of 9.6km, it said.

The earthquake’s intensity, which gauges the actual effect of a seismic event, was highest in Tainan and its neighboring Kaohsiung, where it measured 4 on Taiwan’s 7-tier intensity scale.

The quake also measured an intensity of 3 in Chiayi County and Yunlin County, the CWA said.