TAIPEI (Taiwan News) — After the strongest earthquake recorded in a subduction zone in nearly 50 years struck on Sunday (Oct. 24), the Central Weather Bureau (CWB) warned that aftershocks of magnitude 4 or higher could occur over the coming three days.
At 1:11 p.m. on Sunday, a powerful magnitude 6.5 earthquake struck 22.7 kilometers south of Yilan County Hall, with a focal depth of 66.8 km, and was followed one minute later by a magnitude 5.4 temblor that was centered 16.6 km south-southwest of Yilan County Hall with a focal depth of 67.3 kilometers. The quakes delivered an intensity level of up to 4 in 11 counties and cities and caused several injuries and inflicted minor damage on numerous structures.
During a press briefing that afternoon, Chen Kuo-chang (陳國昌), head of the CWB's Seismological Center, said the magnitude 6.5 earthquake was the largest in the subduction zone 40 km beneath the surface since 1973. The zone is situated between Taiwan and Japan at the west end of the Ryukyu Trench.
Chen pointed out that if the energy is not balanced after the mainshock in the subduction zone, there could be subsequent aftershocks. The seismologist said that aftershocks of magnitude 4.0 or higher are possible over the next three days, but called on the public to not be overly concerned.
He emphasized that although the range of the quake was wide, it was caused by low-frequency waves. Without high-frequency seismic waves, the damage to structures is relatively low, said Chen.
According to CWB statistics, Sunday's magnitude 6.5 temblor was the biggest of the year in Taiwan. There have been two earthquakes that reached magnitude 6.2 this year, one off the coat of Yilan on Feb. 7, and one that struck Hualien County's Shoufeng Township on April 18.
The latter generated higher shock waves than Sunday's quake due to its shallow depth, bringing level 6 shock waves to Hualien that day.