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An Earthquake Swarm Strikes Northeastern and Eastern Taiwan

05 Feb 2018

A swarm of earthquakes hit Hualien, Taiwan, on 4 February 2018 (Source: USGS)

A group of moderate earthquakes struck northeastern and eastern Taiwan on 4 February 2018 at approximately 9 pm (Singapore time). According to Taiwan’s Central Weather Bureau (CWB), the magnitude-5.8 earthquake originated offshore east of the Yilan and Hualien region at a depth of about 16 kilometres (km). 

The image above shows the ground motions produced by the M 5.8 earthquake that took place on 4 February 2018. This is based on the data collected by Taiwan’s P-alert Earthquake Early Warning system and real-time monitoring system. The green dots indicate the areas where the earthquake’s ground motions were felt by the residents. The orange dots show the locations where the ground motions were strong enough to cause alarm. (Source: P-alert real-time monitoring system)

Unlike a typical earthquake sequence, in which a series of smaller aftershocks follow from the larger mainshock, this particular earthquake sequence acted like a swarm where several earthquakes of a similar magnitude occurred within a short span of time. 

This earthquake sequence appeared to have originated from a doublet earthquake (a pair of similar sized, closely occurring earthquakes) measuring 4.6 and 4.3 in magnitudes. They were followed by two further earthquakes (M 5.1 and 5.8) approximately 12 hours later. These two sequences, along with several other smaller quakes, generated noticeable ground motions in Taipei City, which is located more than 80 km from the earthquakes’ epicentres.

Even though the M 5.5 earthquake produced ground motions that were not as strong as the M 5.8 event, the continuous shaking from the resulting tremors sent some residents into panic. (Source: P-alert real-time monitoring system)

Even though the majority of the earthquake shocks from this event are considered to be moderate in strength, they did trigger a number of rockfalls and small-scale landslides in the eastern part of Taiwan. The Suhua Highway that sits in between Yilan and Hualien had its traffic route temporarily blocked as a result. Such rockfalls may continue to occur in the next few weeks, depending on whether the area experiences more quakes. 

The ground motions that shook Taipei is likely to be a result of a phenomenon known as the “basin effect”, where the soft soil in the Taipei Basin could have amplified the earthquake's ground-shaking. Such an effect could prove to be deadly if the earthquake's magnitude is large enough. For example, in November 1986, a Mw 7.4 earthquake originating from a location close to yesterday’s earthquake sequence caused a market building to collapse in Taipei. Last year, Mexico experienced severe damage after a deep intra-slab earthquake took place close to its city.