SRL Focus Section Investigates 2018 Hualien Earthquake Sequence

28 December 2018–A low-cost earthquake early warning system provided a map of expected ground shaking around the epicenter of February’s magnitude 6.4 Hualien earthquake within two minutes of the mainshock’s start, according to a new study by seismologists at the National Taiwan University, which deployed the system.

The predicted shake maps “were in good agreement with the patterns of observed damage in the area,” said study lead author Yih-Min Wu, and individual seismic stations within the system were even able to provide two to eight seconds of lead time for shaking near the mainshock’s epicenter.

The report is one of eleven papers published in a Seismological Research Letters focus section, edited by Wu and Kuo-Fong Ma of National Central University in Taiwan on the damaging February 2018 earthquake sequence, which struck eastern Taiwan. A surface rupture from the earthquake through the city of Hualien on 6 February severely damaged several tall buildings and resulted in 17 deaths and 289 injuries.

Taiwan President Tsai Ing-wen inspects City of Hualien building that collapsed during 6 February 2018 earthquake. | Wikimedia Commons

In October 1951, three damaging earthquakes of magnitude 7 and above occurred near Hualien within 12 hours of each other, and several surface ruptures in the February 2018 event were similar to those of the 1951 earthquakes. The Hualien earthquakes occurred in a tectonic zone where the Philippine Sea Plate and Eurasian tectonic plates collide in complicated ways, prompting researchers to study the mechanisms of the 2018 Hualien earthquake in depth, write Wu and Ma in their preface to the focus section.

The region is likely to see another damaging earthquake relatively soon. Chung-Han Chan of Nanyang Technological University in Singapore and colleagues used two models to estimate the likelihood of another rupture along the Milun fault, which opened up in the 2018 earthquake. One model suggests there is a 53% rupture probability within 50 years, while a second model indicates a probability of 80% within the next 50 years.

Seismic stations located along the Milun fault recorded pulse-like velocities, which can be some of the most destructive seismic waves for medium and high-rise buildings, said Chun-Hsiang Kuo and colleagues. Soil liquefaction, which occurs when strong shaking from an earthquake turns soils into a jelly-like substance, and ground settling were sparse within the city of Hualien, but liquefaction did occur in the Port of Hualien, producing some minor settlement along the wharves there, said Yung-Yen Ko of National Cheng Kung University in Taiwan and colleagues.

Two days after the mainshock, Hao Kuo-Chen of the National Central University and colleagues deployed 70 temporary seismic stations around Hualien for 12 days, detecting 2192 aftershocks that mostly appear to have an extensional (stretching) mechanism, rather than the strike-slip motion and thrust of the mainshock.

Other papers within the focus section analyzed the rupture behavior of the 2018 earthquake sequence, including the fault-to-fault jumping rupture that occurred during the mainshock, and the use of repeating earthquakes to detect changes in the ground around the faults over time. Research teams studied ground deformation and fault rupture for the 2018 quake using several different techniques including strong-motion recordings, Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data, and optical satellite imagery.