Understanding the February 6, 2018, Hualien earthquake sequence through catalog compilation

  • The Hualien earthquake foreshocks exhibited low b-value and high seismicity rates
  • The b-values of the foreshocks were lower than those of the respective aftershocks
  • We established an empirical relations between the aftershock spatial range and Mw

In this study, we compiled the Central Weather Bureau (CWB) data in order to study the Gutenberg–Richter magnitude–frequency slopes (i.e., b-values) and seismicity rates of significant earthquake sequences in the area of Hualien. A total of ten events between 1973 and 2018 were selected for analysis. Using time windows 72 h before and after the main shock, we first examined the existence of detectable foreshocks and then applied the Gutenberg–Richter law and Omori’s law to determine the b-value and seismicity rate, respectively. The compiled results were used to assess the abnormalities and other characteristics of the 2018 Hualien earthquake for their forecast potential. We concluded that seismicity rates alone are not sufficient to forecast whether a greater main shock is forthcoming. The foreshock sequence of the 2018 Hualien earthquake was characterized by a low b-value and a high seismicity rate. Another earthquake with a prominent foreshock sequence occurred in 1990, but it showed a different relationship between the magnitude and the seismicity rate. For both the 1990 and the 2018 Hualien earthquakes, we found that the b-values of the foreshocks were lower than those of the respective aftershocks. The b-values for earthquake sequences are depressed relative to the background seismicity in the area. The mechanisms proposed for temporal variation in b-values are briefly reviewed to explain the observed b-value patterns. Finally, we established an empirical relationship with moment magnitude (Mw) in order to estimate the spatial range of aftershock distributions in the area of Hualien for shallow earthquakes (hypocenter depth ≤ 20 km)with Mw ≥ 5.3.

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