Dominant periods and memory effect of the 2021 earthquake swarm in Hualien, Taiwan.

An earthquake sequence that occurred in the Hualien area during April 7 to August 30, 2021 has been recognized as two swarms by the Central Weather Bureau. Its earthquakes with ML ≤ 6.2 (ML = local magnitude) and focal depths ≤ 25 km were located in an area from 23°46’ N to 24°04’ N and from 121°25’ E to 121°42’ E. The Morlet wavelet technique is applied to analyze the dominant periods of temporal variations in numbers of daily events for the earthquake sequence in two magnitude ranges, i.e., ML ≥ 3 and ML ≥ 4. Results show that the dominant periods are 30.8 and 38.0 days when ML ≥ 3; while the dominant period does not exist when ML ≥ 4. The fluctuation analysis technique in the natural time domain is used to study the memory effect in the swarm for two magnitude ranges, i.e., ML ≥ 3 and ML ≥ 4. Calculated results show that the memory effect is stronger for the time sequence of magnitudes than for that of inter-event times and higher for ML ≥ 3 earthquakes than for ML ≥ 4 events. Consequently, only the short-term corrected memory effect was operative in the earthquake sequence of the Hualien swarms.Key points: To explain the data of the 2021 earthquake swarm in Hualien To describe the Morlet wavelet and fluctuation methods Results show that the memory effect is stronger for the time sequence of magnitudes than for that of inter-event times and higher for ML ≥ 3 events than for ML ≥ 4 ones; and the short- term corrected memory effect was operative in the Hualien swarms [ABSTRACT FROM AUTHOR]