Some statistical characteristics of the 2024 ML7.2 Hualien, Taiwan earthquake and its aftershocks

An M L 7.2 ( M w 7.3) earthquake occurred near Hualien in eastern Taiwan on April 2 (April 3, local time), 2024. Numerous aftershocks immediately followed the mainshock. The spatial distribution and time variation of the mainshock and its aftershocks from April 2 to December 31, 2024, were investigated in this study. Most aftershocks were located around the Lingding fault and Meilun fault in northern Hualien. The primary aftershock sequence occurred from April 2 to April 21, and the secondary one happened on April 22 in the southern part of the narrow and long aftershock zone. The length of the fault inferred from aftershock activity is estimated to be about 80 km. The depth distribution of the number of events shows that the number first increases with depth and peaks at 8 km, then decreases with increasing depth. This depth distribution exhibits a frictional rupture/quasi-plastic (FR/QP) transition. The temporal variations of aftershocks in terms of latitude and focal depth within the aftershock area are also investigated. The b value of the Gutenberg-Richter frequency-magnitude relationship for the Hualien aftershock sequence is 1.05 using the least-squares method and 0.75 using the maximum likelihood method. The former is a better fit to the observations than the latter. The inferred Omori-Utsu law, i.e., n(t) = k/(t + c) p , fits the time variation of aftershocks well for magnitudes M L ≥ M c =3.6. Results show p = 1.05 and c = 0.11 for primary aftershocks and p = 2.70 and c = 1.94 for secondary aftershocks. The p-value for primary aftershocks is lower than that for secondary ones, thus indicating a faster decay of events for the latter than for the former.