Irregular rupture process of the 2022 Taitung, Taiwan, earthquake sequence.

In September 2022, two destructive earthquakes of moment magnitude (M) 6.6 (foreshock) and 7.1 (mainshock) occurred in Taitung County, south-eastern Taiwan. To understand their complex rupture processes, we analysed these earthquakes using the Potency Density Tensor Inversion method, which can stably estimate the rupture propagation process, including fault geometry, without overfitting the data. The analyses revealed that the major rupture of the foreshock propagated towards shallow depth, in a south-southwest direction, following an initial rupture that propagated towards the deeper part of the fault. The mainshock, with its epicentre on the north-northeast side of that of the foreshock, consists of two distinct episodes. During the first episode (0-10 s), the initial rupture propagated north-northeast, through a deep path, followed by the main rupture that propagated bilaterally in a north-northeast and south-southwest direction. The second rupture episode (10-16 s) started near the hypocentre of the mainshock, and the rupture propagated towards the shallow side of the fault. The results suggest that the stress concentration from both the foreshock and mainshock's first rupture episode may have caused the second rupture episode in the high fracture surface energy area between the foreshock and the first rupture episode of the mainshock. The irregular rupture process of the foreshock and mainshock may reflect the heterogeneity of stress and structure in the source region.