Source region of Mw7.4 of 6 Feb. has a complicated 3D lithospheric structure: interaction between the Ryuku subduction and accretionary wedge, the accreted Luzon arc (now forming the coastal range), the Philippines plate, and the Taiwan orogenic prism (central range and foothills).

This E-W section at Hualien latitude only shows a 2D view of the complex 3D structure. If the ~20-40km hypocentral depth of the Mw7.4 is true*, it's difficult to figure out which fault ruptured. Could be one not reaching upper crustal levels. Just hypotheses.

Section from Lallemand 2014,

* Saying "If the ~20-40km depth of the rupture is true" is a big IF as the Taiwan seismic center puts epicenter of the main shock at 15km only. Needs further studies.

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Two magnitude 6+ event happened in 2018 and 2019 in the very same region than the 3 Februaryb 2024 Mw7.4. The 2018 Mw6.4 had a complex (strike-slip + reverse, several fault segments) shallow crustal rupture.

See: and

The Eastern coast of Taiwan is prone to big as this map from Tang and collaborators shows. Main shallow crustal active faults are the Central Range Fault (W-dipping, earthquakes and ruptured zones in red and orange) and the Longitudinal Valley Fault (E-dipping, in blue) 5/n

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