The 4 February 2018 Mw 6.2 Hualien earthquake (Taiwan): geodetic finite-fault rupture model and its potential impact on the occurrence of the 6 February Mw 6.4 event

In early February 2018, a series of earthquakes occurred offshore Hualien. At least 16 M 4.5 earthquakes, including one M 6.1 event, were observed at depths between 3 and 15 km. On 4 February 2018, a moderate-magnitude earthquake (Mw ~6.0–6.2) struck offshore Hualien, followed two days later by a Mw 6.4 event approximately 5 km away. Although occurring in the near-source region of the 6 February Mw  6.4 earthquake and only two days before, this event has been overlooked. For the 4 February 2018 earthquake, we used GNSS and strain time series data to perform a finite source inversion. This analysis shows the rupture of a ~ 25 km × 15 km asperity located on the shallow section (5 to 8 km depth)  of a subhorizontal fault plane with a dominant right-lateral strike-slip mechanism. We also performed the seismological analysis of the aftershocks that occurred between these two days. Aftershocks were clustered in the northeastern and southwestern directions of the 4 Feb event, with northeastern aftershocks situated within the maximum coseismic slip of 0.25–0.30 m, while southwestern aftershocks show a lower slip of 0.05 m. The total seismic moment of the earthquake is 2.4 * 10¹⁸ which corresponds to an earthquake of magnitude 6.18. The average static stress drop Δσ of the rupture is about 4 MPa, which is slightly larger than the characteristic earthquake stress drop of 3 Mpa. We also conducted a Coulomb stress analysis to analyze whether the southwestern aftershocks are linked to potential afterslip. The static Coulomb stress changes resolved onto the hypocenter of the 6 February M6.4 Hualien earthquake are approximately 0.1 MPa, which suggests that the latter may have been clock-advanced through static stress transfer.