Rupture dynamics of a moderate intraplate earthquake sequence in the Târgu Jiu region, Romania

Abstract

A significant seismic sequence occurred in 2023 in the Târgu Jiu region (Romania), at the boundary between the Getic Depression and the Southeastern Carpathians. The sequence began with two moderate mainshocks (Mw 4.9 and Mw 5.4) located in the active crust (~ 20 km depth) and was followed by intense aftershock activity lasting more than one year. This well-recorded sequence provides an opportunity to investigate rupture dynamics in a complex intraplate extensional setting. We combine relocated hypocenters, focal-mechanism solutions, waveform inversion results, and simplified self-similar dynamic rupture modeling to explore the fault geometry, slip distribution, and stress conditions associated with the mainshocks. The rupture scenarios suggest predominantly unilateral upward rupture propagation toward northwest along a southeast-dipping normal fault. The modeled ruptures are characterized by stress drops of ~ 3.6 MPa (Mw 4.9) and ~ 4.7 MPa (Mw 5.4), and source duration of ~ 1.3 s and ~ 2.0 s, respectively. The simulated rupture evolution indicates heterogeneous slip, with peak slip values of ~ 0.05 m and ~ 0.12 m concentrated near the nucleation area. Possible rupture directivity effects toward the northwest are qualitatively compatible with the modeled waveform patterns and the asymmetric aftershock distribution. The spatial distribution of more than 4400 aftershocks highlights the role of complex pre-existing crustal structures in controlling rupture propagation and seismicity in the region and possible fluid-driven effects.