## 2 Data and Methodology of Inversion  
### Data Description  
The Targu Jiu earthquake sequences have their epicenters located in a distinctive area at the interface between the South Carpathian curvature and the Getic Depression(South Carpathian Foredeep). The seismic sequence of 2023 began on 13 February with a significant double shock of magnitudes Mw = 4.8 and Mw = 5.4, followed by over 4000 aftershocks, with magnitudes ranging from 1.1 to 4.6 [12; 13; 14]. Considering that the layout of the seismic network during the seismic sequence was not the most suitable for a seismic tomography study, the dataset was extended with more than 1000 seismic events that occurred in the regions surrounding the studied area recorded since 2016. The dataset used in the study consists of 5323 earthquakes (see Figure 2) that occurred in the Targu Jiu region and its surroundings from 2016 until December 2023. More than 3900 earthquakes are derived from the seismic sequence that started on 13 February 2023, in the northwest part of Targu Jiu.  
The initial locations were performed using the LocSAT [28] algorithm embedded within the Antelope v5.14 software [29], which is operated by the National Institute for Earth Physics (NIEP) for routine earthquake locations. A total number of 108,016 picks with 51,598 P-data and 56,418 S-data from 133 seismic stations (Figure 2) were processed. The Romanian Seismic Network (RSN) [30] operated by the NIEP, designed to monitor and analyze the seismicity of Romania, covers the entire country surface with an average density of 30 km inter-station distance. The RSN's capabilities are further strengthened by collaboration protocols with neighboring countries, allowing for NIEP to acquire and archive seismic data from additional seismic stations located outside Romania [31]. This cross-border integration enhances the network's ability to detect and analyze borders or regional seismic events. At present, NIEP is an EIDA (European Integrated Data Archive) node collecting, archiving and providing data for the neighboring countries: \"[[https://www.orfeus-eu.org/data/eida/nodes/NIEP/](https://www.orfeus-eu.org/data/eida/nodes/NIEP/)]([https://www.orfeus-eu.org/data/eida/nodes/NIEP/](https://www.orfeus-eu.org/data/eida/nodes/NIEP/)) (accessed on 21 May 2025)\". Most of the  
Figure 2: Location of seismic events and vertical sections used for seismic tomography in the study region. Black dots are the events. Colored triangles are seismic stations (blue for RSN, green for AdriaArray and red for outside the border stations).  
seismic stations selected for our study, shown in Figure 3, are part of the RSN. Some stations belong to the AdriaArray project [32] which aimed to investigate the entire Adriatic plate in southeastern Europe by covering the area with a dense temporary seismic network.  
All these events have been reviewed and included in the ROMPLUS earthquake catalog [33; 34], respecting localization quality criteria such as RMS < 1 s or GAP < 120, where GAP represents the maximum azimuthal gap between stations surrounding an event, used to evaluate the reliability of its location. For the tomographic inversion, we applied additional criteria to select the input dataset: at least 6 picks per event and the absolute values of residuals after source location in the 1D model should not exceed 0.7 s. The final dataset consists of 105,976 arrival times from 5281 events with 50,533 P-wave and 55,443 S-wave arrival times. Figure 3 shows the ray coverage of the dataset.