## 4 Discussion  
Analyzing the distribution of P-wave (\(\mathrm{Vp}\)) and S-wave (\(\mathrm{Vs}\)) velocities, distinct geophysical properties can be identified. First, a general NE-SW trend which separates two regions of contrast in velocity. The regions in the north and northeast of the maps, with high \(\mathrm{Vp}\) and \(\mathrm{Vs}\) values, indicate the presence of denser and more rigid rocks, possibly associated with consolidated geological units such as magmatic rock masses or closed and inactive  
Figure 13: P wave velocity anomalies in vertical sections (**left**), S wave velocity anomalies (**middle**) and \(\mathrm{Vp/Vs}\) ratio (**right**). The locations of the profiles are shown on top in the horizontal section for P and S. Dots indicate the relocated sources at distances less than 20 km from the profile. Blue triangles are seismic stations, black line is the border line and blue line are rivers. \begin{table}
\begin{tabular}{c c c} \hline \hline
**No. of Iteration** & \
\begin{tabular}{c} **RMS of P** \\ **Residuals (s)** \\ \end{tabular} &
\begin{tabular}{c} **RMS of S** \\ **Residuals (s)** \\ \end{tabular} \\ \hline
1 & 0.3149 & 0.4755 \\ \hline
2 & 0.2293 & 0.2812 \\ \hline
3 & 0.2067 & 0.2353 \\ \hline
4 & 0.1965 & 0.2207 \\ \hline
5 & 0.1915 & 0.2142 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The RMS residuals reduction for the reference 1D model. faults. Areas with low Vp values may indicate the presence of less consolidated rocks, areas of intense fracturing, or the presence of fluids (groundwater, magmatic fluids, or gases). The NE-SW alignment crossing the epicentral area of the sequence of 2023 suggests the presence of an active fault oriented along this alignment. A significant low Vs anomaly overlaps with the central area of the map, where multiple earthquakes are located, suggesting the presence of weakly consolidated rocks with intense fracturing, as also observed in the representation of the main faults in the area (according to Project: CEEX Project No. 647/2005, Map of the Main Fault Systems in Romania). These negative anomalies also extend to the south and southeast to the Getic Depression, which is a sedimentary basin filled with 4-5 km of Paleozoic units [52]. These units consist of loose formations with lower seismic velocities compared to the older and well-compacted Danubian units to the north. Seismic waves propagate more slowly through sedimentary formations [53], but surface waves can be significantly amplified [54]. This amplification effect increases the potential for destruction, as surface waves tend to cause more damage to buildings and infrastructure. Moreover, the numerous faults that cross the Getic Depression in a W-E direction represent a secondary factor that justifies the slow seismic velocities in this sedimentary basin. The finger-like shape of the anomalies (the shape of a three-fingered hand) visible in Figure 12 is reflecting an artificial modulation of the distribution of the anomalies due to the uneven ray coverage (the fact that the events are tightly clustered in the center and the stations are distributed over a large area giving a radial system of rays that does not provide sufficient ray intersections (see Figure 3)). S-wave velocities are more sensitive to the presence of fluids, so their decrease in these regions may indicate the presence of fluids in crustal fissures. The Vp/Vs ratio in the central area is high, with values \(\geq\)1.8-1.9, which may suggest the presence of fluids in fractured rocks, contributing to a stronger decrease in S-wave velocity than in P-wave velocity. According to Figure 12, this area matches with the seismically active region, suggesting that fluids [55] play a role in fault rupture and slip processes. In the first stage of the sequence process, the hypocenters concentrated at lower depth (around 20 km) and then propagated upward [56].