Geophysics Based Ideas on Structural Setting of 2023 Seismic Sequence SW South Carpathians, Romania

RATIONALE

Following the 2023 February 6 disastrous earthquakes (M_w7.8 & M_w 7.7) that struck southern & central Türkiye, and northern & western Syria, an intense crust seismic sequence was triggered southwestern South Carpathians, Romania.

Hosted by Gorj County, north Târgu Jiu city, its capital, the sequence started with twin unusual high magnitude earthquake (M_L5.2 on February 13, and M_L5.7 on February 14) and continued for months with more than 4000 seismic events.

The area was known for quasi-continuous seismic activity, but surprising was the unusual intensity of earthquakes triggered and time extent of the sequence, which had a strong societal impact by scarring the population and provoking economic loss.

The paper brings some geophysics inferred ideas about tectonic circumstances for triggering and maintaining the long-lasting seismic sequence.

 

METHOD

Objective of the undertaken research was twofold: (i) to outline the overall tectonic setting of the area were the seismic sequence occurred, and (ii) to unveil more detailed structural circumstances of the largest magnitude earthquakes.

The approach was mainly based on gravity data mining and interpretation. In a first step, various filters were applied to the complete Bouguer anomaly, like low pass filtering and upward continuation for separating regional and local effects, horizontal and vertical gradients, for emphasising the faults track, etc.

In a second step, more advanced data processing, including inversion and forward modelling was conducted especially in areas of interest.

For better interpreting/understanding the obtained results, data provided by other geophysical investigations, like e.g., geomagnetism, seismology, or seismic tomography were also employed.

 

RESULTS

Among the main results it is worth mentioning: (i) overall location of seismicity along the northern flank of the westernmost segment of Getic Depression; (ii) earthquakes triggering mainly along faults striking WSW – ENE, as crustal echoes of the lithospheric contact between Moesian microplate (MoP) and Intra-alpine microplate (IaP); (iii) some earthquakes were also triggered along sub meridional faults.

The most active area appears as a highly fractured zone, overlaying an underground mass excess with high magnetisation, echoed by a gravity high associated with a geomagnetic anomaly. The basalt dykes cropping out in the area suggest the in-depth presence of mafic/ultramafic intrusive. The assumption is supported by the existence of a hidden high velocity body unveiled by seismic tomography.

 

FINAL REMARKS

To conclude, the unusual intense earthquakes of the Gorj sequence were likely triggered by a sudden increase of tectonic stress in the area due to an acceleration of the Black Sea microplate acting upon MoP. The NW push was WSW redirected along the transform contact between MoP and IaP. Among the others, the strain activated a seismic prone structure generated by the magmatic “diapirism” of an in-depth hidden mafic intrusive, likely belonging to Severine Nappe. The uplift of the mafic dome, had intensively fractured its crustal roof, creating a complex fault system along which earthquakes were triggered.