Repeating crustal and intermediate-depth earthquakes from the Vrancea seismic region.

Vrancea seismic zone is the most important seismically active region in Romania, representing the main source of seismic hazard in the area and neighbouring countries. The largest significant earthquakes of the past century, M 7.7 and 7.4 in 1940 and 1977, caused major and widespread destruction. The intermediate-depth earthquakes from Vrancea have a particular space distribution, being constrained to a compact volume (60-180 km in depth and 20x50 km areal extent) and falling into the category of, so called, “seismic nests”, with a peculiar and not well understood seismogenic mechanisms.

We present first results of the repeating events identification for both the crustal and intermediate-depth activity from the Vrancea seismic region, obtained by multi-channel waveform-similarity (cross-correlation - cc) analysis for earthquakes extracted from the ROMPLUS catalogue. The analysed events cover the time-period of about 2.5 years (August 2016 – December 2018) and contains 1229 earthquakes with magnitude of 2.5 – 5.9. Of these events 630 correspond to crustal and 599 – to the intermediate-depth earthquakes. Our analysis identifies 37 families of similar events (cc > 0.7) with the largest one composed of 30 events. We observe that most of the repeating events families are located in the deeper portion of the Vrancea intermediate-depth seismic volume, at the depth greater than 100 km. The identified most active family corresponds to the average depth of 140 km. Most of the intermediate-depth families are characterized by the activity that is persistent in time over the analysed time-period. For the crustal seismicity, we identified over 35 similar event families, most of which corresponding to the short-lived activations of the local (fault) structures.

In the presentation, we will discuss how these results, in relation to the structural composition and tectonic regime of the region, can help to better understand the depth-dependent activity of the Vrancea seismic zone and whether there exists a potential correlation between the crustal and intermediate-depth seismicity.