The source mechanism of the seismic events during the sequence of the moderate-size crustal earthquake of November 22, 2014 of Vrancea region (Romania)

Abstract

The moderate-size earthquake (ML 5.7) which occurred on November 22, 2014 in Vrancea region (Romania), is the largest crustal event instrumentally recorded. Its aftershock sequence lasted around 70 days, 222 earthquakes with M L ≥ 0.1 being located using the records collected by the Romanian seismic network. The seismic sequence occurred mainly in the lower crust (depths greater than 25 km), and the epicenter distribution-along a NNE-SSW direction-follows the orientation of the Vrancea crustal earthquakes alignment. The spatio-temporal distribution of the seismic activity, as well as the seismic energy release during the seismic sequence are analysed in detail, and the focal mechanisms of the largest events-34 shocks with local magnitude ≥ 1.8-are determined using reliable P-wave polarities and amplitude ratios. Taking into consideration that the moderate-size shock on November 22, 2014 is the strongest instrumentally recorded crustal earthquake in the region, its focal mechanism provides highly relevant seismological information on the deformation field in front of the Carpathian bend. The obtained fault plane solution indicates normal faulting with a dominant dip-slip component; both nodal planes being oriented NW-SE.

Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting

“…It is worth noticing that for the event that occurred in 12012015, 06:08 (GMT), M L = 4.2, our results indicate a reverse focal mechanism, with the nodal planes NE-SW oriented, which contradicts the normal fault plane solution determined by [32]. We assume that the fault mechanism determined by the above study is flawed due to the event location, which in their study is reported as being located ~40 km towards the north, and at a greater depth (with ~20 km) as compared to the location provided by the ROMPLUS catalogue.…”

“…For the mainshock, the fault plane solution indicates normal faulting with both nodal planes NW-SE oriented, in the direction of the Peceneaga-Camena Fault zone. Our determination shows a good similarity with the solutions computed by several international centers (INGV, USGS, GFZ), as well as with the solution of [32]. The compression axis is plunging almost vertically, while the extension axis is almost horizontal W-E oriented.…”

“…The overall mechanisms highlight a wide variety for earthquakes associated or triggered by the 2014 Mărăs , es , ti seismic sequence, although most of these events occurred in the Focs , ani Basin, a thick Neogene foredeep sediment layer as thick as 13 km located near the Carpathian bend zone [70]. Although the region is relatively narrow, this variety of fault plane solutions is not uncommon for the SE Carpathians foredeep area as shown by other studies [7,31,32,54,55] due to the geological complexity of this region, pointed out on one hand by the collision of three major units with different geometries and characteristics, all representing cratonic continental platforms [82,83] and on the other hand by the significant subsidence, although it is still uncertain whether these faults influenced the lateral variations in the thin-skinned thrusting kinematics [83].…”

“…The previous investigations dealing with the 2014 Mărăşeşti sequence limited themselves to the evaluation of the macro-seismic effects [31] and the computation of the focal mechanisms for the main event and 32 aftershocks using P-wave polarities and amplitude ratios [32].…”

A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration

“…The fault plane solutions are obtained using the FOCMEC code 77 using the velocity model of 7 and integrated in the SEISAN software 78 . These solutions were estimated from P-wave polarities for events with at least 10 records, filtered based on the weight assigned to the P onset (discrete classed from 0, best, to 3, worse) keeping only polarities for picks with weights 0 and 1 and rejecting those with weight 2 and 3, therefore solutions' confidence level is reliable 79 . The location of the seismic stations and seismic events can be seen in Fig.…”