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

Anica Otilia Placinta,Ioan Munteanu,Felix Borleanu,Mircea Radulian,Emilia Popescu

doi:10.3390/acoustics3020019

Abstract

Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower crust. The mainshock triggered a significant increase in the number of small-magnitude events spread over an unusually large area in the ECAB. The paper’s goal is to compute the source parameters of the earthquakes that occurred during the aforementioned sequence, by empirical application of Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple methods and seismicity evolution at regional scale is investigated. Our results highlight a still active deformation regime at the edge of the EE Craton, while the source parameters reveal a complex fracture of the mainshock and a very high-stress drop.

Highlights

The crustal seismicity developed in front of the SE Carpathians bending zone (SEC) is spread over the entire area between the Neogene Trotusand Intra-Moesian faults (Figure 1)
This is reflected in the variability of the focal mechanisms that show consistent stress regime with different localization: normal faulting on the eastern and southern flank of the Focsani Basin (FB), thrust faulting in the area adjacent to the Vrancea subcrustal earthquakes and strike-slip solutions preferably observed along the Peceneaga-Camena Fault [2]
By increasing the number of seismic stations in the area, we succeeded in increasing the results accuracy and in adding complementary information that could improve the seismotectonic characteristics of the region

Summary

Introduction

The crustal seismicity developed in front of the SE Carpathians bending zone (SEC) is spread over the entire area between the Neogene Trotusand Intra-Moesian faults (Figure 1). Crustal earthquakes are generated into a complex structure with significant lateral inhomogeneities involving different kinematics [1]. This is reflected in the variability of the focal mechanisms that show consistent stress regime with different localization: normal faulting on the eastern and southern flank of the Focsani Basin (FB), thrust faulting in the area adjacent to the Vrancea subcrustal earthquakes and strike-slip solutions preferably observed along the Peceneaga-Camena Fault [2]. The crustal seismicity in the SEC partly overlaps with the intermediate-depth earthquakes generated in a depth range of 60–180 km, beneath the Vrancea Zone (Figures 1 and 2). TRhRe—abAbmreavria,tiPoGnsOaRre—aPsofgoollaonwesl:e,VTRL—B—VrTanocpeaaluZosenies,mIMicFs—tatIinotnras.-Moesian Fault, PCF—Peceneaga-Camena Fault, AMRR—Amara, PGOR—Pogoanele, TLB—Topalu seismic stations

Methods

Results

Conclusion