Seismic scattering and absorption mapping from intermediate-depth earthquakes reveals complex tectonic interactions acting in the Vrancea region and surroundings (Romania)

The present study was performed during a stay at the University of Münster financed by a grant awarded by the German Academic Exchange Service (DAAD) in 2014. Data used in the present study were provided by the National Institute for Earth Physics (Romania) and processed within the National Data Centre in Magurele. Seismic Analysis Code (SAC) (Goldstein and Snoke, 2005) and GMT (Wessel et al., 2013) codes were used. We thank the College of Physical Sciences (University of Aberdeen) and the Santander Mobility Award for providing travel grant to LDS to complete this manuscript. We are grateful as well to the anonymous reviewer for his useful remarks which helped us to improve the paper.Peer reviewedPostprin

Evidence of strong lateral inhomogeneous structure beneath SE Carpathians and specific mantle flow patterns

The Vrancea region, in the South-Eastern Carpathians (Romania), represents a unique case among the seismic areas in the world taking into account the extreme concentration and persistence of seismicity and tectonic stress field. Our goal is to show that the particular shape of attenuation and shear wave splitting properties can be interpreted in the light of the decoupling and slab retreat processes, which hypothetically induce a specific configuration of the upper mantle flow. Delamination and break-off processes combined with retrograde motion of the slab imply lateral asymmetry in flow geometry and geotectonic properties. Particularly relevant is the strike-parallel flow localized in front of the downgoing slab, in contrast with the steeply dip flow along the slab in the back side. The local upper mantle flows around the descending seismic active body explains the contrast of the seismic wave attenuation in the back-arc region against fore-arc region and the strong anisotropy anomaly observed in the South-East Carpathians (time delays of 1.5–2 s). Outside the epicentral area, the coherent pattern of the shear wave splitting follows the trends of the deformation field as outlined by the GPS measurements, in favour of a strong coupling between the surface and mantle processes

The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database, and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt

The seismic attenuation signature of collisional orogens and sedimentary basins within the Carpathian Orogen

Sedimentary basins in collisional settings result from interactions within and between lithospheric plates and sublithospheric mantle. Imaging their structure brings fundamental constraints to both the extraction of hy-drocarbon or geothermal resources and seismic hazard analyses, especially in seismogenic areas affected by fluid percolation. Seismic attenuation is highly sensitive to stress, fluid saturation, and fluid-rock interaction and can often constrain small changes in the Earth's matrix better than seismic velocity. Here, we separate different attenuation mechanisms (scattering and absorption) at multiple frequencies and map them in space to constrain the properties of the Carpathian Orogen and the surrounding basins. The separation is achieved by determining S-wave peak delay times and late-time coda quality factors based on first-order Tikhonov inversion and analytical sensitivity kernels. We analysed 366 small-to-moderate crustal local earthquakes (0.7 < ML < 5.8) recorded by permanent and temporary stations operated by the Romanian Seismic Network between 2008 and 2021. Scattering and absorption appear to be frequency-dependent and highly heterogeneous throughout the region. High scattering and absorption characterise the Vrancea Seismic Zone, located in the Eastern Carpathian bend region, at all frequencies, likely due to high-stress rate and fluid inclusions. The seismically-active bend of the collisional orogen also shows high absorption and high scattering, particularly at low frequencies (similar to 3 Hz). Low scattering and high absorption features are observed across the Danubian section of the South Carpathians, marking the contact with the Pannonian Basin, which sits on top of a thin and highly-extended lithosphere. A transition from high to low-scattering regimes with increasing frequencies could mark small-scale heterogeneous structures in the Transylvanian Basin, an elevated sedimentary unit surrounded by high topography, comprising Cretaceous Neogene sediments deposited on top of oceanic ophiolites