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

Abstract

The Vrancea region, located at the southeastern edge of the Carpathians arc bend, is a region of intense seismicity, whose major earthquakes produce hazard in southeastern Europe. Despite the consequent focus of the geophysical and geological community on providing accurate structural and dynamical models of Vrancea, these are still subject to numerous controversies and debates. In the present study, we use intermediate-depth seismicity recorded by the broadband stations of the Romanian Seismic Network between 2009 and 2011 to measure S-wave peak delay times and late-time coda quality factors. After mapping these two quantities in space, a cluster analysis provides a quantitative structural interpretation of the region in terms of different attenuation mechanisms affecting the seismic wave field, i.e. seismic scattering and seismic absorption. The results show that scattering is higher west and northwest of Vrancea, while absorption dominates in the Focsani Basin, located in the forearc region. In general, we obtain higher absorption in stable regions, with patterns emphasized at high-frequency affected by the presence of hydrocarbons and natural gas reservoirs in the upper crustal layers. Regions characterized by active seismicity and structural heterogeneity show higher scattering, spatially correlated with the highest velocity contrasts and the lowest density. The high-frequency scattering/absorption contrasts obtained using the cluster analysis depict a southwest-to-northeast lithospheric contrast, following the epicentral trend of Vrancea earthquakes, and characteristic of either lithospheric subduction or delamination. Low-frequency cluster analysis results, sampling deeper Earth layers, mark a unique high-absorption trend perpendicular to the epicentral trend, feasibly linked to Neogene volcanism, and induced by the back-arc mantle upwelling. Its most recent expression is Ciomadul volcano, located at the northwestern limit of the absorption trend.

Upper mantle deformation signatures of craton–orogen interaction in the Carpathian–Pannonian region from SKS anisotropy analysis

“…Beneath the East Carpathians, upwelling of low-Vp asthenosphere has been proposed and supported with independent seismic measurements (e.g. Ren et al, 2012;Borleanu et al, 2017). The upwelling hypothesis (Gögüş et al, 2016;Maţenco, 2017;Şengül Uluocak et al, 2019) is also supported by the occurrence of post-collisional volcanism (Seghedi et al, 2011), and the observed high heat flux values (up to mW/m 2 locally, Demetrescu and Veliciu, 1991).…”

Seismic wave simulation using a 3D printed model of the Los Angeles Basin

“…We note that seismic attenuation, i.e., 1/Q, would also contribute to frequency-dependent reduction of the ground motion, but the effect is secondary. Sedimentary basins generally have a relatively high attenuation, or low Q 24 – 26 , resulting in gradual decrease in seismic energies with distance. This would lead to a progressive decline in high frequency contents both within and across basins, which cannot explain our observations of the instantaneous depletion of high-frequency energy at the edges of basins (Fig.…”

Fast Changes in Seismic Attenuation of the Upper Crust due to Fracturing and Fluid Migration: The 2016–2017 Central Italy Seismic Sequence

“…High peak delays and high Q c −1 detect intrusions in mountain chains and magmatic systems (De Siena et al, 2016). These parameters are reliable markers of fluid-induced fracturing leading to earthquakes at the regional scale (Borleanu et al, 2017). In fault networks like those ruptured by the AVN, Napolitano et al (2020) found that high-scattering and highabsorption patterns obtained by increasing frequencies map the migration of the historical seismic events from the 16th century until the Pollino seismic sequence.…”

“…Takahashi et al (2007) first used peak delay times to map volcanic areas in northeast Japan. High peak delays mark fractured volumes across the Pyrenees , the eastern portion of the Siletz terrane in the Western US (De Siena et al, 2016), and the Vrancea region in Romania (Borleanu et al, 2017). Within fault networks, peak delay increases are visible at all frequencies when waves cross fractured geological volumes, as the carbonates of the Pollino seismic gap, in Southern Italy (Napolitano et al, 2020).…”

“…Q c −1 strongly depends on the frequency in active tectonic regions (Sato et al, 2012). At low frequencies, high Q c −1 anomalies map surface geology and sedimentary basins in the Pyrenees , the Alps (Mayor et al, 2016), and Vrancea (Borleanu et al, 2017). At higher frequencies, the change in coda composition from surface to body waves can increase depth sensitivity (De Siena et al, 2016;Mayor et al, 2016;Gabrielli et al, 2020).…”