The mantle transition zone (MTZ) is a critical feature of the Earth's mantle, sensitive to both thermal and waterrelated influences due to surface geodynamics. This study investigates the MTZ using topside P-wave reflections derived from ambient seismic noise, an often underutilized resource in the investigation of upper mantle processes. Using a database of inter-station cross-correlations from the broadband Romanian National Seismic Network, we extract these topside reflections with a processing method inspired from common-conversion point stacking: we bin the data according to reflection points in a gridded approach, align them using an adjoint tomography model of Europe, and phase-weight stack them to estimate empirical Green's functions for each grid. We then calculate the travel-time difference between observed and expected arrival times and iteratively adjust the discontinuity depths in the velocity model to match the time delays. Comparisons with two regional tomography models reveal several key correlations, including a depressed 660 km discontinuity beneath a highvelocity MTZ region, often interpreted as an accumulation of cold, recycled oceanic slabs. The Vrancea slab is also observed to penetrate the MTZ, significantly uplifting the 410 km discontinuity. Using Clapeyron slopes, temperature anomalies from these depth deviations are estimated, providing insights into the extent of slab subduction and its influence on upper mantle deformation and thermal anomalies.
The study area is located in the western branch of the Southern Carpathians in Romania, between the Olt River and the Danube. The data on seismicity recorded before 1999–2000 indicate a sporadic seismic activity, mostly of small magnitude. A few events of magnitude above 5 (maximum magnitude Mw 5.7) have been reported through historical data, which are inherently imprecise. Recently, several significant crustal sequences occurred in the region in 2011–2012 (eastern part of Târgu Jiu city), 2011 and 2013 (Haţeg basin), 2014–2015 (Caransebeş–Mehadia basin), 2020 (Orşova basin). The most spectacular sequence was recorded at the contact between Southern Carpathians and Getic depression, in the northern part of Târgu Jiu city, starting in February with two shocks of magnitude above 5, and continuing for more than one year with more than 4000 aftershocks. The new recorded data are particularly valuable for interpreting the present-day seismotectonics and for redefining the regional seismic hazard. The motivation of the present study naturally came out from the need to elucidate what makes the regional tectonics able to suddenly activate an unexpected level of seismicity for an area previously characterized by apparently sporadic and insignificant seismic activity. In order to characterize the stress field, we collected all the earthquake focal mechanisms available from source catalogues that we considered to be of acceptable quality. For inversion, we defined 8 individualized clusters of earthquakes associated either to active tectonic faults, or to intramontane extensional basins, for which we assumed that the individual focal mechanisms follow the trends of a homogeneous stress field. The results show a clear model of mostly extensional stress field responsible for generating the earthquake swarms during the last decades. The configuration of the horizontal stress directions reveals a complex tectonic setting following a main NE-SW trending associated with the transcurrent movement along the active faults in the Danubian region (e.g., Cerna–Jiu fault, Timok fault) and significant rotations toward a NW-SE alignment in the intramontane basins and in the Getic depression. It reveals an overall extensional stress field predominance in the Southern Carpathians.
Accurate characterization of shallow groundwater systems is essential for hydrogeological assessment, groundwater management, and geotechnical planning. Seismic refraction is a commonly used non-invasive method for subsurface characterization. However, its effectiveness is strongly dependent on lithological conditions and the associated P-wave velocity contrast. This study evaluates the capability of seismic refraction to delineate shallow saturated zones in three contrasting geological environments in Romania: an alluvial setting (Câmpul cu Maci), a loess-clay-dominated setting (Seismologilor), and a siliciclastic and evaporitic environment (Năruja). Seismic data were acquired using 24- and 48-channel systems and processed through first-arrival traveltime tomography. The results show that at Câmpul cu Maci and Năruja, a clear velocity increase from approximately 400–600 m/s to over 900–1300 m/s allows delineation of the top of the saturated zone at depths of approximately 4.5-6 m. In contrast, in clay-rich deposits, no significant velocity contrast is observed, and the top of the saturated zone remains undetectable. To complement the seismic refraction tomography results, MASW-derived shear-wave velocity profiles were analyzed along two profiles, providing additional constraints on near-surface stiffness and lithological variability. The comparison of the three sites shows that detectability of the shallow saturated zone using seismic refraction is primarily controlled by lithology rather than by water-table depth. These results highlight the importance of integrating geological information into the interpretation of seismic refraction data.
We study productivity disparities across metropolitan and non‐metropolitan areas in Great Britain. Spatial disparities in productivity are large and persistent. Using a development accounting framework, we show that differences in area size and in the spatial distribution of human and business capital are key explanatory factors. A combination of area size and human capital explains 30% of the productivity variance, increasing to 43–57% once we add measures of business capital stocks. Applying our framework to a case study of Greater Manchester, we show that large increases in both types of capital are needed to narrow productivity disparities with London, illustrating the scale of the challenge for policies aimed at reducing spatial inequality.
The 24 June 2026 Venezuela earthquake doublet occurred along a complex transform-fault system. Teleseismic P waves encompassing both cataloged events were analyzed using potency density tensor inversion with seven structure models. All models indicate predominantly eastward rupture over approximately 220 km, including several stages of supershear propagation. Three apparent rupture stalls coincided with changes in the inferred fault orientation, suggesting that fault bends or transitions between fault segments may have influenced rupture propagation. A high-slip-rate subevent occurred about 90 km behind the apparent rupture front 37 s after the origin time of the first event.Whether this subevent reflects back-propagating rupture or a relatively independent rupture episode remains unresolved, but rapid eastward rupture appears to have initiated or resumed afterward. This compound rupture process raises the possibility that parts of the source region experienced repeated strong shaking.
