Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

We present the regional ground-motion prediction equations for peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (PSA), and seismic intensity (MSK scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania. The prediction equations were constructed using the stochastic technique on the basis of the regional Fourier amplitude spectrum (FAS) source scaling and attenuation models and the generalised site amplification functions. Values of considered ground motion parameters are given as the functions of earthquake magnitude, depth and epicentral distance. The developed ground-motion models were tested and calibrated using the available data from the large Vrancea earthquakes. We suggest to use the presented equations for the rapid estimation of seismic effect after strong earthquakes (Shakemap generation) and seismic hazard assessment, both deterministic and probabilistic approaches.

We use ambient noise tomography to investigate the crust and uppermost mantle structure beneath the Carpathian-Pannonian region of Central Europe. Over 7500 Rayleigh wave empirical Green's functions are derived from interstation cross-correlations of vertical component ambient seismic noise recordings (2005-2011) using a temporary network of 54 stations deployed during the South Carpathian Project (2009-2011), 56 temporary stations deployed in the Carpathian Basins Project (2005)(2006)(2007) and 100 permanent and regional broad-band stations. Rayleigh wave group velocity dispersion curves (4-40 s) are determined using the multiple-filter analysis technique. Group velocity maps are computed on a grid of 0.2 • × 0.2 • from a non-linear 2-D tomographic inversion using the subspace method. We then inverted the group velocity maps for the 3-D shear wave velocity structure of the crust and uppermost mantle beneath the region. Our shear wave velocity model provides a uniquely complete and relatively high-resolution view of the crustal structure in the Carpathian-Pannonian region, which in general is validated by comparison with previous studies using other methods to probe the crustal structure. At shallow depths (<10 km) we find relatively high velocities below where basement is exposed (e.g. Bohemian Massif, Eastern Alps, most of Carpathians, Apuseni Mountains and Trans-Danubian Ranges) whereas sedimentary areas (e.g. Vienna, Pannonian, Transylvanian and Foçsani Basins) are associated with low velocities of well defined depth extent. The mid to lower crust (16-34 km) below the Mid-Hungarian Line is associated with a broad NE-SW trending relatively fast anomaly, flanked to the NW by an elongated low-velocity region beneath the Trans-Danubian Ranges. In the lowermost crust and uppermost mantle (between 30 and 40 km), relatively low velocities are observed beneath the Bohemian Massif and Eastern Alps but the most striking features are the broad low velocity regions beneath the Apuseni Mountains and most of the Carpathian chain, which likely is explained by relatively thick crust. Finally, most of the Pannonian and Vienna Basin regions at depths >30 km are relatively fast, presumably related to shallowing of the Moho consequent on the extensional history of the Pannonian region.

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Abstract. The purpose of this paper is to study the interaction of the Vrancea seismic activity (Romania) in space as result of Coulomb, static stress transfer during M = 7+ events. In this area, three large events occurred in 1977, 1986 and 1990 at mid-lower, lithospheric depths and with similar focal mechanisms. Assuming elastic rheology for the deforming rocks it is suggested that frictional sliding on pre-existing fault produced the 1986 M = 7.1 event (depth 131 km), that was possibly triggered by the 1977 M = 7.4 event (depth 94 km). We calculated a static stress transfer of 0.52-0.78 bar to the hypocentre of the 1986 event. On the contrary, the occurrence of the 1990 event is uncertain: it is located inside the relaxed (shadow) zone of the combined 1977 and 1986 static stress field considering an azimuth for maximum compression of N307 • E. It follows that, the 1990 earthquake most likely represents an unbroken patch (asperity) of the 1977 rupture plane that failed due to loading. However, if a different compression azimuth is assumed (N323 • E) then the 1990 event was also possibly triggered by static stress transfer of the 1977 and 1986 events (combined). Our modeling is a first-order approximation of the kind of earthquake interaction we might expect at intermediate lithospheric depths (80-90 to 130-140 km). It is also suggested that static stress transfer may explain the clustering of Vrancea earthquakes in space by the rupturing of two (possibly three) NW-dipping major zones of weakness (faults) which accommodate the extension (vertical elongation) of the slab.

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