ALBERT

Description: Receiver functions of teleseismic waveforms recorded at four Hungarian permanent broadband stations have been analyzed using semilinearized and stochastic inversion methods to estimate the crustal thickness and S wave velocity structure in the Pannonian Basin. The results of both inversion methods agree well with the crustal thicknesses obtained by previous seismic refraction and reflection studies in the regions which are densely covered with seismic lines (28 and 27 km in westernmost and southern Hungary, respectively) and suggest a thicker crust compared to what was known before beneath the Transdanubian and Northern Ranges (34 and 33 km, respectively). The comparison of the one-dimensional shear wave velocity models derived by the different inversion methods shows that, in case of simple, smoothly varying structures, the results do not differ significantly and can be regarded as absolute velocities. Otherwise, the recovered velocity gradients agree, but there are differences in the absolute velocity values. The back-azimuthal variations of both radial and tangential receiver functions are interpreted as dipping structure and as waves sampling different geological areas. The signature of the deep structure on low-frequency receiver functions suggests a strong velocity contrast at the 670-km discontinuity. The vanishing 410-km boundary may be attributed to the remnant of a subducted oceanic slab with increased Poisson’s ratio in the transition zone.

Description: The depths of the 410 and 660 km phase transitions provide critical information on the thermal state of the mantle transition zone (MTZ) and, by implication, on the circulation of the upper mantle. We analyze converted seismic waves to produce a high resolution image of these discontinuities beneath the Pannonian Basin of Central Europe. Models to explain the extension of the basin in the Middle Miocene involve subduction roll-back and gravitational instability of an over-thickened lithosphere. We find that cold dense material appears to have accumulated in the MTZ, consistent with the idea of supply from several subduction episodes, or upper mantle overturn. Variation in depth of the 660 km discontinuity by as much as 40 km suggests that cold lithospheric mantle is ponding on top of this phase transition. Where the phase transition is deepest, eventual transfer of this material into the lower mantle may be expected to develop.

Description: Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (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. Although 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 quiet period 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 human activities.