Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures

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

Simulation of Seismic?Wave Propagation through Geometrically Complex Basins: The Dead Sea Basin

The Dead Sea Transform (DST) is the source for some of the largest earthquakes in the eastern Mediterranean. The seismic hazard presented by the DST threatens the Israeli, Palestinian, and Jordanian populations alike. Several deep and structurally complex sedimentary basins are associated with the DST. These basins are up to 10 km deep and typically bounded by active fault zones. The low seismicity of the DST, the sparse seismic network, and limited coverage of sedimentary basins result in a critical knowledge gap. Therefore, it is necessary to complement the limited instrumental data with synthetic data based on computational modeling, in order to study the effects of earthquake ground motion in these sedimentary basins. In this research we performed a 2D ground?motion analysis in the Dead Sea Basin (DSB) using a finite?difference code. Cross sections transecting the DSB were compiled for wave propagation simulations. Results indicate a complex pattern of ground?motion amplification affected by the geometric features in the basin. To distinguish between the individual contributions of each geometrical feature in the basin, we developed a semiquantitative decomposition approach. This approach enabled us to interpret the DSB results as follows: (1) Ground?motion amplification as a result of resonance occurs basin?wide due to a high impedance contrast at the base of the uppermost layer; (2) Steep faults generate a strong edge?effect that further amplifies ground motions; (3) Sub?basins cause geometrical focusing that may significantly amplify ground motions; and (4) Salt diapirs diverge seismic energy and cause a decrease in ground?motion amplitude

Geometrical focusing as a mechanism for significant amplification of ground motion in sedimentary basins: analytical and numerical study

We study the geometrical and material conditions which lead to focusing of seismic waves traveling across a concave velocity interface representing the boundary of a sedimentary basin within a denser rock. We approximate, using geometrical analysis for plane-waves, the combination of interface eccentricities and velocity ratios for which the seismic rays converge to a near surface region of the basin. 2-D finite difference modeling is used to compute Peak Ground Velocity (PGV) and spectral amplification across the basin. We show that effective geometrical focusing occurs for a narrow set of eccentricities and velocity ratios, where seismic energy is converged to a region of ±±\pm 0.5 km from surface. This mechanism leads to significant amplification of PGV at the center of the basin, up to a factor of 3; frequencies of the modeled spectrum are amplified up to the corner frequency of the source. Finally, we suggest a practical method for evaluating the potential for effective geometrical focusing in sedimentary basins

Seismoacoustic Coupled Signals From Earthquakes in Central Italy: Epicentral and Secondary Sources of Infrasound

In this study we analyze infrasound signals from three earthquakes in central Italy. The Mw 6.0 Amatrice, Mw 5.9 Visso, and Mw 6.5 Norcia earthquakes generated significant epicentral ground motions that couple to the atmosphere and produce infrasonic waves. Epicentral seismic and infrasonic signals are detected at I26DE; however, a third type of signal, which arrives after the seismic wave train and before the epicentral infrasound signal, is also detected. This peculiar signal propagates across the array at acoustic wave speeds, but the celerity associated with it is 3 times the speed of sound. Atmosphere-independent backprojections and full 3-D ray tracing using atmospheric conditions of the European Centre for Medium-Range Weather Forecasts are used to demonstrate that this apparently fast-arriving infrasound signal originates from ground motions more than 400 km away from the epicenter. The location of the secondary infrasound patch coincides with the closest bounce point to I26DE as depicted by ray tracing backprojections.Applied Geophysics and Petrophysic

The 2010 Haiti earthquake revisited: An acoustic intensity map from remote atmospheric infrasound observations

In the days following the January 12, 2010 Mw 7 Haiti earthquake the shaking intensity near the epicenter was overestimated and the spatial extent of the potentially damaging shaking was underestimated. This was due to the lack of seismometers in the near-source region at the time of the earthquake. Besides seismic waves, earthquakes generate infrasound, i.e., inaudible acoustic waves in the atmosphere. Here we show that infrasound signals, detected at distant ground-based stations, can be used to generate a map of the acoustic intensity, which is proportional to the shaking intensity. This is demonstrated with infrasound from the 2010 Haiti earthquake detected in Bermuda, over 1700 km away. Wavefront parameters are retrieved in a beamforming process and are backprojected to map the measured acoustic intensity to the source region. The backprojection process accounts for horizontal advection effects due to winds and inherent uncertainties with regard to the time of detection and the back azimuth resolution. Furthermore, we resolve the ground motion polarity in the epicentral region and use synthetics generated by an extended infrasound source model to support this result. Infrasound measurements are conducted globally for the verification of the Comprehensive Nuclear-Test-Ban Treaty and although the network was designed to provide global coverage for nuclear explosions in the atmosphere, it is shown in this paper that there is also global coverage for the estimation of acoustic shaking intensity. In this study, we lay the groundwork that can potentially make infrasound-based ShakeMaps a useful tool alongside conventional ShakeMaps and a valuable tool for earthquake disaster mitigation in sparsely monitored regions.Applied Geophysics and Petrophysic

Long-Term Infrasonic Monitoring of Land and Marine-Terminating Glaciers in Greenland

A period of 18 years of infrasonic recordings was analyzed from a microbarometer array (I18DK) in northwestern Greenland, near Qaanaaq. A huge number of infrasonic detections, over 700,000, have been made in I18DKs soundscape during the Arctic summers. Simultaneously identified were both calving events from marine-terminating glaciers and discharge related acoustics from a land-terminating glacier. This infrasonic activity is correlated to sea-surface and atmospheric temperature, respectively. Inter-yearly to daily variations were retrieved showing a strong variability in infrasonic detection rates and hence glacier activity. The highest number of infrasonic detections were found in recent years from the land-terminating glacier. The latter is supported by actual discharge measurements and partly by a discharge model. It is concluded that monitoring infrasound from glaciers can complement other techniques to remotely and passively get insights into glacier dynamics with high temporal and spatial resolution.Applied Geophysics and Petrophysic

CLEAN beamforming for the enhanced detection of multiple infrasonic sources

The detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals which conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e., following the Bartlett or Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a ’clean’ spectrum. A data-driven stopping criterion for CLEAN is proposed that relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data-set and is applied to data from multiple IMS infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic, that would have remained unidentified if conventional methods had been applied.Accepted Author ManuscriptApplied Geophysics and Petrophysic

Research Facilities for Europe’s Next Generation Gravitational-Wave Detector Einstein Telescope

peer reviewedThe Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy) and E-TEST in Liége (Belgium) and its surroundings. The ARC pursues the investigation of a large cryostat, equipped with dedicated low-vibration cooling lines, to test full-scale cryogenic payloads. The installation will be gradual and interlaced with the payload development. ETpathfinder aims to provide a low-noise facility that allows the testing of full interferometer configurations and the interplay of their subsystems in an ET-like environment. ETpathfinder will focus amongst others on cryogenic technologies, silicon mirrors, lasers and optics at 1550 and 2090 nm and advanced quantum noise reduction schemes. The SarGrav laboratory has a surface lab and an underground operation. On the surface, the Archimedes experiment investigates the interaction of vacuum fluctuations with gravity and is developing (tilt) sensor technology for the Einstein Telescope. In an underground laboratory, seismic characterisation campaigns are undertaken for the Sardinian site characterisation. Lastly, the Einstein Telecope Euregio meuse-rhine Site & Technology (E-TEST) is a single cryogenic suspension of an ET-sized silicon mirror. Additionally, E-TEST investigates the Belgian–Dutch–German border region that is the other candidate site for Einstein Telescope using boreholes and seismic arrays and hydrogeological characterisation. In this article, we describe the Einstein Telescope, the low-frequency part of its science case and the four research facilities