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.

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In the days following the January 12, 2010 M_w 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.

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Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures tomitigate 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 thelongest and most prominent global anthropogenic seismicnoise reduction on record. Although the reduction isstrongestatsurfaceseismometersinpopulatedareas, this seismic quiescence extends for many kilometersradially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signalsfrom subsurface seismic sources that would have been concealed in noisier times and to benchmark sources ofanthropogenic noise. A strong correlation between seismic noise and independent measurements of humanmobility suggests that seismology provides an absolute, real-time estimate of human activities.@en

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.@en

The ShakeMap is a key component in the initial relief efforts following an earthquake disaster. It depicts the distribution of shaking intensity in the epicentral region and is used to guide emergency responders to the region. In regions where seismic instrumentation is limited, such ShakeMaps are poorly constrained and can take days to generate. We show, that pseudo-ShakeMaps that indicate the relative shaking intensity, can be generated, within minutes, from enhanced processing and modeling of infrasound. Furthermore, the source mechanism can be retrieved. This is illustrated with infrasound from the 2010 Mw 7.0 Port-au-Prince, Haiti earthquake, detected in Bermuda, over 1700 km away from Haiti. The pseudo-ShakeMap and focal mechanism retrieved in this study are in good agreement with the USGS estimated ShakeMap and the Global CMT moment tensor. Such observations are made possible by: (1) An advanced array processing technique that enables the detection of coherent wavefronts, even when amplitudes are below the noise level, and (2) Backprojection of observed pressure perturbations to ground motions in the epicentral region while accounting for advection effects in the atmosphere. We support our observations with an example using the Rayleigh integral to generate synthetic waveforms from four quadrants of an earthquake focal mechanism. Synthetics are then processed to retrieve the relative sense of motion in each quadrant. The current infrasound networks routinely detect earthquakes and allow for an unprecedented global coverage. This makes infrasound as an earthquake mitigation technique feasible for the first time.@en

The 2017 North Korean nuclear test gave rise to seismic and low-frequency acoustic signals, that is, infrasound. The infrasonic signals are due to seismo-acoustic coupling and have been detected on microbarometer array I45RU in the Russian Federation at 401 km from the test site. I45RU is part of the International Monitoring System for the verification of the Comprehensive Nuclear-Test-Ban Treaty. We analyze the seismo-acoustic coupling by making use of array-processing and backprojection techniques. The backprojections show that infrasound radiation is not confined to the epicentral region. More distant regions are found to be consistent with locations of topography, sedimentary basins, and underwater evanescent sources. The backprojections can be used to estimate the average infrasonic propagation speed through the atmosphere. We discuss these findings in the context of infrasound propagation conditions during the sixth nuclear test. It is suggested that propagation from the test site to I45RU may have occurred along unexpected paths instead of typical stratospheric propagation. We present several scenarios that could be considered in the interpretation of the observations. Electronic Supplement: Details on signal characterization and infrasound propagation conditions.

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In this study we analyze infrasound signals from three earthquakes in central Italy. The M_w 6.0 Amatrice, M_w 5.9 Visso, and M_w 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.

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The Eastern Mediterranean contains two active tectonic borders: the Dead Sea Transform (DST) and the Cyprus Arc. Along the DST system of faults several deep sedimentary basins are found. Due to moderate seismicity and sparse spatial coverage of the Israel Seismic Network, ground motion analysis atop the basins is limited. In this paper, we present the results of two different approaches for ground motion amplification analysis. For the Amiaz Basin (Dead Sea region) we performed 3-D, high resolution numerical model using different source types and locations. We use the abundance of seismically induced clastic dikes to constrain the results of the numerical model and show the complexity of structure-source interaction. Specifically, we show the importance of seismic shielding by high velocity structures. For the Zevulun Valley, which underlays the densely populated and industrialized area of Haifa Bay, we present the results of seismic monitoring campaign using a transportable array. We calculate spectral amplification factors using out of basin reference stations and show the limitations of standard 1-D analysis.@en

The Zevulun Valley (ZV) is a sedimentary basin underlying the heavily populated and industrialized petrochemical hub of Haifa Bay, Israel. With active tectonic faults at close range and a mixture of large population and vulnerable facilities, the seismic risk in the ZV is high. However, until now the national seismic network in Israel only included rock stations with no measurements supporting the expected difference between the ZV and its surroundings. Moreover, a detailed analysis of ground motions atop sedimentary basins using earthquakes data was never conducted in Israel for any basin. In this paper, we present a dataset collected during a 16 months monitoring campaign with a transportable network deployed in the ZV. For the first time in Israel we simultaneously recorded earthquake (3.1 < Mw < 5.5) ground motions at basin- and reference-sites. Spectral ratios reveal amplification factors tangibly higher than those previously reported by horizontal-to-vertical-spectral-ratio (HVSR) techniques and 2-D modeling. In particular, the deeper parts of the valley exhibit ground motion amplification up to a factor of 8 at frequencies lower than 1 Hz. Comparison of the measured spectral ratios with the results of 1-D linear-elastic analysis shows partial correlation reflecting the complexity of the sub-surface structure.

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We study the propagation of seismic waves, the resulting ground motions, and their amplification atop sedimentary structures underlying continental passive margins. We employ a set of generic models with increasing complexity within a framework of a 3D numerical scheme. The basic geological structure and velocity model were derived from the subsurface of the Israeli coastal plain where soft sediments form a wedge over the stiffer bedrock and fill subsurface canyons that incise deep into the bedrock. Ground motions were modeled for both seaside and landside seismic sources. We show that for a landside source, peak ground velocities (PGVs) atop a sedimentary wedge are amplified by a maximum factor of 2.6 and on average by a factor of 1.6, relative to a reference model. This amplification is mainly due to the ellipticity of Rayleigh waves in the soft sediment layer. Spatial distribution of amplification factors shows that sedimentary wedges do not exhibit a prominent edge effect. Atop sediment-filled canyons and landside source, PGV are amplified by a maximum factor of 3.3, relative to a reference model, along the exposed part of the canyon. The PGV amplification factor in the canyon relative to adjacent hard-rock site is up to 2.4. PGV amplification atop the sediment-filled canyons is mainly due to the geometrical focusing of SH waves. Based on our findings, we present a simplified ground-motion amplification map for the Israeli coastal plain.

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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 ± 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.

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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 groundmotion 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.

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