Search

SUMMARY 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 that 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 the 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, which 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 International Monitoring System 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.

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.

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., P.K. was funded by a Royal Society University Research Fellowship (URF\R1\180377). P.B. and M.J. acknowledge support from the International Training Course “Seismology and Seismic Hazard Assessment” funded by the GeoForschungsZentrum Potsdam (GFZ) and the German Federal Foreign Office through the German Humanitarian Assistance program (grant S08-60 321.50 ALL 03/19). P.B also acknowledges financial support from the Boğaziçi University Research Fund (BAP 15683). O.F.C.d.O acknowledges funding from a Young Investigator Grant from the Human Frontier Science Program (HFSP - project number RGY0072/2017). C.P.E. and E.S. acknowledge funding from the HELPOS Project “Hellenic Plate Observing System” (MIS 5002697). L.E. and S.S.-K. acknowledge funding from a VIDI project from the Dutch Research Council (NWO project number 864.14.005). G.A.F. acknowledges contributions from the Observatorio San Calixto, which is supported by the Air Force Technical Application Center (AFTAC). C.R.L. acknowledge funding from the NSF Graduate Research Fellowship Program (grant No. DGE‐1745301). V.-H.M. and R.D.P. acknowledge support from grant CONACYT-299766. R.D.P. acknowledges support from the UNAM-DGAPA postdoctoral scholarship. J.O. acknowledges support from the Agencia Nacional de Investigación y Desarrollo (Scholarship ANID-PFCHA / Doctorado Nacional / 2020-21200903). S.P. acknowledges financial support from the Natural Environment Research Council (NE/R013144/1). A.E.R. acknowledges support from the K.H. Renlund foundation. M.K.S. acknowledges the New Zealand Earthquake Commission (EQC Project No 20796). H.X. acknowledges support from a Multidisciplinary Research on the Coronavirus and its Impacts (MRCI) grant from UC Santa Barbara. The Australian Seismometers in Schools data used in this research are supported by AuScope, enabled by the Australian Commonwealth NCRIS program. A.O. acknowledges support from the project RESIST, funded by the Belgian Federal Science Policy (contract SR/00/305) and the Luxembourg National Research Fund

When an earthquake occurs, it is important to rapidly assess the severity of the consequences. The distribution of shaking intensity around the epicenter, known as the ShakeMap, is a key component in this process and is crucial for guiding first responders to the region. Whereas earthquake source characteristics, e.g., location and magnitude, can be rapidly determined using distant seismic stations, ground motion measurements from stations in the near-source region are needed to generate an adequate ShakeMap. When few or no seismometers exist in the region, ground motions are only estimated and the ShakeMap can be grossly inaccurate.Besides seismic waves, earthquakes generate infrasound, i.e., inaudible acoustic waves in the atmosphere. Due to the low frequency nature of infrasound, and facilitated by waveguides in the atmosphere, signals propagate over long ranges with limited attenuation and are detected at ground-based stations. Here we show, that acousto-ShakeMaps, indicating the relative shaking intensity, can be rapidly generated using remotely detected infrasound. We illustrate this with infrasound from the 2010 Mw 7.0 Port-au-Prince, Haiti earthquake, detected in Bermuda, over 1700 km away from Haiti.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) A backprojection technique that maps infrasound detections in time to their origin on the Earth's surface.Infrasound measurements are conducted globally for the verification of the Comprehensive Nuclear-Test-Ban Treaty and together with regional infrasound networks allow for an unprecedented global coverage. This makes infrasound as an earthquake disaster mitigation technique feasible for the first time.

Inaudible sound, i.e., infrasound, from glaciers is generated by glacier run-off and during calving events. Such sounds can be continuously monitored with microbarometer arrays. Changes in the rate of events can be retrieved with a resolution of a few seconds. Applying array processing techniques enables the identification of individual glaciers over ranges of tens of kilometers. We concentrated on the remote region around Qaanaaq in northwestern Greenland and found coherent infrasound of at least five glaciers over a period of 16 years. Knowledge on the dynamical behavior of these remote glaciers, and other glaciers, is important for assessing hazards due to glacier melt and calving, mass balance of ice sheets and consequently sea level rise. Here we use a novel technique involving passive infrasound measurements to show that remote land and sea terminating glaciers behave differently in terms of their temporal behavior during the seasons and years. Strong fluctuations in infrasonic activity are found over time, with the highest activity rates in 2019. Increased activity over the years of the land- ans sea-terminated glaciers is retrieved. We anticipate that monitoring glacier infrasound can contribute to complementary knowledge of the behavior of remote glaciers, as the glacial dynamics can be passively observed on a fine temporal scale.

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. Electronic Supplement: Animations of the three components of the ground‐motion models.

We develop a generic finite‐fault source model for simulation of large earthquakes: the distributed slip model (DSM). Six geometric and seven kinematic parameters are used to describe a smooth pseudo‐Gaussian slip distribution, such that slip decays from peak slip within an elliptical rupture patch to zero at the borders of the patch. The DSM is implemented to initiate seismic‐wave propagation in a finite‐difference code. Radiation pattern and spectral characteristics of the DSM are compared with those of commonly used finite‐fault models, that is, the classical Haskell’s model (HM) and the modified HM with radial rupture propagation (HM‐RRP). The DSM accounts for directivity effects in the fault‐parallel direction, as well as fault‐normal ground motions, and overcomes the unrealistic uniform slip and stress singularities of the Haskell‐type models. We show the potential of the DSM to estimate the ground motions of strong earthquakes. We use this model to initiate seismic‐wave propagation during the 1927 M L 6.25 Jericho earthquake and compare calculated macroseismic intensities to reported intensities at 122 localities. The root mean square of intensity residuals is 0.68, with 56% of the calculated intensities matching the reported intensities and 98% of the calculated intensities within a single unit from the reported intensities. The DSM is an essential step toward robust ground‐motion prediction in earthquake‐prone regions with a long return period and limited instrumental coverage. Online Material: Animation of rupture and wave propagation.

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.

Activities of the Monowai Volcanic centre have been observed as seismic waves recorded at Rarotonga station (RAR), as well as hydroacoustic waves recorded at Ascension Island (H10). Beamforming has been performed on the hydroacoustic data to verify the source and study the effect of performing calculations with a best beam. The lag time that is calculated by cross correlating seismic and hydroacoustic data. With the known source-receiver distance, the sound speed and hence the temperature can be calculated. Finally, the lag time is studied as a function of time in order to observe seasonal changes. In general, travel times between the source and hydrostation H10 do increase with decreasing temperature. In the Southern Hemisphere winter, however, an unexpected decrease of lag time is observed. This might be explained by a different propagation path due to the formation of sea ice.

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.

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.