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

Arrival angles of teleseismic fundamental mode Rayleigh waves across the AlpArray

The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions

Ambient-noise tomography of the wider Vienna Basin region

We present a new 3-D shear-velocity model for the top 30 km of the crust in the wider Vienna Basin region based on surface waves extracted from ambient-noise cross-correlations. We use continuous seismic records of 63 broad-band stations of the AlpArray project to retrieve interstation Green’s functions from ambient-noise cross-correlations in the period range from 5 to 25 s. From these Green’s functions, we measure Rayleigh group traveltimes, utilizing all four components of the cross-correlation tensor, which are associated with Rayleigh waves (ZZ, RR, RZ and ZR), to exploit multiple measurements per station pair. A set of selection criteria is applied to ensure that we use high-quality recordings of fundamental Rayleigh modes. We regionalize the interstation group velocities in a 5 km × 5 km grid with an average path density of ∼20 paths per cell. From the resulting group-velocity maps, we extract local 1-D dispersion curves for each cell and invert all cells independently to retrieve the crustal shear-velocity structure of the study area. The resulting model provides a previously unachieved lateral resolution of seismic velocities in the region of ∼15 km. As major features, we image the Vienna Basin and Little Hungarian Plain as low-velocity anomalies, and the Bohemian Massif with high velocities. The edges of these features are marked with prominent velocity contrasts correlated with faults, such as the Alpine Front and Vienna Basin transfer fault system. The observed structures correlate well with surface geology, gravitational anomalies and the few known crystalline basement depths from boreholes. For depths larger than those reached by boreholes, the new model allows new insight into the complex structure of the Vienna Basin and surrounding areas, including deep low-velocity zones, which we image with previously unachieved detail. This model may be used in the future to interpret the deeper structures and tectonic evolution of the wider Vienna Basin region, evaluate natural resources, model wave propagation and improve earthquake locations, among others

Shear-wave velocity structure beneath the Dinarides from the inversion of Rayleigh-wave dispersion

Highlights • Rayleigh-wave phase velocity in the wider Dinarides region using the two-station method. • Uppermost mantle shear-wave velocity model of the Dinarides-Adriatic Sea region. • Velocity model reveals a robust high-velocity anomaly present under the whole Dinarides. • High-velocity anomaly reaches depth of 160 km in the northern Dinarides to more than 200 km under southern Dinarides. • New structural model incorporating delamination as one of the processes controlling the continental collision in the Dinarides. The interaction between the Adriatic microplate (Adria) and Eurasia is the main driving factor in the central Mediterranean tectonics. Their interplay has shaped the geodynamics of the whole region and formed several mountain belts including Alps, Dinarides and Apennines. Among these, Dinarides are the least investigated and little is known about the underlying geodynamic processes. There are numerous open questions about the current state of interaction between Adria and Eurasia under the Dinaric domain. One of the most interesting is the nature of lithospheric underthrusting of Adriatic plate, e.g. length of the slab or varying slab disposition along the orogen. Previous investigations have found a low-velocity zone in the uppermost mantle under the northern-central Dinarides which was interpreted as a slab gap. Conversely, several newer studies have indicated the presence of the continuous slab under the Dinarides with no trace of the low velocity zone. Thus, to investigate the Dinaric mantle structure further, we use regional-to-teleseismic surface-wave records from 98 seismic stations in the wider Dinarides region to create a 3D shear-wave velocity model. More precisely, a two-station method is used to extract Rayleigh-wave phase velocity while tomography and 1D inversion of the phase velocity are employed to map the depth dependent shear-wave velocity. Resulting velocity model reveals a robust high-velocity anomaly present under the whole Dinarides, reaching the depths of 160 km in the north to more than 200 km under southern Dinarides. These results do not agree with most of the previous investigations and show continuous underthrusting of the Adriatic lithosphere under Europe along the whole Dinaric region. The geometry of the down-going slab varies from the deeper slab in the north and south to the shallower underthrusting in the center. On-top of both north and south slabs there is a low-velocity wedge indicating lithospheric delamination which could explain the 200 km deep high-velocity body existing under the southern Dinarides

Crustal Thinning From Orogen to Back-Arc Basin: The Structure of the Pannonian Basin Region Revealed by P-to-S Converted Seismic Waves

We present the results of P-to-S receiver function analysis to improve the 3D image of the sedimentary layer, the upper crust, and lower crust in the Pannonian Basin area. The Pannonian Basin hosts deep sedimentary depocentres superimposed on a complex basement structure and it is surrounded by mountain belts. We processed waveforms from 221 three-component broadband seismological stations. As a result of the dense station coverage, we were able to achieve so far unprecedented spatial resolution in determining the velocity structure of the crust. We applied a three-fold quality control process; the first two being applied to the observed waveforms and the third to the calculated radial receiver functions. This work is the first comprehensive receiver function study of the entire region. To prepare the inversions, we performed station-wise H-Vp/Vs grid search, as well as Common Conversion Point migration. Our main focus was then the S-wave velocity structure of the area, which we determined by the Neighborhood Algorithm inversion method at each station, where data were sub-divided into back-azimuthal bundles based on similar Ps delay times. The 1D, nonlinear inversions provided the depth of the discontinuities, shear-wave velocities and Vp/Vs ratios of each layer per bundle, and we calculated uncertainty values for each of these parameters. We then developed a 3D interpolation method based on natural neighbor interpolation to obtain the 3D crustal structure from the local inversion results. We present the sedimentary thickness map, the first Conrad depth map and an improved, detailed Moho map, as well as the first upper and lower crustal thickness maps obtained from receiver function analysis. The velocity jump across the Conrad discontinuity is estimated at less than 0.2 km/s over most of the investigated area. We also compare the new Moho map from our approach to simple grid search results and prior knowledge from other techniques. Our Moho depth map presents local variations in the investigated area: the crust-mantle boundary is at 20–26 km beneath the sedimentary basins, while it is situated deeper below the Apuseni Mountains, Transdanubian and North Hungarian Ranges (28–33 km), and it is the deepest beneath the Eastern Alps and the Southern Carpathians (40–45 km). These values reflect well the Neogene evolution of the region, such as crustal thinning of the Pannonian Basin and orogenic thickening in the neighboring mountain belts

The Ebreichsdorf 2013 earthquake series: location, interaction and wave propagation

Aufgrund der Lage Wiens in einer seismisch aktiven Region ist es wichtig, diese Seismizität gut zu bestimmen. Das seismologische Netz Österreichs besteht aus hochwertigen Stationen, jedoch sind die Stationen zu weit voneinander entfernt, um Lage und Tiefe von kleineren Erdbeben genau zu charakterisieren. Deshalb versuche ich in dieser Doktorarbeit die Menge der der verwendeten Daten zur Lokalisierung zu erhöhen. Dafür wurde ein umfassender Datensatz der Erdbebenserie in Ebreichsdorf von 2013 zusammengestellt, ergänzt durch die Erdbebenserie von 2000. Zwei Lokalisierungsmethoden werden verwendet: die absolute Lage wird mit NonLinLoc von Lomax et al. (2000) bestimmt. Danach wird HypoDD von Waldhauser und Ellsworth (2000) verwendet, um das raumzeitliche Muster der Erdbebenserie zu berechnen. Nach dieser Analyse wird die Wechselwirkung zwischen den Ereignissen untersucht. Da aufgrund der Netzgeometrie die Lagegenauigkeit beschränkt ist, wird versucht diese durch regionale Tiefenphasen zu verbessern, sofern diese identifiziert werden können. Dafür wurden Daten des seismischen Arrays GERES verwendet. Um die Wellenausbreitung um das Wiener Becken besser zu verstehen, werden weiters synthetische Seismogramme mit regionalen Herdmechanismen und einem lokal angepassten Geschwindigkeitsmodell berechnet. NonLinLoc setzt die Epizentren in den Südosten von Ebreichsdorf und zeigt eine längliche Verteilung von Südwesten nach Nordosten. Mit der Relokalisierung mit HypoDD verkleinert sich die Fläche und der Tiefenbereich, auf dem die Beben auftreten. Auch die Hauptbeben sind nun kollokalisiert, wie auch die Kohärenzmessungen bestätigen. Die Modellierung von Coulomb-Bruchspannungen zeigt, dass Coulomb-Spannungsübertragung nicht stark genug ist, um die Nachbeben auszulösen. Ein ergänzende Vergleich mit der 2000-Serie ergibt, dass sich die Hauptbeben 4 km entfernt im Nordosten befinden. Die anschließende Analyse der 2013 Erdbeben in GERES, identifiziert die Phasen PmP und PbP, aber nicht ihre entsprechenden Tiefenphasen. Dies steht im Einklang mit den synthetischen Daten. Prüfung der modellierten Seismogramme zeigt des weiteren, dass die Wellenausbreitung stark vom Herdmechanismus beeinflußt wird. Auch eine langsame oberflächennahe Schicht, wie ein Sedimentbecken, beeinflusst die Sichtbarkeit von Tiefenphasen stark negativ.Due to Vienna’s location in a seismogenic region, it is important to characterize seismicity in the vicinity well. The Austrian seismological network is built of very high quality stations, but those are spread too widely to determine earthquake depths accurately for small earthquakes. Therefore I attempt to optimize use of the network and available data for locating earthquakes, with a comprehensive dataset from the Ebreichsdorf 2013 earthquake series, complemented by data from earlier events in the area. Two location methods are applied: one by Lomax et al. (2000) to get precise absolute location of the series. Afterwards, HypoDD by Waldhauser and Ellsworth (2000) is used to get a better picture of the spatio-temporal pattern of the series. Following this analysis the interaction between the events is examined. Due to the network geometry hypocentral depths are not as well-constrained as lateral location. Regional depth phases can improve depth estimation if they can be identified. To detect those phases data from the GERES seismic array were processed additionally. To understand RDP propagation in this area, synthetic seismograms are calculated with regional focal mechanisms and a locally adapted velocity model. Location with NonLinLoc puts the epicentres to the south-east of Ebreichsdorf and reveals a south-west to north-east pattern. After relocation with hypoDD, the earthquake series clusters on a smaller area and depth range. Also the main shocks collocate, which was confirmed by coherence measurements. The application of Coulomb failure stress modelling for analysing interaction between the 2013 main shocks shows that Coulomb stress transfer from the main shocks does not explain aftershock triggering. Complementary comparison with the 2000 series indicates that the previous main shocks are located 4 km apart to the north-east. Subsequent analysis of the 2013 earthquakes at GERES, identifies clear arrivals of PmP and PbP, but not of their corresponding depth phases, which is consistent with the synthetic data. Examination of the modelled seismograms also shows, that RDP propagation is dependent not only on depth but is also strongly influenced by the focal mechanism. Also a low-velocity top-layer, like a sedimentary basin, renders RDP invisible, as it complicates the resulting seismograms considerably

Influence of network configuration on earthquake localization

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersZsfassung in engl. SpracheDie rezente seismische Aktivität um Wien weist auf eine noch immer aktive Tektonik im Wiener Becken hin. Die Kenntnis wahrscheinlicher maximaler Magnituden im Großraum Wien ist für die zu erwartende Erdbebengefährdung von großer Bedeutung.Eine Abschätzung kann über die Ausdehnung von Bruchflächen gemacht werden, welche oberflächennahe bekannt sind, in den tieferen seismogenen Zonen aber nur durch Erdbeben kartiert werden können. Aktuelle Standardlokalisierung bieten jedoch keine ausreichende Genauigkeit.Durch die Verwendung zusätzlicher Stationen im Rahmen des Projektes ALPAACT und eines 3D-Geschwindigkeitmodells wird die Ortungsgenauigkeit erhöht.Manche Stationen stehen aber nur zeitlich begrenzt zur Verfügung, die Ortungsgenauigkeit soll jedoch erhalten bleiben. Dazu müssen systematische Einflüsse auf die Lokalisierung aufgedeckt und Verbesserungsmöglichkeiten untersucht werden.Als erstes wurde der Abbau von Stationen des ALPAACT Netzes simuliert.Im zweiten Fall wurde die Änderung der Netzwerkkonfiguration aufgrund von Stationswegfall durch geringere Magnituden modelliert. Dafür wurden Detektionsschwellen für jede Station über die tatsächlich gemachten Ankunftszeitbeobachtungen gebildet.Anschließend wurden die Einflüsse in der Laufzeitberechnung durch Offset und Station ermittelt und mit einer verbesserten Laufzeitberechnung neu lokalisiert. Da nur eine teilweise Verbesserung erreicht werden konnte, wurde Fehler in den beobachteten Ankunftszeiten simuliert. Die Fehler in den Ankunftszeiten entsprachen den tatsächlich beobachteten Lageänderungen.Durch die verbesserte Laufzeitberechnung konnte die Standardabweichung der Laufzeitresiduen um mehr als 20 % reduziert werden. Die relative Lageänderungen konnten teilweise verbessert werden, im Mittel jedoch nicht. Mit der verbesserten Laufzeitberechnung weichen die tektonischen Erdbeben nur noch um weniger als 2.7 km von einer Mittellinie mit einer Länge von 49 km im südlichen Wiener Becken ab. Außerdem ist nun ein Abwärtstrend der Bebetiefen in Richtung Nordosten sichtbar.Für eine genauere Lokalisierung ist eine verbesserte Beobachtung der Ankunftszeiten notwendig. Ein größerer Datensatz an Erdbeben für die Ermittlung von Offset- und Stationskorrektur wäre für zukünftige Untersuchungen ebenfalls förderlich.The recent seismic activity around Vienna suggests still-active tectonics in the Vienna Basin. Knowledge of maximum credible earthquake in the metropolitan area of Vienna is of great importance for estimating seismic hazard. One possibility to do so is to estimate the size of fault surfaces, which, however, with the current standard earthquake localization can not be mapped sufficiently.Through the use of additional stations in the ALPAACT project and an enhanced 3D velocity model, the location accuracy is increased. Some stations are only available for limited time, but the location accuracy is to be preserved. This requires detecing systematic influences in localization and afterwards invesitgating possible improvements.Two changes in the network configuration were investigated. First, the dismantling of some stations of the ALPAACT network was simulated. In the second case, the change in network configuration due to station outage at lower magnitudes was modeled. Detection thresholds were established for each station based on actual observed arrival times.Subsequently, the effects on localisation by station and offset were determined. The calculated travel times were corrected and a new localisation was determined. Since only a partial improvement was achieved, errors in the observed arrival times were simulated.The errors in the arrival times correspond to the actual observed changes in position.Because of improved travel time calculation the standard deviation of travel time residuals was reduced by more than 20%. The relative position changes were partially improved, but the mean was not. With the improved travel-time calculation the located earthquakes differ by less than 2.7 to a center line with a length of 49 km.Furthemore a downward trend in the earthquake depths to the northeast is visible.For better earthquake localization improved picking of the arrival times is necessary. A larger data set of earthquakes for the determination of the offset-term and station-term would be benificial for further investigations.iv9

The 2013 earthquake series in the Southern Vienna Basin: location

Eastern Austria is a region of low to moderate seismicity, and hence the seismological network coverage is relatively sparse. Nevertheless accurate earthquake location is very important, as the area is one of the most densely populated and most developed areas in Austria. In 2013 a series of earthquakes with magnitudes up to 4.2 was recorded in the Southern Vienna Basin. With portable broadband, semi-permanent, and permanent installed seismic sensors from different institutions it was possible to record the main- and aftershocks with an unusual multitude of close-by seismic stations. In this study we combine records from all available stations up to 240 km distance in one dataset. First, we stabilize the location with three stations deployed in the epicentral area. The higher network density moves the location of smaller magnitude events closer to the main shocks, with respect to preliminary locations achieved by permanent and semi-permanent networks. Then we locate with NonLinLoc using consistent picks, a 3-D velocity model and apply station corrections. This second approach results in stable epicenters, for limited and even changing station availability.This dataset can then be inspected more closely for the presence of regional phases, which then can be used for more accurate localizations and especially depth estimation. Further research will address directivity effects and the asymmetry in earthquake intensity observed throughout the area, using double differences and cross-correlations.ZAMGORFEU

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 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.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 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 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. acknowledges funding from the NSF Graduate Research Fellowship Program (grant 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 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

Shear wave splitting in the Alpine region

To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics