Urban Seismology: on the origin of earth vibrations within a city
TL;DR: The main results include the evidence that urban seismometers can be used as a easy-to-use, robust monitoring tool for road traffic and subway activity inside the city, and the interest to understand the propagation of seismic waves generated by those rather particular sources.
Abstract: Urban seismology has become an active research field in the recent years, both with seismological objectives, as obtaining better microzonation maps in highly populated areas, and with engineering objectives, as the monitoring of traffic or the surveying of historical buildings. We analyze here the seismic records obtained by a broad-band seismic station installed in the ICTJA-CSIC institute, located near the center of Barcelona city. Although this station was installed to introduce visitors to earth science during science fairs and other dissemination events, the analysis of the data has allowed to infer results of interest for the scientific community. The main results include the evidence that urban seismometers can be used as a easy-to-use, robust monitoring tool for road traffic and subway activity inside the city. Seismic signals generated by different cultural activities, including rock concerts, fireworks or football games, can be detected and discriminated from its seismic properties. Beside the interest to understand the propagation of seismic waves generated by those rather particular sources, those earth shaking records provide a powerful tool to gain visibility in the mass media and hence have the opportunity to present earth sciences to a wider audience.
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Royal Observatory of Belgium1, Imperial College London2, University of Auckland3, Royal Holloway, University of London4, National Autonomous University of Mexico5, Swiss Seismological Service6, University of Helsinki7, United States Geological Survey8, Central Institution for Meteorology and Geodynamics9, University of Costa Rica10, Royal Netherlands Meteorological Institute11, Kandilli Observatory and Earthquake Research Institute12, University of Potsdam13, University of Catania14, National Institute of Geophysics and Volcanology15, University of Cologne16, University of Savoy17, King's College, Aberdeen18, Dublin Institute for Advanced Studies19, Delft University of Technology20, Spanish National Research Council21, Institute for Geosciences and Natural Resources22, Mediterranean University23, Norwegian Geotechnical Institute24, University of Alaska Fairbanks25, University of Strasbourg26, University of Lausanne27, University of Bristol28, Princeton University29, University of Tehran30, Boston College31, California Institute of Technology32, Stanford University33, University of British Columbia34, Search for extraterrestrial intelligence35, Ludwig Maximilian University of Munich36, Australian National University37, McGill University38, University of Maine39, University of California, Riverside40, University of Chile41, University of Oxford42, BBN Technologies43, Institut de Physique du Globe de Paris44, Victoria University of Wellington45, University of Patras46, University of Bergen47, University of California, Berkeley48, Institut d'Estudis Catalans49, University of Michigan50, University of California, Santa Barbara51
TL;DR: The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record and suggests that seismology provides an absolute, real-time estimate of human activities.
Abstract: 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.
202 citations
TL;DR: In this paper, the authors demonstrate how fiber-optic distributed acoustic sensing (DAS) connected to a telecommunication cable beneath Palo Alto, CA, successfully monitored traffic over a 2-month period, including major reductions associated with COVID-19 response.
Abstract: Throughout the recent COVID-19 pandemic, real-time measurements about shifting use of roads, hospitals, grocery stores, and other public infrastructure became vital for government decision makers. Mobile phone locations are increasingly assimilated for this purpose, but an alternative, unexplored, natively anonymous, absolute method would be to use geophysical sensing to directly measure public infrastructure usage. In this paper, we demonstrate how fiber-optic distributed acoustic sensing (DAS) connected to a telecommunication cable beneath Palo Alto, CA, successfully monitored traffic over a 2-month period, including major reductions associated with COVID-19 response. Continuous DAS recordings of over 450,000 individual vehicles were analyzed using an automatic template-matching detection algorithm based on roadbed strain. In one commuter sector, we found a 50% decrease in vehicles immediately following the order, but near Stanford Hospital, the traffic persisted. The DAS measurements correlate with mobile phone locations and urban seismic noise levels, suggesting geophysics would complement future digital city sensing systems.
71 citations
TL;DR: In this paper, the authors demonstrate that near-surface velocity changes induced by the excavation of a basement construction can be monitored using existing fiber optic infrastructure in a noisy urban environment using distributed acoustic sensing (DAS).
Abstract: Urban subsurface monitoring requires high temporal-spatial resolution, low maintenance cost, and minimal intrusion to nearby life. Distributed acoustic sensing (DAS), in contrast to conventional station-based sensing technology, has the potential to provide a passive seismic solution to urban monitoring requirements. Based on data recorded by the Stanford Fiber Optic Seismic Observatory, we demonstrate that near-surface velocity changes induced by the excavation of a basement construction can be monitored using existing fiber optic infrastructure in a noisy urban environment. To achieve the satisfactory results, careful signal processing comprising of noise removal and source signature normalization are applied to raw DAS recordings. Repeated blast signals from quarry sites provide free, unidirectional, and near-impulsive sources for periodic urban seismic monitoring, which are essential for increasing the temporal resolution of passive seismic methods. Our study suggests that DAS will likely play an important role in urban subsurface monitoring.
69 citations
Posted Content•
TL;DR: Gravity coupling between two gold spheres of 1 millimetre radius is shown, thereby entering the regime of sub-100-milligram sources of gravity and opening the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions and provide a path towards exploring the quantum nature of gravity.
Abstract: We demonstrate gravitational coupling between two gold spheres of approximately 1mm radius and 90mg mass. By periodically modulating the source mass position at a frequency f=12.7mHz we generate a time-dependent gravitational acceleration at the location of the test mass, which is measured off resonance in a miniature torsional balance configuration. Over an integration time of 350 hours the test mass oscillator enables measurements with a systematic accuracy of 4E-11m/s^2 and a statistical precision of 4E-12m/s^2. This is sufficient to resolve the gravitational signal at a minimal surface distance of 400mum between the two masses. We observe both linear and quadratic coupling, consistent in signal strength with a time-varying 1/r gravitational potential. Contributions of non-gravitational forces could be kept to less than 10% of the observed signal. We expect further improvements to enable the isolation of gravity as a coupling force for objects well below the Planck mass. This opens the way for precision tests of gravity in a new regime of isolated microscopic source masses.
51 citations
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...[62] Dı́az, J....
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TL;DR: In this article, the authors show that gravity coupling between two gold spheres of 1 millimetre radius can be observed as a consequence of the nonlinearity of the gravitational potential, and they extend the parameter space of gravity measurements to small, single source masses and low gravitational field strengths.
Abstract: Gravity is the weakest of all known fundamental forces and poses some of the most important open questions to modern physics: it remains resistant to unification within the standard model of physics and its underlying concepts appear to be fundamentally disconnected from quantum theory1-4. Testing gravity at all scales is therefore an important experimental endeavour5-7. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond8. Here we show gravitational coupling between two gold spheres of 1 millimetre radius, thereby entering the regime of sub-100-milligram sources of gravity. Periodic modulation of the position of the source mass allows us to perform a spatial mapping of the gravitational force. Both linear and quadratic coupling are observed as a consequence of the nonlinearity of the gravitational potential. Our results extend the parameter space of gravity measurements to small, single source masses and low gravitational field strengths. Further improvements to our methodology will enable the isolation of gravity as a coupling force for objects below the Planck mass. This work opens the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions9-11 and provide a path towards exploring the quantum nature of gravity12-15.
50 citations
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TL;DR: This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation and has potential applications in other fields.
Abstract: The late seismic coda may contain coherent information about the elastic response of Earth. We computed the correlations of the seismic codas of 101 distant earthquakes recorded at stations that were tens of kilometers apart. By stacking cross-correlation functions of codas, we found a low-frequency coherent part in the diffuse field. The extracted pulses have the polarization characteristics and group velocities expected for Rayleigh and Love waves. The set of cross-correlations has the symmetries of the surface-wave part of the Green tensor. This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation. Because it relies on general properties of diffuse waves, this result has potential applications in other fields.
1,139 citations
27 Sep 1950-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: In this paper, it was shown that in an infinite wave train there is in general a second-order pressure variation at infinite depth which is applied equally over the whole fluid and is associated with no particle motion.
Abstract: In the past it has been considered unlikely that ocean waves are capable of generating microseismic oscillations of the sea bed over areas of deep water, since the decrease of the pressure variations with depth is exponential, according to the first-order theory generally used. However, it was recently shown by Miche that in the second approximation to the standing wave there is a second-order pressure variation which is not attenuated with depth and which must therefore ultimately predominate over the first-order pressure variations. In §§ 2 and 3 of the present paper the general conditions under which second-order pressure variations of this latter type will occur are considered. It is shown that in an infinite wave train there is in general a second-order pressure variation at infinite depth which is applied equally over the whole fluid and is associated with no particle motion. In the case of two progressive waves of the same wave-length travelling in opposite directions this pressure variation is proportional to the product of the (first-order) amplitudes of the two waves and is of twice their frequency. The pressure variation at infinite depth is found to be closely related to changes in the potential energy of the wave train as a whole. By introducing the two-dimensional frequency spectrum of the motion it is shown that in the general case variations in the mean pressure over a wide area only occur when the spectrum contains wave groups of the same wave-length travelling in opposite directions. (These are called opposite wave groups.) In § 4 the effect of the compressibility of the water is considered by evaluating the motion of an opposite pair of waves in a heavy compressible fluid to the second order of approximation. In place of the pressure variation at infinite depth, waves of compression are set up, and there is resonance between the bottom and the free surface when the depth of water is about (1/2 n + 1/4) times the length of a compression wave ( n being an integer). The motion in a surface layer whose thickness is of the order of the length of a Stokes wave is otherwise unaffected by the compressibility. Section 5 is devoted to the question whether the second-order pressure variations in surface waves are capable of generating microseisms of the observed order of magnitude. By considering the displacement of the sea bed due to a concentrated force at the upper surface of the water it is shown that the effect of resonance will be to increase the disturbance by a factor of the order of 5 over its value in shallow water. The results of §§ 3 and 4 are used to derive an expression for the vertical displacement of the ground in terms of the frequency characteristics of the waves. The displacement from a storm area of 1000 sq.km, is estimated to be of the order of 6.5μ at a distance of 2000 km. Ocean waves may therefore be the cause of microseisms, provided that there is interference between groups of waves of the same frequency travelling in opposite directions. Suitable conditions of wave interference may occur at the centre of a cyclonic depression or possibly if there is wave reflexion from a coast. In the latter case the microseisms are likely to be smaller, except perhaps locally. Confirmation of the present theory is provided by the observations of Bernard and Deacon, who discovered independently that the period of the microseisms is in many cases about half that of the ocean waves associated with them.
1,050 citations
TL;DR: In this article, the authors discuss the existing scientific literature in order to gather all the available information dealing with the origin and the nature of the ambient seismic noise wavefield, and reveal an overall agreement about the origin of seismic noise and its frequency dependence.
552 citations
TL;DR: In this article, an alternative approach based on spectral transfer functions and the local energy-balance equation of the seismic field is proposed, which enables a rigorous analysis of the statistical aspects of the problem, which could be treated only approximately and under restrictive conditions in terms of the far-field representations used previously.
Abstract: Theories of the origin of microseisms have in the past generally been expressed in terms of the Green functions of the elastic systems considered. An alternative approach based on spectral transfer functions and the local energy-balance equation of the seismic field is proposed. The method enables a rigorous analysis of the statistical aspects of the problem, which could be treated only approximately and under restrictive conditions in terms of the far-field representations used previously. Three suggested origins of microseisms are considered: (1) the action of ocean waves on coasts, originally proposed by Wiechert; (2) atmospheric pressure fluctuations, as suggested by Gherzi, Scholte, and others; and (3) nonlinear interactions between ocean waves as proposed by Longuet-Higgins. In all cases appreciable microseisms are generated only by Fourier components of the random exciting fields that have the same phase velocities as free modes of the elastic system. The effect of pressure fluctuations associated with turbulence in the atmosphere is found to be negligible. The theory for Wiechert's and Longuet-Higgins' mechanisms is in good agreement with recent measurements by Haubrich et al.
551 citations