6,981 research outputs found
COVID-19 societal response captured by seismic noise in China and Italy
Seismic noise with frequencies above 1 Hz is often called cultural noise and
is generally correlated quite well with human activities. Recently, cities in
mainland China and Italy imposed lockdown restrictions in response to COVID-19,
which gave us an unprecedented opportunity to study the relationship between
seismic noise above 1 Hz and human activities. Using seismic records from
stations in China and Italy, we show that seismic noise above 1 Hz was
primarily generated by the local transportation systems. The lockdown of the
cities and the imposition of travel restrictions led to a ~4-12 dB energy
decrease in seismic noise in mainland China. Data also show that different
Chinese cities experienced distinct periods of diminished cultural noise,
related to differences in local response to the epidemic. In contrast, there
was only ~1-6 dB energy decrease of seismic noise in Italy, after the country
was put under a lockdown. The noise data indicate that traffic flow did not
decrease as much in Italy, but show how different cities reacted distinctly to
the lockdown conditions
A physical model for seismic noise generation from sediment transport in rivers
Measuring sediment flux in rivers remains a significant problem in studies of landscape evolution. Recent studies suggest that observations of seismic noise near rivers can help provide such measurements, but the lack of models linking observed seismic quantities to sediment flux has prevented the method from being used. Here, we develop a forward model to describe the seismic noise induced by the transport of sediment in rivers. The model provides an expression for the power spectral density (PSD) of the Rayleigh waves generated by impulsive impacts from saltating particles which scales linearly with the number of particles of a given size and the square of the linear momentum. After incorporating expressions for the impact velocity and rate of impacts for fluvially transported sediment, we observe that the seismic noise PSD is strongly dependent on the sediment size, such that good constraints on grain size distribution are needed for reliable estimates of sediment flux based on seismic noise observations. The model predictions for the PSD are consistent with recent measurements and, based on these data, a first attempt at inverting seismic noise for the sediment flux is provided
A Semi-classical calculus of correlations
The method of passive imaging in seismology has been developped recently in
order to image the earth crust from recordings of the seismic noise. This
method is founded on the computation of correlations of the seismic noise. In
this paper, we give an explicit formula for this correlation in the
"semi-classical" regime. In order to do that, we define the power spectrum of a
random field as the ensemble average of its Wigner measure, this allows
phase-space computations: the pseudo-differential calculus and the ray theory.
This way, we get a formula for the correlation of the seismic noise in the
semi-classcial regime with a source noise which can be localized and non
homogeneous. After that, we show how the use of surface guided waves allows to
image the earth crust.Comment: To appear in a special issue "Imaging and Monitoring with Seismic
Noise" of the series "Comptes Rendus G\'eosciences", from the French
"Acad\'emie des sciences
Long term study of the seismic environment at LIGO
The LIGO experiment aims to detect and study gravitational waves using ground
based laser interferometry. A critical factor to the performance of the
interferometers, and a major consideration in the design of possible future
upgrades, is isolation of the interferometer optics from seismic noise. We
present the results of a detailed program of measurements of the seismic
environment surrounding the LIGO interferometers. We describe the experimental
configuration used to collect the data, which was acquired over a 613 day
period. The measurements focused on the frequency range 0.1-10 Hz, in which the
secondary microseismic peak and noise due to human activity in the vicinity of
the detectors was found to be particularly critical to interferometer
performance. We compare the statistical distribution of the data sets from the
two interferometer sites, construct amplitude spectral densities of seismic
noise amplitude fluctuations with periods of up to 3 months, and analyze the
data for any long term trends in the amplitude of seismic noise in this
critical frequency range.Comment: To be published in Classical and Quantum Gravity. 24 pages, 15
figure
Vibration induced phase noise in Mach-Zehnder atom interferometers
The high inertial sensitivity of atom interferometers has been used to build
accelerometers and gyrometers but this sensitivity makes these interferometers
very sensitive to the laboratory seismic noise. This seismic noise induces a
phase noise which is large enough to reduce the fringe visibility in many
cases. We develop here a model calculation of this phase noise in the case of
Mach-Zehnder atom interferometers and we apply this model to our thermal
lithium interferometer. We are thus able to explain the observed dependence of
the fringe visibility with the diffraction order. The dynamical model developed
in the present paper should be very useful to further reduce this phase noise
in atom interferometers and this reduction should open the way to improved
interferometers
Toward Forecasting Volcanic Eruptions using Seismic Noise
During inter-eruption periods, magma pressurization yields subtle changes of
the elastic properties of volcanic edifices. We use the reproducibility
properties of the ambient seismic noise recorded on the Piton de la Fournaise
volcano to measure relative seismic velocity variations of less than 0.1 % with
a temporal resolution of one day. Our results show that five studied volcanic
eruptions were preceded by clearly detectable seismic velocity decreases within
the zone of magma injection. These precursors reflect the edifice dilatation
induced by magma pressurization and can be useful indicators to improve the
forecasting of volcanic eruptions.Comment: Supplementary information:
http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary
video:
http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av
Time Domain Classification and Quantification of Seismic Noise
Currently several efforts are undertaken in seismology to retrieve information about the underground from ambient seismic noise (e.g. Curtis et al. 2006; Shapiro et al. 2005; Sens-Schönfelder & Wegler 2006). Such studies are especially interesting in areas where traditional seismic methods are complicated such as remote areas with poor access and cities. E.g. a large number of passive seismic measurements in urban environments are undertaken with the aim to provide the required underground information for seismic hazard assessment. Seismological research must significantly improve the understanding of (urban) seismic noise to successfully and reliably apply these new methods in urban environments (Bonnefoy-Claudet et al. 2006; Campillo 2006). A good knowledge of the seismic noise conditions and contributing noise sources are crucial to select adequate time windows of available long-term data or to design short-term measurements.
We present a statistical classification scheme in the time domain to quantify and characterise seismic noise. The character of seismic noise (e.g. Gaussian distributed or dominated by single signals) is represented by only six noise classes. This approach allows us to easily visualise the seismic noise properties (amplitude and statistical properties). Furthermore, it provides a reduced dataset from broadband seismic waveforms to analyse temporal and spatial changes of seismic noise conditions
Noise generation in the solid Earth, oceans, and atmosphere, from non-linear interacting surface gravity waves in finite depth
Oceanic pressure measurements, even in very deep water, and atmospheric
pressure or seismic records, from anywhere on Earth, contain noise with
dominant periods between 3 and 10 seconds, that is believed to be excited by
ocean surface gravity waves. Most of this noise is explained by a nonlinear
wave-wave interaction mechanism, and takes the form of surface gravity waves,
acoustic or seismic waves. Previous theoretical works on seismic noise focused
on surface (Rayleigh) waves, and did not consider finite depth effects on the
generating wave kinematics. These finite depth effects are introduced here,
which requires the consideration of the direct wave-induced pressure at the
ocean bottom, a contribution previously overlooked in the context of seismic
noise. That contribution can lead to a considerable reduction of the seismic
noise source, which is particularly relevant for noise periods larger than 10
s. The theory is applied to acoustic waves in the atmosphere, extending
previous theories that were limited to vertical propagation only. Finally, the
noise generation theory is also extended beyond the domain of Rayleigh waves,
giving the first quantitative expression for sources of seismic body waves. In
the limit of slow phase speeds in the ocean wave forcing, the known and
well-verified gravity wave result is obtained, which was previously derived for
an incompressible ocean. The noise source of acoustic, acoustic-gravity and
seismic modes are given by a mode-specific amplification of the same
wave-induced pressure field near the zero wavenumber.Comment: Paper accepted for publication in the Journal of Fluid Mechanic
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