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  • 1
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    Asymmetry of coda cross-correlation function: dependence of the epicentre location (2015)
    Oxford University Press
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    Publication Date: 2015-04-04
    Description: We analyze the cross-correlation function (CCF) of coda of earthquakes, which is used to retrieve the Green's function between two stations as well as the CCF of ambient noise. We select 74 Hi-net stations located in eastern Japan and 66 earthquakes to calculate the CCF. For each earthquake, we calculate the CCFs between possible pairs for the frequency bands of 0.1–0.2 Hz, 0.2–0.4 Hz and 0.4–0.8 Hz. Then we stack the CCFs for different earthquakes at each pair to obtain the average CCF. Although the correlation coefficients between the average and each CCFs are lower than 0.5 for most of the earthquakes, we obtain the propagating Rayleigh wave trace from average CCFs. We focus on the ratio of the amplitude in the positive lag time of the CCF to that in the negative lag time. CCFs for different earthquakes show different ratios which depend on the angle between the path of two stations and the epicentre. The amplitude in the lag time corresponding to the signal travelling from the near source station to the far source station is larger than that in the opposite lag time. Therefore the energy flux is not isotropic even in the coda and the energy from the source side is dominant. We average the ratios of pairs whose absolute values of angles are less than 45°. The average ratios are 0.5 at 0.1–0.2 Hz. For higher frequencies, the ratio is not clear because of the bad signal-to-noise ratio. According to the diffusion model, the ratio is predicted as 0.6. Therefore, the coda is represented as the diffusion state in 0.1–0.2 Hz with our observation setting.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    Influence of seismic anisotropy on the cross correlation tensor: numerical investigations (2015)
    Oxford University Press
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    Publication Date: 2015-03-11
    Description: Temporal changes in seismic anisotropy can be interpreted as variations in the orientation of cracks in seismogenic zones, and thus as variations in the stress field. Such temporal changes have been observed in seismogenic zones before and after earthquakes, although they are still not well understood. In this study, we investigate the azimuthal polarization of surface waves in anisotropic media with respect to the orientation of anisotropy, from a numerical point of view. This technique is based on the observation of the signature of anisotropy on the nine-component cross-correlation tensor (CCT) computed from seismic ambient noise recorded on pairs of three-component sensors. If noise sources are spatially distributed in a homogeneous medium, the CCT allows the reconstruction of the surface wave Green's tensor between the station pairs. In homogeneous, isotropic medium, four off-diagonal terms of the surface wave Green's tensor are null, but not in anisotropic medium. This technique is applied to three-component synthetic seismograms computed in a transversely isotropic medium with a horizontal symmetry axis, using a spectral element code. The CCT is computed between each pair of stations and then rotated, to approximate the surface wave Green's tensor by minimizing the off-diagonal components. This procedure allows the calculation of the azimuthal variation of quasi-Rayleigh and quasi-Love waves. In an anisotropic medium, in some cases, the azimuth of seismic anisotropy can induce a large variation in the horizontal polarization of surface waves. This variation depends on the relative angle between a pair of stations and the direction of anisotropy, the amplitude of the anisotropy, the frequency band of the signal and the depth of the anisotropic layer.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Toward 4D Noise-Based Seismic Probing of Volcanoes: Perspectives from a Large-N Experiment on Piton de la Fournaise Volcano (2016)
    Seismological Society of America (SSA)
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    Publication Date: 2016-01-08
    Description: Noise-based seismology is proving to be a complementary approach to active-source or earthquake-based methods for imaging and monitoring the Earth’s interior. Until recently, however, noise-based imaging and monitoring relied mostly on the inversion of surface waves reconstructed from correlations of mostly microseismic seismic noise (around 5 s of period). Compared to body-wave tomography and considering similar wavelengths, surface-wave tomography succeeds in retrieving lateral subsurface velocity contrasts but is less efficient in resolving velocity perturbations at depth. Recently, the use of large- N seismic arrays has proven to be of great benefit for extracting body waves from noise correlations by stacking over a large number of receiver pairs and by applying array processing. In this article, we describe a recent large- N array experiment that we conducted on Piton de la Fournaise (PdF). Our main goal was to extract body waves traveling directly in the vicinity of the active magma reservoir located at ~2.5 km depth below the summit crater using noise correlations between arrays of seismic sensors. Within this article, we provide technical information about the VolcArray experiment, which consisted of the deployment of 300 seismic nodes during one month on PdF. We also present data-quality measurements and show how the short-period seismometers that we used compare to standard broadband seismic records. Finally, we show noise array beamforming results to study the content of seismic noise at frequencies between 1 and 12 Hz and discuss the ability to use these data to recover body waves between arrays from the correlation of ambient seismic noise.
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 4
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    Seismic anisotropy and its precursory change before eruptions at Piton de la Fournaise volcano, La Réunion (2015)
    Wiley
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    Publication Date: 2015-04-13
    Description: Piton de la Fournaise Volcano exhibits frequent eruptions preceded by seismic swarms and is a good target to test hypotheses about magmatically induced variations in seismic wave properties. We use the permanent station network and a portable broadband network to compare seismic anisotropy measured via shear wave splitting with geodetic displacements, ratios of compressional to shear velocity (Vp/Vs), earthquake focal mechanisms and ambient noise correlation analysis of surface wave velocities, and to examine velocity and stress changes from 2000 through 2012. Fast directions align radially to the central cone and parallel to surface cracks and fissures, suggesting stress-controlled cracks. High Vp/Vs ratios under the summit compared with low ratios under the flank suggest spatial variations in the proportion of fluid-filled versus gas-filled cracks. Secular variations of fast directions (ϕ) and delay times (dt) between split shear waves are interpreted to sense changing crack densities and pressure. Delay times tend to increase while surface wave velocity decreases before eruptions. Rotations of ϕ may be caused by changes in either stress direction or fluid pressure. These changes usually correlate with GPS baseline changes. Changes in shear wave splitting measurements made on multiplets yield several populations with characteristic delay times, measured incoming polarizations and fast directions, which change their proportion as a function of time. An eruption sequence on 14 October 2010 yielded over 2000 shear wave splitting measurements in a 14-hour period, allowing high-time-resolution measurements to characterize the sequence. Stress directions from a propagating dike model qualitatively fit the temporal change in splitting.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Timing of a large volcanic flank movement at Piton de la Fournaise Volcano using noise-based seismic monitoring and ground deformation measurements (2013)
    Oxford University Press
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    Publication Date: 2013-10-09
    Description: In volcanic domains, magma transport and pressure build-up induce high stress–strain perturbations in the surrounding volcanic edifice that may lead to volcanic flank movements and possible instability. In this study, we focus on the 2007 March–April episode of volcanic activity at Piton de la Fournaise (PdF) Volcano, La Réunion Island. This episode was associated with a large volume of emitted lava (240 x 10 6 m 3 ) and a 340-m caldera collapse. We present observations of continuous seismic velocity changes measured using cross-correlations of ambient seismic noise over 10 yr at PdF. Overall, we observe a large velocity reduction starting a few days prior to the major 2007 April 2 eruption. Comparison of seismic velocity change measurements with observations of deformation from InSAR and GPS shows that the seismic velocity reduction coincided with a widespread flank movement starting at the time of injection of magma to feed an initial eruption, a few days before the 2007 April 2 eruption. We emphasize the potential of noise-based seismic velocity change measurements, together with geodetic observations, to detect and monitor possibly hazardous slope instabilities.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 6
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    First Results from the UnderVolc High Resolution Seismic and GPS Network Deployed on Piton de la Fournaise Volcano (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-01-01
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 7
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    MSNoise, a Python Package for Monitoring Seismic Velocity Changes Using Ambient Seismic Noise (2014)
    Seismological Society of America (SSA)
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    Publication Date: 2014-05-04
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 8
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    Continuous Kurtosis-Based Migration for Seismic Event Detection and Location, with Application to Piton de la Fournaise Volcano, La Reunion (2014)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2014-02-04
    Description: We present an automatic earthquake detection and location technique based on migration of continuous waveform data. Data are preprocessed using a kurtosis estimator in order to enhance the first arrival information, then migrated onto a predefined search grid using precalculated P -wave travel times, and finally stacked. Local maxima in the resulting 4D space–time grid indicate the locations and origin times of seismic events. We applied our technique to earthquake swarms occurring on Piton de la Fournaise volcano, La Réunion, France. We located 5000 events from 12 different swarms that occurred between 2009 and 2011. Our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level. Multiplet analysis of the detected events and subsequent double-difference relocation produce sharper images of the earthquake swarms. Our code, Waveloc, is released in open source. Online Material: Figures of seismicity distributions from Waveloc, synthetic test, and stack amplitude values versus magnitudes.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 9
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    Monitoring reservoir response to earthquakes and fluid extraction, Salton Sea geothermal field, California (2018)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2018-01-11
    Description: Continuous monitoring of in situ reservoir responses to stress transients provides insights into the evolution of geothermal reservoirs. By exploiting the stress dependence of seismic velocity changes, we investigate the temporal evolution of the reservoir stress state of the Salton Sea geothermal field (SSGF), California. We find that the SSGF experienced a number of sudden velocity reductions (~0.035 to 0.25%) that are most likely caused by openings of fractures due to dynamic stress transients (as small as 0.08 MPa and up to 0.45 MPa) from local and regional earthquakes. Depths of velocity changes are estimated to be about 0.5 to 1.5 km, similar to the depths of the injection and production wells. We derive an empirical in situ stress sensitivity of seismic velocity changes by relating velocity changes to dynamic stresses. We also observe systematic velocity reductions (0.04 to 0.05%) during earthquake swarms in mid-November 2009 and late-December 2010. On the basis of volumetric static and dynamic stress changes, the expected velocity reductions from the largest earthquakes with magnitude ranging from 3 to 4 in these swarms are less than 0.02%, which suggests that these earthquakes are likely not responsible for the velocity changes observed during the swarms. Instead, we argue that velocity reductions may have been induced by poroelastic opening of fractures due to aseismic deformation. We also observe a long-term velocity increase (~0.04%/year) that is most likely due to poroelastic contraction caused by the geothermal production. Our observations demonstrate that seismic interferometry provides insights into in situ reservoir response to stress changes.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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  • 10
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    Detecting seismic activity with a covariance matrix analysis of data recorded on seismic arrays (2016)
    Oxford University Press
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    Publication Date: 2016-01-17
    Description: Modern seismic networks are recording the ground motion continuously at the Earth's surface, providing dense spatial samples of the seismic wavefield. The aim of our study is to analyse these records with statistical array-based approaches to identify coherent time-series as a function of time and frequency. Using ideas mainly brought from the random matrix theory, we analyse the spatial coherence of the seismic wavefield from the width of the covariance matrix eigenvalue distribution. We propose a robust detection method that could be used for the analysis of weak and emergent signals embedded in background noise, such as the volcanic or tectonic tremors and local microseismicity, without any prior knowledge about the studied wavefields. We apply our algorithm to the records of the seismic monitoring network of the Piton de la Fournaise volcano located at La Réunion Island and composed of 21 receivers with an aperture of ~15 km. This array recorded many teleseismic earthquakes as well as seismovolcanic events during the year 2010. We show that the analysis of the wavefield at frequencies smaller than ~0.1 Hz results in detection of the majority of teleseismic events from the Global Centroid Moment Tensor database. The seismic activity related to the Piton de la Fournaise volcano is well detected at frequencies above 1 Hz.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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