Anatomy of the high-frequency ambient seismic wave field at the TCDP borehole.

International audienceThe Taiwan Chelungpu-fault Drilling Project (TCDP) installed a vertical seismic array between 950 and 1270 m depth in an active thrust fault environment. In this paper we analyze continuous noise records of the TCDP array between 1 and 16 Hz. We apply multiple array processing and noise correlation techniques to study the noise source process, properties of the propagation medium, and the ambient seismic wave field. Diurnal amplitude and slowness patterns suggest that noise is generated by cultural activity. The vicinity of the recording site to the excitation region, indicated by a narrow azimuthal distribution of propagation directions, leads to a predominant ballistic propagation regime. This is evident from the compatibility of the data with an incident plane wave model, polarized direct arrivals of noise correlation functions, and the asymmetric arrival shape. Evidence for contributions from scattering comes from equilibrated earthquake coda energy ratios, the frequency dependent randomization of propagation directions, and the existence of correlation coda waves. We conclude that the ballistic and scattered propagation regime coexist, where the first regime dominates the records, but the second is weaker yet not negligible. Consequently, the wave field is not equipartitioned. Correlation signal-to-noise ratios indicate a frequency dependent noise intensity. Iterations of the correlation procedure enhance the signature of the scattered regime. Discrepancies between phase velocities estimated from correlation functions and in-situ measurements are associated with the array geometry and its relative orientation to the predominant energy flux. The stability of correlation functions suggests their applicability in future monitoring efforts

Origin of deep ocean microseisms by using teleseismic body waves.

International audienceRecent studies of oceanic microseisms have concentrate on fundamental-mode surface waves. Extraction of fundamental-mode Rayleigh and Love wave Green functions from station-station correlations of ambient seismic noise has recently been demonstrated to be a very powerful tool for imaging of the Earth's crust and uppermost mantle. In this study we concentrate on energetic arrivals in two frequency bands around the primary (14s) and the secondary (7s) microseismic peaks that appear at near-zero times in noise cross-correlations. Thanks to a polarisation analysis of data from the the ETSE network (Turkey), we identify this "near-zero time" signal as an upcoming P wave in the secondary microseismic frequency band (5-10s). In a second step, analysing noise cross-correlations from three different arrays ( in Yellowstone, in Turkey and in Kyrgyzstan), we determine the origin of these signals by means of beamforming analysis and its projection on the Earth. Our results show that, in the 0.1-0.3 Hz frequency band, the energetic "near-zero" time arrivals in seismic noise cross-correlations are mainly formed by teleseismic P, PP, and PKP waves. Generation of this ambient body waves in the secondary microseismic band presents a marked seasonal behaviour with sources located in southern and northern oceans during summer and winter, respectively. Moreover, body wave array analysis is accurate enough to confirm that significant amount of the microseism energy is generated far from the coast in deep oceans

Spatial coherence of the seismic wavefield continuously recorded by the USArray

International audienceWe use a method based on the array covariance matrix eigenvalues to study the level of spatial coherence and of isotropy of the seismic wavefield continuously recorded during 2010 by the USArray. First, we observe that the raw data are often dominated by local sources. To remove their influence, we apply spectral and temporal normalizations to the input signals. We notice that this widely used preprocessing in ambient-noise seismology does not fully homogenize the seismic wavefield and that some strongly coherent arrivals persist. Among these persistent signals generated by teleseismic sources we detect (1) seismic waves emitted by strong earthquakes, (2) a nearly continuous quasi-monochromatic signal at 26 s period, and (3) multiday coherent wave trends in the spectral band of oceanic microseisms (0.07-0.2 Hz). For the latter, beamforming analysis shows that while most of the signals are composed of surface waves, some are dominated by body waves likely generated in the deep ocean

Interaction of microseisms with crustal heterogeneity: A case study from the San Jacinto fault zone area

International audienceWe perform a multicomponent analysis to evaluate the validity and limits of noise imaging in the San Jacinto fault zone (SJFZ) area. Estimates of noise propagation and scattering length scales in the area are combined with a noise correlation-based analysis of variability of noise constituents, excitation regions, and propagation patterns. We evaluate the quality of correlation-phase and -amplitude imaging of tectonic features in the context of observed noise properties. Statistical properties of a regional high-resolution 3-D velocity model indicate that propagation of double-frequency microseism Rayleigh waves is sensitive to medium heterogeneity in the southern California plate boundary area. The analysis of noise correlation functions constructed from records of a regional seismic network suggests stable excitation of microseisms along the southern California coastline. The proximity to the source region together with randomization properties of the heterogeneous medium govern the scattered yet anisotropic character of the wave field. Insignificant travel time errors resulting from the associated imperfect reconstruction of interstation Green's function estimates allow the resolution of a velocity contrast across the SJFZ from noise correlations. However, attenuation estimates are biased by the anisotropic propagation directions. The interaction of the ambient surface wave field with medium heterogeneity facilitates imaging of the velocity structure, but the inversion of the amplitude pattern is limited since it is dominated by wave field instead of medium properties

Three-dimensional shear velocity anisotropic model of Piton de la Fournaise Volcano (La Réunion Island) from ambient seismic noise,

International audienceWe cross correlate 4 years of seismic noise from the seismic network of Piton de la Fournaise Volcano (La Réunion Island) to measure the group velocity dispersion curves of Rayleigh and Love waves. We average measurements from vertical and radial components to obtain 577 Rayleigh wave dispersion curves. The transverse components provided 395 Love wave dispersion curves. We regionalize the group velocities measurements into 2-D velocity maps between 0.4 and 8 s. Finally, we locally inverted these maps for a pseudo 3-D anisotropic shear-velocity model down to 3 km below the sea level using a Neighborhood Algorithm. The 3-D isotropic shear-wave model shows three distinct high-velocity anomalies surrounded by a low-velocity ring. The anomaly located below the present “Plaine des Sables” could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location. The second high-velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity. The third high-velocity anomaly located below the “Grandes Pentes” and the “Grand Brûlé” areas and is an imprint of the solidified magma chamber of the dismantled “Les Alizés” Volcano. Radial anisotropy shows two main anomalies: positive anisotropy above sea level highlighting the recent edifice of Piton de la Fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of Piton de la Fournaise dominated by intrusions of vertical dykes

Ambient noise surface wave tomography to determine the shallow shear velocity structure at Valhall: depth inversion with a Neighbourhood Algorithm

International audienceS U M M A R Y This study presents a depth inversion of Scholte wave group and phase velocity maps obtained from cross-correlation of 6.5 hr of noise data from the Valhall Life of Field Seismic network. More than 2 600 000 vertical-vertical component cross-correlations are computed from the 2320 available sensors, turning each sensor into a virtual source emitting Scholte waves. We used a traditional straight-ray surface wave tomography to compute the group velocity map. The phase velocity maps have been computed using the Eikonal tomography method. The inversion of these maps in depth are done with the Neighbourhood Algorithm. To reduce the number of free parameters to invert, geological a priori information are used to propose a power-law 1-D velocity profile parametrization extended with a gaussian high-velocity layer where needed. These parametrizations allowed us to create a high-resolution 3-D S-wave model of the first 600 m of the Valhall subsurface and to precise the locations of geological structures at depth. These results would have important implication for shear wave statics and monitoring of seafloor subsidence due to oil extraction. The 3-D model could also be a good candidate for a starting model used in full-waveform inversions

Detecting seismic activity with a covariance matrix analysis of data recorded on seismic arrays

International audienceModern 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