Ambient Noise Cross-correlation Research Articles

On the estimation of attenuation from the ambient seismic field: inferences from distributions of isotropic point scatterers

S U M M A R Y Cross-correlation of ambient seismic noise recorded by two seismic stations may result in an estimate of the Green’s function between those two receivers. Several authors have recently attempted to measure attenuation based on these interferometric, receiver–receiver surface waves. By now, however, it is well established that the loss of coherence of the crosscorrelation as a function of space depends strongly on the excitation of the medium. In fact, in a homogeneous dissipative medium, uniform excitation is required to correctly recover attenuation. Applied to fundamental-mode ambient seismic surface waves, this implies that the cross-correlation will decay at the local attenuation rate only if noise sources are distributed uniformly on the Earth’s surface. In this study we show that this constraint can be relaxed in case the observed loss of coherence is due to multiple scattering instead of dissipation of energy. We describe the scattering medium as an effective medium whose phase velocity and rate of attenuation are a function of the scatterer density and the average strength of the scatterers. We find that the decay of the cross-correlation in the effective medium coincides with the local attenuation of the effective medium in case the scattering medium is illuminated uniformly from all angles. Consequently, uniform excitation is not a necessary condition for the correct retrieval of scattering attenuation. We exemplify the implications of this finding for studies using the spectrally whitened cross-correlation to infer subsurface attenuation.

Crustal shear-wave velocity structure of northeastern Tibet revealed by ambient seismic noise and receiver functions

The Tibetan plateau is formed by the persistent convergence between the Indian and Eurasian plates. The northeastern Tibetan plateau is undergoing young deformation that has been noticed for a long time. We conduct a passive-source seismic profile with 22 stations in NE Tibet in order to investigate the crustal shear-wave velocity structure and its relationship with tectonic processes. In this paper we obtain the Rayleigh-wave phase velocity dispersion data among all station pairs within the period bandwidth of 5–20s from the method of ambient noise cross-correlations. Phase velocity variations correlate well with surface geological boundaries and tectonic features, for instance, low phase velocity beneath the Songpan–Ganzi block and the Guide basin. We also compute P-wave receiver functions based on the selected teleseismic events with similar ray parameters, and perform the joint inversion of surface wave dispersion data and receiver functions to obtain the 2-D crustal shear-wave velocity structure along the profile. The inversion results show that low shear-wave velocities beneath the Songpan–Ganzi block are widespread in the middle-to-lower crust. In together with high crustal Vp/Vs ratios and high temperature suggested by the P-wave velocities obtained from the active-source seismic study, we suggest that the low velocity zone beneath the Songpan–Ganzi block is probably attributed to partial melting. Across the North Kunlun fault, there is no crustal LVZ found beneath the Kunlun block. This structural difference may have already existed before the collision of the two blocks, or due to limit of the northward extension for the crustal LVZ across the North Kunlun fault.

Identification of surface wave higher modes using a methodology based on seismic noise and coda waves

Dispersion analysis of Rayleigh waves is performed to assess the velocity of complex structures such as sedimentary basins. At short periods several modes of the Rayleigh waves are often exited. To perform a reliable inversion of the velocity structure an identification of these modes is thus required. We propose a novel method to identify the modes of surface waves. We use the spectral ratio of the ground velocity for the horizontal components over the vertical component (H/V) measured on seismic coda. We then compare the observed values with the theoretical H/V ratio for velocity models deduced from surface wave dispersion when assuming a particular mode. We first invert the Rayleigh wave measurements retrieved from ambient noise cross-correlation with the assumptions that (1) the fundamental mode and (2) the first overtone are excited. Then we use these different velocity models to predict theoretical spectral ratios of the ground velocity for the horizontal components over the vertical component (H/V). These H/V ratios are computed under the hypothesis of equipartition of a diffuse field in a layered medium. Finally we discriminate between fundamental and higher modes by comparing the theoretical H/V ratio with the H/V ratio measured on seismic coda. In an application, we reconstruct Rayleigh waves from cross-correlations of ambient seismic noise recorded at seven broad-band stations in the Valley of Mexico. For paths within the soft quaternary sediments basin, the maximum energy is observed at velocities higher than expected for the fundamental mode. We identify that the dominant mode is the first higher mode, which suggests the importance of higher modes as the main vectors of energy in such complex structures.

Waveform modeling and inversion of ambient noise cross-correlation functions in a coastal ocean environment.

Theoretical studies have shown that cross-correlation functions (CFs) of time series of ambient noise measured at two locations yield approximations to the Green's functions (GFs) that describe propagation between those locations. Specifically, CFs are estimates of weighted GFs. In this paper, it is demonstrated that measured CFs in the 20-70 Hz band can be accurately modeled as weighted GFs using ambient noise data collected in the Florida Straits at ∼100 m depth with horizontal separations of 5 and 10 km. Two weighting functions are employed. These account for (1) the dipole radiation pattern produced by a near-surface source, and (2) coherence loss of surface-reflecting energy in time-averaged CFs resulting from tidal fluctuations. After describing the relationship between CFs and GFs, the inverse problem is considered and is shown to result in an environmental model for which agreement between computed and simulated CFs is good.

Variability of the coherent arrivals extracted from low-frequency deep-ocean ambient noise correlations

Correlation processing of ocean noise can be used to develop totally passive ocean monitoring methods. Using various hydrophone pair orientations, this study investigates the frequency dependence, seasonal variability, and emergence rate of coherent arrivals from cross-correlations of low frequency ambient noise (f < 40 Hz) recorded on triangular hydrophones arrays. These arrays are located at five existing hydroacoustic stations of the International Monitoring System (IMS), situated in the deep-sound channel, and distributed across the Atlantic, Pacific, and Indian Ocean basins. For the majority of studied sites, persistent and fast-emerging coherent arrivals are reliably obtained if the axis connecting the selected hydrophone pair has a direct line-of-sight with regions of the globe containing stable and diffuse noise sources (e.g., polar-ice or seismic noise). Furthermore, for this favorable orientation, the emergence rate of coherent arrivals extracted between hydrophone pairs separated by long ranges (here ∼130 km) can be approximated based on measurements made between hydrophone pairs separated by short ranges (∼2 km) in the Atlantic Ocean. Hence, results from this study, obtained using existing hydrophone configurations of the IMS hydroacoustic stations, could be used to guide the placement of other hydrophone arrays over the globe for future long-range passive ocean monitoring experiments.

Velocity models and site effects at Kawah Ijen volcano and Ijen caldera (Indonesia) determined from ambient noise cross-correlations and directional energy density spectral ratios

Two different ambient seismic noise techniques allow the determination of velocity models at Ijen caldera and Kawah Ijen volcano. At Ijen caldera, two average velocity models are constrained down to a depth of 3km by inverting the stacked dispersion curve obtained by ambient noise cross-correlation. In addition, four local velocity models were obtained by inverting the horizontal to vertical spectral ratio of the directional energy densities of ambient noise (DEDHVR) i.e., computed through auto-correlation of ambient noise. This set of velocity models is then compared to the geology of the caldera and the local discontinuities are evidenced. At Kawah Ijen volcano, a map of local dominant frequencies is provided by the DEDHVR measurements recorded at 87 sites around Kawah Ijen volcano. The horizontality of the shallow layers and the great outcrop of the crater rim facilitate the interpretation of the DEDHVRs. Furthermore, the inversion of several juxtaposed DEDHVRs provided a complete 2-D velocity model of the flank of the crater. Unlike other stratovolcanoes, the Kawah Ijen appears to have a remarkably homogeneous structure. The DEDHVR technique is used for the first time in a volcanic environment and shows a great potential for an accurate and fast computation of the velocity model on volcanoes.

Stationary‐phase integrals in the cross correlation of ambient noise

AbstractThe cross correlation of ambient signal allows seismologists to collect data even in the absence of seismic events. “Seismic interferometry” shows that the cross correlation of simultaneous recordings of a random wavefield made at two locations is formally related to the impulse response between those locations. This idea has found many applications in seismology, as a growing number of dense seismic networks become available: cross‐correlating long seismic records, the Green's function between instrument pairs is “reconstructed” and used, just like the seismic recording of an explosion, in tomography, monitoring, etc. These applications have been accompanied by theoretical investigations of the relationship between noise cross correlation and the Green's function; numerous formulations of “ambient noise” theory have emerged, each based on different hypotheses and/or analytical approaches. The purpose of this study is to present most of those approaches together, providing a comprehensive overview of the theory. Understanding the specific hypotheses behind each Green's function recipe is critical to its correct application. Hoping to guide nonspecialists who approach ambient noise theory for the first time, we treat the simplest formulation (the stationary‐phase approximation applied to smooth unbounded media) in detail. We then move on to more general treatments, illustrating that the “stationary‐phase” and “reciprocity theorem” approaches lead to the same formulae when applied to the same scenario. We show that a formal cross correlation/Green's function relationship can be found in complex, bounded media and for nonuniform source distributions. We finally provide the bases for understanding how the Green's function is reconstructed in the presence of scattering obstacles.

Potential of ambient seismic noise techniques to monitor the St. Gallen geothermal site (Switzerland)

AbstractThe failures of two recent deep geothermal energy projects in Switzerland (Basel, 2006; St. Gallen, 2013) have again highlighted that one of the key challenges for the successful development and operation of deep underground heat exchangers is to control the risk of inducing potentially hazardous seismic events. In St. Gallen, after an injection test and two acid injections that were accompanied by a small number of micro‐earthquakes (ML&lt;0.2), operators were surprised by an uncontrolled gas release from the formation (gas kick). The “killing” procedures that had to be initiated following standard drilling procedures led to a ML3.5 earthquake. With ambient seismic noise cross correlations from nine stations, we observe a significant loss of waveform coherence that we can horizontally and vertically constrain to the injection location of the fluid. The loss of waveform coherence starts with the onset of the fluid injections 4 days prior to the gas kick. We interpret the loss of coherence as a local perturbation of the medium. We show how ambient seismic noise analysis can be used to assess the aseismic response of the subsurface to geomechanical well operations and how this method could have helped to recognize the unexpected reservoir dynamics at an earlier stage than the microseismic response alone, allowed.

Choice of regularization in adjoint tomography based on two-dimensional synthetic tests

We present synthetic tests of 2-D adjoint tomography of surface wave traveltimes obtained by the ambient noise cross-correlation analysis across the Czech Republic. The data coverage may be considered perfect for tomography due to the density of the station distribution. Nevertheless, artefacts in the inferred velocity models arising from the data noise may be still observed when weak regularization (Gaussian smoothing of the misfit gradient) or too many iterations are considered. To examine the effect of the regularization and iteration number on the performance of the tomography in more detail we performed extensive synthetic tests. Instead of the typically used (although criticized) checkerboard test, we propose to carry out the tests with two different target models—simple smooth and complex realistic models. The first test reveals the sensitivity of the result on the data noise, while the second helps to analyse the resolving power of the data set. For various noise and Gaussian smoothing levels, we analysed the convergence towards (or divergence from) the target model with increasing number of iterations. Based on the tests we identified the optimal regularization, which we then employed in the inversion of 16 and 20 s Love-wave group traveltimes.

The impact of the 24 March 2011 Myanmar earthquake (MS 7.2) on seismic structure of the Yunnan region

The temporal and spatial variations of the velocity structure 60days before and after the March 24, 2011 Myanmar M7.2 earthquake in the Yunnan region are studied by using the ambient noise cross-correlation method with the continuous seismic waveform records. In addition, the response ratio of strain energy release is obtained using moderate and small earthquakes, and the Coulomb stress changes along different faults are calculated using the source parameters of the Myanmar earthquake. There are three regions where the velocities increased, the area from the Zemuhe–Xiaojiang fault zone to the Yuanmou–Lvzhi fault zone, the southern segment of the Honghe fault zone, and the area from the Nantinghe fault zone northward extending to the Dayingjiang fault zone. The seismic activity and the Coulomb stress of these regions are also enhanced. By contrast, there are several regions where velocities decreased, such as the area from the Nantinghe fault zone southward extending to the Simao region. The seismic activity and the Coulomb stress of these regions are decreased as well. The consistency of the variation indicates some relationship between the variation and the Myanmar earthquake.

Recent advances in imaging crustal fault zones: a review

Crustal faults usually have a fault core and surrounding regions of brittle damage, forming a low-velocity zone (LVZ) in the immediate vicinity of the main slip interface. The LVZ may amplify ground motion, influence rupture propagation, and hold important information of earthquake physics. A number of geophysical and geodetic methods have been developed to derive high-resolution structure of the LVZ. Here, I review a few recent approaches, including ambient noise cross-correlation on dense across-fault arrays and GPS recordings of fault-zone trapped waves. Despite the past efforts, many questions concerning the LVZ structure remain unclear, such as the depth extent of the LVZ. High-quality data from larger and denser arrays and new seismic imaging technique using larger portion of recorded waveforms, which are currently under active development, may be able to better resolve the LVZ structure. In addition, effects of the along-strike segmentation and gradational velocity changes across the boundaries between the LVZ and the host rock on rupture propagation should be investigated by conducting comprehensive numerical experiments. Furthermore, high-quality active sources such as recently developed large-volume air-gun arrays provide a powerful tool to continuously monitor temporal changes of fault-zone properties, and thus can advance our understanding of fault zone evolution.

Directionality of ambient noise on the Juan de Fuca plate: implications for source locations of the primary and secondary microseisms

Based on cross-correlations of ambient seismic noise computed using 61 ocean bottom seismometers (OBSs) within the Juan de Fuca (JdF) plate from the Cascadia Initiative experiment and 42 continental stations near the coast of the western United States, we investigate the locations of generation of the primary (11–20 s period) and secondary (5–10 s period) microseisms in the northern Pacific Ocean by analysing the directionality and seasonality of the microseism (Rayleigh wave) signals received in this region. We conclude that (1) the ambient noise observed across the array is much different in the primary and secondary microseism bands, both in its azimuthal content and seasonal variation. (2) The principal secondary microseism signals propagate towards the east, consistent with their generation in deep waters of the North Pacific, perhaps coincident both with the region of observed body wave excitation and the predicted wave–wave interaction region from recent studies. (3) The primary microseism, as indicated by observations of the azimuthal dependence of the fundamental mode Rayleigh wave as well as observations of precursory arrivals, derives significantly from the shallow waters of the eastern Pacific near to the JdF plate but also has a component generated at greater distance of unknown origin. (4) These observations suggest different physical mechanisms for generating the two microseisms: the secondary microseisms are likely to be generated by non-linear wave–wave interaction over the deep Pacific Ocean, while the primary microseism may couple directly into the solid earth locally in shallow waters from ocean gravity waves. (5) Above 5 s period, high quality empirical Green's functions are observed from cross-correlations between deep water OBSs and continental stations, which illustrates that microseisms propagate efficiently from either deep or shallow water source regions onto the continent and are well recorded by continental seismic stations.

Seismic anisotropy indicates ridge‐parallel asthenospheric flow beneath the Eastern Lau Spreading Center

AbstractSeismic anisotropy beneath the Eastern Lau Spreading Center (ELSC) is investigated using both Rayleigh waves and shear waves, using data from the 2009–2010 ELSC ocean bottom seismograph experiment. Phase velocities of Rayleigh waves determined by ambient noise cross correlation are inverted for azimuthally anisotropic phase velocity maps. Splitting of S waves from five intermediate and deep focus earthquakes was determined by waveform analysis. Taken together, Rayleigh wave and S wave data indicate that significant (~2%) anisotropy extends to at least 300 km depth. Both data sets indicate a fast direction aligned within a few degrees of the N10°E striking ELSC and somewhat oblique to the N25°E strike of the neighboring volcanic arc. We therefore describe the fast direction as spreading perpendicular, not convergence perpendicular and interpret it as due to ridge‐parallel flow of the asthenosphere. However, the region arcward (east) of the ELSC has the stronger anisotropy, suggesting that the strongest flow gradients may occur in the region between the spreading center and the arc, in contrast to being centered beneath the ELSC. Fluids released from the underlying plate may produce anisotropic hydrous materials, but more importantly lower the viscosity, thus enhancing along‐strike flow. Both could contribute to an along‐strike fast direction signature. Seafloor spreading diminishes south of the seismic array, ceasing altogether south of latitude 25°S (500 km south of the array center), a region dominated by much slower tectonic extension, suggesting that asthenosphere is inflowing from the north to accommodate the increase in asthenospheric volume associated with the seafloor spreading.

Feasibility of Passive Tomography of Extended Defects Using Ambient Elastic Noise Correlation

Using guided waves in structural health monitoring seems interesting because of their capability to propagate over long distances. This paper shows that tomography algorithms using guided waves in plate-like structures and cross-correlation of ambient noise can be combined to process the passive tomography of a structure. This can make a very robust and low power SHM system. An active tomography image has been performed with a time-of-flight tomography algorithm using experimental data produced by piezoelectric transducers. Comparison between active signals and passive signals which come from cross-correlation of ambient noise produced by compressed air sprayed on an aluminum plate shows that it is possible to detect time-of-flight passively.

Ground Truth Location of Earthquakes by Use of Ambient Seismic Noise From a Sparse Seismic Network: A Case Study in Western Australia

The estimated Green’s function (EGF) extracted from the ambient seismic noise cross-correlation function (NCF) enables valuable calibration of surface wave propagation along the path connecting seismic stations. Such calibration is adopted in a new method for ground truth location of earthquakes, achieved from the location relative to a seismic station. The surface wave group travel times were obtained from the NCFs between a station near the earthquake and remote stations. The differential travel times from the NCFs and the surface wave of the earthquake were used in a relative location procedure. When this method was applied to earthquake location with only six seismic stations in western Australia, the location of the Mw 4.1 Kalannie (September 21, 2005) earthquake was found to be accurate to within 2 km compared with the ground truth location with InSAR for which azimuth coverage of seismic stations is preferable. Synthetic tests suggest that the group travel time is slightly affected by focal mechanism and focal depth, thus unknown earthquake source parameters did not introduce substantial bias to earthquake location with the group travel time method.

Upper crustal azimuthal anisotropy across the contiguous U.S. determined by Rayleigh wave ellipticity

AbstractConstraints on upper crustal seismic anisotropy provide insight into the local stress orientation and structural fabric, but such constraints are scarce except in areas with dense recordings of local seismicity. We investigate directionally dependent Rayleigh wave ellipticity, or Rayleigh wave H/V (horizontal to vertical) amplitude ratios, between 8 and 20 s period across USArray to infer azimuthal anisotropy in the upper crust across the contiguous U.S. To determine the H/V ratios, we use all available multicomponent ambient noise cross correlations between all USArray stations operating between 2007 and 2013. In many locations, the observed H/V ratios are clearly back azimuth dependent with a 180° periodicity, which allows the fast directions and amplitudes of upper crustal anisotropy to be determined. The observed patterns of anisotropy correlate well with both near‐surface geological features (e.g., the Intermountain Seismic Belt and Appalachian‐Ouachita collision belt) and a previous stress model.

Body waves revealed by spatial stacking on long-term cross-correlation of ambient noise

Theoretical and experimental studies indicate that complete Green’s Function can be retrieved from cross-correlation in a diffuse field. High SNR (signal-to-noise ratio) surface waves have been extracted from cross-correlations of long-duration ambient noise across the globe. Body waves, not extracted in most of ambient noise studies, are thought to be more difficult to retrieve from regular ambient noise data processing. By stacking cross-correlations of ambient noise in 50 km inter-station distance bins in China, western United States and Europe, we observed coherent 20–100 s core phases (ScS, PKIKPPKIKP, PcPPKPPKP) and crustal-mantle phases (Pn, P, PL, Sn, S, SPL, SnSn, SS, SSPL) at distances ranging from 0 to 4 000 km. Our results show that these crustal-mantle phases show diverse characteristics due to different substructure and sources of body waves beneath different regions while the core phases are relatively robust and can be retrieved as long as stations are available. Further analysis indicates that the SNR of these body-wave phases depends on a compromise between stacking fold in spatial domain and the coherence of pre-stacked cross-correlations. Spatially stacked cross-correlations of seismic noise can provide new virtual seismograms for paths that complement earthquake data and that contain valuable information on the structure of the Earth. The extracted crustal-mantle phases can be used to study lithospheric heterogeneities and the robust core phases are significantly useful to study the deep structure of the Earth, such as detecting fine heterogeneities of the core-mantle boundary and constraining differential rotation of the inner core.

Long‐term dynamics of Piton de la Fournaise volcano from 13 years of seismic velocity change measurements and GPS observations

AbstractWe study the Piton de la Fournaise (PdF) volcano dynamics through the observation of continuous seismic velocity changes from 2000 to 2013. We compute the cross correlations of ambient seismic noise recorded at more than 30 short‐period and broadband stations of the UnderVolc temporary seismic experiment and of the PdF volcano observatory network. The velocity changes are estimated from the travel time delay measured on the cross correlations computed between pairs of stations. We average the relative velocity changes for all pairs of stations and obtain a time series of the velocity change of Piton de la Fournaise volcano over 13 years. From the period 0.5 to 4 s, the depth sensitivity of the velocity change is ranging from approximately 100 m to 2500 m. A slow decrease of velocity is measured from 2000 and ends with a major eruption that occurred in April 2007. This eruptive episode is followed by an increase of the velocity. These long‐term changes are compared to the deformation of the Piton de la Fournaise edifice estimated from geodetic measurements. An analysis of baseline change between GPS stations indicates an inflation of the volcanic edifice prior to April 2007 followed by a deflation since then. This deflation predominantly affects the terminal cone. Seismic velocity changes and deformation have similar long‐term trends with velocity decrease observed during inflation and velocity increase during deflation. However, the velocity change magnitude is about 2 orders of magnitude greater than the deformation. This suggests nonlinear relation between velocity changes and deformation.

Waveform inversion of ambient noise cross-correlation functions in a coastal ocean environment

Approximations to Green's functions have been obtained by cross-correlating concurrent records of ambient noise measured on near-bottom instruments at 5 km range in a 100 m deep coastal ocean environment. Inversion of the measured cross-correlation functions presents a challenge as neither ray nor modal arrivals are temporally resolved. We exploit both ray and modal expansion of the wavefield to address the inverse problem using two different parameterizations of the seafloor structure. The inverse problem is investigated by performing an exhaustive search over the relevant parameter space to minimize the integrated squared difference between computed and measured correlation function waveforms. To perform the waveform-based analysis described, it is important that subtle differences between correlation functions and Green's functions are accounted for. [Work supported by NSF and ONR.]

Crustal structure of Damavand volcano, Iran, from ambient noise and earthquake tomography

We investigated the shear wave velocity structure beneath the Damavand volcano, Iran, using analysis of Rayleigh wave empirical Green’s functions obtained from the cross-correlation of 1-year seismic ambient noise and 26 local earthquakes. After the observed phase, velocity dispersion curves were inverted to obtain phase velocity tomographic maps; the synthetic phase velocity dispersion curves were then estimated for each evenly spaced geographic grid point. Estimated synthetic phase velocities were inverted using the nonlinear damped least-squares inversion method to obtain a quasi-3D shear wave velocity for the study area. Analysis of obtained quasi-3D shear wave velocity model reveals the presence of three distinct low or high anomalies in the upper 5 km. A low-velocity layer (VS ~ 2.8 km/s) was observed in the upper ~2 km of the crust that includes the sedimentary sequence of carbonate, siliciclastic, and volcanic rocks. According to our results, there is an indication of a high-velocity layer (VS ≥ 3.0 km/s) at the depth range of 2 to 5 km, which may indicate the presence of dense, cooled magma. There is a clear indication of intrusion of the low-velocity anomaly/body into the higher velocity layer, in the depth range of approximately 3.0 to 4.5 km. The corresponding low-velocity body is interpreted as a hot magma chamber associated with the young eruption of Damavand. The chamber is located southwest of the crater, which stretches from the west to beneath the Damavand cone.