Добірка наукової літератури з теми "Seismic tomography, Ambient noise correlations, Australian Crust"

SUMMARY We present an improved crustal Vs model and Moho depth map using ambient noise wave-equation tomography. The so-called ‘ambient noise wave-equation tomography’ is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation, which accounts for 3-D and finite-frequency effects. We use cross-correlations of up to 4 yr of continuous vertical-component ambient seismic noise recordings from 304 high-quality broad-band stations in the Alpine region. We use model LSP_Eucrust1.0 obtained from traditional ambient noise tomography as initial model, and we iteratively improve the initial model by minimizing frequency-dependent phase traveltime differences between the observed and synthetic waveforms of Rayleigh waves in the period range 10–50 s. We obtain the final model after 15 iterations with ∼65 per cent total misfit reduction compared to the initial model. At crustal depth, the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies. At Moho and upper-mantle depth, the final model corrects an obvious systematic velocity shift of the initial model. The resulting isovelocity Moho map confirms a Moho step along the external side of the external crystalline massifs of the northwestern Alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern Alps. Ambient noise wave-equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory.

Abstract Ambient-noise tomography (ANT) has become a well-established method to image the crust and uppermost mantle structures in the past 15 yr. Having a good estimate of uncertainties of phase velocity dispersion measurements in ANT is critical as they can guide the level of data fitting in tomography. However, to date, there are still no systemic studies to evaluate these uncertainties. In this study, we obtain cross correlations with different stacking durations from 17 yr of ambient-noise data recorded at 120 stations in the United States. We analyze the variations of signal-to-noise ratio (SNR) and phase velocities of cross correlations. We find that the uncertainties of phase velocities are affected by SNRs, interstation distances, and stacking durations. However, none of those three variables can be solely used as a proxy to estimate the uncertainties of phase velocity measurements. Based on our analysis, we graphically present empirical relations of uncertainties of phase velocity measurements as a function of SNR, interstation distance, and stacking duration. These relations can be employed as a guide to estimate phase velocity uncertainties in applications of ANT, assisting in evaluating the reliability of resulting models from ANT.

Abstract. We applied seismic ambient noise surface wave tomography to estimate Rayleigh wave empirical Green's functions from cross-correlations to study crust and uppermost mantle structure beneath the Makran region in south-east Iran. We analysed 12 months of continuous data from January 2009 through January 2010 recorded at broadband seismic stations. We obtained group velocity of the fundamental mode Rayleigh-wave dispersion curves from empirical Green's functions between 10 and 50 s periods by multiple-filter analysis and inverted for Rayleigh wave group velocity maps. The final results demonstrate significant agreement with known geological and tectonic features. Our tomography maps display low-velocity anomaly with south-western north-eastern trend, comparable with volcanic arc settings of the Makran region, which may be attributable to the geometry of Arabian Plate subducting overriding lithosphere of the Lut block. At short periods (<20 s) there is a pattern of low to high velocity anomaly in northern Makran beneath the Sistan Suture Zone. These results are evidence that surface wave tomography based on cross correlations of long time-series of ambient noise yields higher resolution group speed maps in those area with low level of seismicity or those region with few documented large or moderate earthquake, compare to surface wave tomography based on traditional earthquake-based measurements.

SUMMARY This study presents the first detailed analysis of ambient noise tomography in an area of the continental upper crust in the Cantabrian Mountains (NW Spain), where a confluence of crustal scale faults occurs at depth. Ambient noise data from two different seismic networks have been analysed. In one side, a 10-short-period station network was set recording continuously for 19 months. A second set of data from 13 broad-band stations was used to extend at depth the models. The phase cross-correlation processing technique was used to compute in total more than 34 000 cross-correlations from 123 station pairs. The empirical Green's functions were obtained by applying the time–frequency, phase-weighted stacking methodology and provided the emergence of Rayleigh waves. After measuring group velocities, Rayleigh-wave group velocity tomographic maps were computed at different periods and then they were inverted in order to calculate S-wave velocities as a function of depth, reaching the first 12 km of the crust. The results show that shallow velocity patterns are dominated by geological features that can be observed at the surface, particularly bedding and/or lithology and fracturing associated with faults. In contrast, velocity patterns below 4 km depth seem to be segmented by large structures, which show a velocity reduction along fault zones. The best example is the visualization in the tomography of the frontal thrust of the Cantabrian Mountains at depth, which places higher velocity Palaeozoic rocks over Cenozoic sediments of the foreland Duero basin. One of the major findings in the tomographic images is the reduction of seismic velocities above the area in the crust where one seismicity cluster is nucleated within the otherwise quiet seismic area of the range. The noise tomography reveals itself as a valuable technique to identify shear zones associated with crustal scale fractures and hence, lower strain areas favourable to seismicity.

Abstract. We study the crustal structure under the Eastern and Southern Alps using ambient noise tomography. We use cross-correlations of ambient seismic noise between pairs of 71 permanent stations and 19 stations of the Eastern Alpine Seismic Investigation (EASI) profile to derive new 3D shear velocity models for the crust. Continuous records from 2014 and 2015 are cross-correlated to estimate Green's functions of Rayleigh and Love waves propagating between the station pairs. Group velocities extracted from the cross-correlations are inverted to obtain isotropic 3D Rayleigh- and Love-wave shear-wave velocity models. Our models image several velocity anomalies and contrasts and reveal details of the crustal structure. Velocity variations at short periods correlate very closely with the lithologies of tectonic units at the surface and projected to depth. Low-velocity zones, associated with the Po and Molasse sedimentary basins, are imaged well to the south and north of the Alps, respectively. We find large high-velocity zones associated with the crystalline basement that forms the core of the Tauern Window. Small-scale velocity anomalies are also aligned with geological units of the Austroalpine nappes. Clear velocity contrasts in the Tauern Window along vertical cross sections of the velocity model show the depth extent of the tectonic units and their bounding faults. A mid-crustal velocity contrast is interpreted as a manifestation of intracrustal decoupling in the Eastern Alps that accommodated eastward escape of the Alcapa block.

SUMMARY Stacking of ambient noise correlations is a crucial step to extract empirical Green's functions (EGFs) between station pairs. The traditional method is to linearly stack all short-duration cross-correlation functions (CCFs) over a long period of time to obtain final stacks. It requires at least several months of ambient noise data to obtain reliable phase velocities at periods of several to tens of seconds from CCFs. In this study, we develop a new stacking method named root-mean-square ratio selection stacking (RMSR_SS) to reduce the time duration required for the recovery of EGFs from ambient noise. In our RMSR_SS method, rather than stacking all short-duration CCFs, we first judge if each of the short-duration CCF constructively contributes to the recovery of EGFs or not. Then, we only stack those CCFs which constructively contribute to the convergence of EGFs. By applying our method to synthetic noise data, we demonstrate how our method works in enhancing the signal-to-noise ratio of CCFs by rejecting noise sources which do not positively contribute to the recovery of EGFs. Then, we apply our method to real noise data recorded in western USA. We show that reliable and accurate phase velocities can be measured from 15-d long ambient noise data using our RMSR_SS method. By applying our method to ambient noise tomography (ANT), we can reduce the deployment duration of seismic stations from several months or years to a few tens of days, significantly improving the efficiency of ANT in imaging crust and upper-mantle structures.

SUMMARY In the summer of 2017, we deployed 174 three-component nodal geophones along a 130 km west–east line across the central Oregon forearc lasting about 40 d. Our goal was to evaluate the possibility of imaging the lithospheric structure in detail with a dense but short-duration sampling of passive seismic signals. In this study, we used passive recordings from the nodal array and the previous CASC93 broad-band array along the same line to calculate noise cross-correlations. Fundamental Rayleigh wave signals were observed in the cross-correlations between 3 and 15 s period. To enhance the signal and simultaneously measure the phase velocity, we employed a double beamforming method. At each period and location, a source beam and a receiver beam were selected and the cross-correlations between the two were shifted and stacked based on the presumed local velocities. A 2-D grid search was then used to find the best velocities at the source and receiver location. Multiple velocity measurements were obtained at each location by using different source and receiver pairs, and the final velocity and uncertainty at each location were determined using the mean and the standard deviation of the mean. All available phase velocities across the profile were then used to invert for a 2-D shear wave crustal velocity model. Well resolved shallow slow velocity anomalies are observed corresponding to the sediments within the Willamette Valley, and fast velocity anomalies are observed in the mid-to-lower crust likely associated with the Siletzia terrane. We demonstrate that the ambient noise double beamforming method is an effective tool to image detailed lithospheric structures across a dense and large-scale (&gt;100 km) temporary seismic array.

ABSTRACT Seismic records during energetic storms, similar to earthquake signals, carry information about the Earth’s interior. However, the utility of these records is rare in seismic imaging, because signal onsets in storm-generated microseisms are difficult to pick. The exciting seismic sources are also unsteady. Here, we present a new method for extracting accurate Rayleigh-wave dispersion curves from broadband seismic recordings made during the passage of hurricanes by incorporating Shen et al. (2012) cross-correlation strategy with Wang et al. (2019) frequency–Bessel transform array stacking technique. We show that surface-wave dispersion curves are observed in the seismic recordings made of four hurricanes. Compared with dispersion curves from hurricanes traveling onshore or in the deepwater, dispersion curves have higher signal-to-noise ratios from hurricanes moving near the coast or along the continental shelf. The average dispersion curve obtained from stacked cross correlations of all the four hurricanes agrees closely with that obtained from yearly ambient noise. We utilize the average dispersion curve from hurricanes to obtain a reliable shear-wave velocity (VS) model. Our VS model matches that derived from annual ambient seismic noise results with an average difference less than 0.1 km/s in the crust and uppermost mantle. This study suggests that hurricane-based dispersion curves can be an effective supplement to surface-wave tomography.

SUMMARY The Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for &gt;300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.

SUMMARY Traditional two-station ambient noise interferometry estimates the Green’s function between a pair of synchronously deployed seismic stations. Three-station interferometry considers records observed three stations at a time, where two of the stations are considered receiver–stations and the third is a source–station. Cross-correlations between records at the source–station with each of the receiver–stations are correlated or convolved again to estimate the Green’s function between the receiver–stations, which may be deployed asynchronously. We use data from the EarthScope USArray in the western United States to compare Rayleigh wave dispersion obtained from two-station and three-station interferometry. Three three-station interferometric methods are distinguished by the data segment utilized (coda-wave or direct-wave) and whether the source–stations are constrained to lie in stationary phase zones approximately inline with the receiver–stations. The primary finding is that the three-station direct wave methods perform considerably better than the three-station coda-wave method and two-station ambient noise interferometry for obtaining surface wave dispersion measurements in terms of signal-to-noise ratio, bandwidth, and the number of measurements obtained, but possess small biases relative to two-station interferometry. We present a ray-theoretic correction method that largely removes the bias below 40 s period and reduces it at longer periods. Three-station direct-wave interferometry provides substantial value for imaging the crust and uppermost mantle, and its ability to bridge asynchronously deployed stations may impact the design of seismic networks in the future.

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. The TASMAL experiment 2003-2005 spans the expected location of the transition between Precambrian and Phanerozoic Australia. The 20 portable broadband stations were exploited in receiver function studies to extract S wave crustal structure through the inversion of stacked receiver functions using the Neighbourhood Algorithm. There is no clear crustal transition associated with the presence of Tasman Line. The Precambrian Cratons tend to exhibit crustal thicknesses close to 40 km but such values are also found in some Phanerozoic sites. ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. The TASMAL experiment covered a significant fraction of the Australian continent with a simultaneous deployment of portable broadband stations. From these continuous records, it has proved possible to extract very clear Rayleigh wave signals for station separations up to 2000 km, and to demonstrate the frequency dependent variations in group velocity behaviour. The combination of the paths between the 20 stations localize such behaviour, but detailed images needed more data. The entire archive of portable broadband data recorded by RSES was mined, and combined with data from permanent stations to provide more than 1100 estimates of interstation Green's functions within Australia. Group velocity analysis as function of frequency was followed by nonlinear tomography with the Fast Marching Method. The resulting images of group velocity patterns as a function frequency show pronounced regions of lowered group velocities, most of which match regions of thick sediment. The frequency dependence is not consistent with just sedimentary structure and low midcrustal velocities, most likely due to elevated temperatures, are also needed. ¶ The surface wave portion of the interstation Green's function is the most energetic, and is normally all that seen in ambient noise studies. However, in the coda of events record at the broadband Warramunga seismic array in the Northern Territory, the P and S body wave components also emerge. The characteristics of these arrivals match those observed from nearby small earthquakes. The stacked cross-correlation is the normal approach to enhance Green's function information from ambient noise, but a broader spectral band width with the same phase response can be found by spectral division. It appears advantageous to compare both approaches and select the best result, since very little modifications to procedures are needed. ¶ The properties of the ambient noise at a single station have been investigated in the logarithmic spectral domain and a station dependent signal can be extracted by stacking. The signal appears to be related to the local structure beneath the station, and when fully characterized may provide a new means of investigating structure.

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. ... ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. ¶ ...