SUMMARY Ambient noise tomography is a rapidly emerging field of seismological research. This paper presents the current status of ambient noise data processing as it has developed over the past several years and is intended to explain and justify this development through salient examples. The ambient noise data processing procedure divides into four principal phases: (1) single station data preparation, (2) cross-correlation and temporal stacking, (3) measurement of dispersion curves (performed with frequency‐time analysis for both group and phase speeds) and (4) quality control, including error analysis and selection of the acceptable measurements. The procedures that are described herein have been designed not only to deliver reliable measurements, but to be flexible, applicable to a wide variety of observational settings, as well as being fully automated. For an automated data processing procedure, data quality control measures are particularly important to identify and reject bad measurements and compute quality assurance statistics for the accepted measurements. The principal metric on which to base a judgment of quality is stability, the robustness of the measurement to perturbations in the conditions under which it is obtained. Temporal repeatability, in particular, is a significant indicator of reliability and is elevated to a high position in our assessment, as we equate seasonal repeatability with measurement uncertainty. Proxy curves relating observed signal-to-noise ratios to average measurement uncertainties show promise to provide useful expected measurement error estimates in the absence of the long time-series needed for temporal subsetting.

/pdf/processing-seismic-ambient-noise-data-to-obtain-reliable-3tbf11lhfy.pdf

Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements

splitting observations are interpreted in terms of the strain-induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anlsotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of q. For stable continental regions, this is interpreted as "fossil" anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present-day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent "underplating" or conductive growth of the thermal boundary layer. One of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes. Most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed (by convective instability or delamination) and replaced by warm asthenosphere. These models, however, are incompatible with the evidence for preserved coherent lithospheric deformation; rather, the deformed mantle appears to have been heated in place. We suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation.

/pdf/shear-wave-splitting-and-subcontinental-mantle-deformation-3tk4a7yxgq.pdf

Shear wave splitting and subcontinental mantle deformation

The number of broadband three-component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the crustal thicknesses and V_p/V_s ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P-to-S converted phase trades off strongly with the crustal V_p/V_s ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H-V_p/V_s domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and V_p/V_s ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average V_p/V_s ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900322

Moho depth variation in southern California from teleseismic receiver functions

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98RG02075

Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?

Seismic anisotropy beneath continents is analyzed from shear-wave splitting recorded at more than 300 continental seismic stations. Anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity. The observed coherence with crustal deformation implies that the mantle plays a major, if not dominant, role in orogenies. No evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are coupled to general mantle circulation.

/pdf/seismic-anisotropy-beneath-the-continents-probing-the-depths-4f88qj5fdh.pdf

SEISMIC ANISOTROPY BENEATH THE CONTINENTS: Probing the Depths of Geology

SUMMARY We consider the behaviour of shear-wave splitting parameters that are made under the assumption of a single layer of anisotropic material when in fact two layers of differing anisotropic properties are present. It is shown that the resulting apparent splitting parameters are still meaningful quantities and in fact can (at fixed frequency) be written as trigonometric functions of the splitting parameters of the individual layers. These expressions reveal many properties of the apparent splitting parameters. For example, the apparent fast polarization 4, and delay time 6t, exhibit systematic variations as a function of incoming polarization with r/2 periodicity. The derived expressions can be used to invert for the individual properties of the two layers in many circumstances. We show that several stations along the San Andreas fault system display the properties of two layers, with the top layer being parallel to the local strike of the San Andreas fault. Finally, the derived expressions for the apparent splitting parameters are shown to be easily generalized to multiple layers.

/pdf/the-interpretation-of-shear-wave-splitting-parameters-in-the-198umcqybo.pdf

The Interpretation of Shear‐Wave Splitting Parameters In the Presence of Two Anisotropic Layers

We examine the effects on receiver functions of transverse anisotropy and of dipping isotropic boundaries. Splitting of the Moho Ps phase predicts the anisotropy from a postulated 10-km-thick layer of highly anisotropic crustal material only when other phases can be well isolated and when allowance is made for the rotation of the incident energy out of the plane of energy propagation expected for isotropic models. We examine azimuthal variations of the synthetic radial and transverse receiver functions. For both transversely anisotropic layers and dipping isotropic boundaries, radial receiver functions are symmetric and transverse receiver functions are antisymmetric about a given back azimuth (the horizontal projection of the axis of symmetry or the dip direction). Differences include the following: (1) Transverse energy from dipping boundaries arrives at the time of the initial P phase no matter at what depth the dipping boundaries occur, while transverse energy arrives at the time of the initial P phase for anisotropic material only when the anisotropy is at the surface. (2) Transversely anisotropic systems with horizontal symmetry axes have waveforms with 180° periodicity as a function of back azimuth, while dipping symmetry axes or dipping boundaries just have 360° periodicity. A consequence of the symmetry is that stacking the initial P and Ps phases from transverse receiver functions for back azimuths that are 180° apart tends to decrease the arrivals from shallowly dipping isotropic boundaries and doubles the arrivals from anisotropic layers with horizontal symmetry axes. We examine receiver functions for station SNZO in New Zealand, above a subducting plate dipping at 15° beneath the station. Opposite polarities of transverse receiver functions separated by 180° in back azimuth suggest that either dipping isotropic layers or dipping axes of symmetry of anisotropic layers are present. The 15° dip of the subduction zone is insufficient to explain the energy on the transverse receiver functions by the isotropic structures previously determined from refraction and earthquake travel time inversion. However, modifying the isotropic models for the region by anisotropy determined via shear wave splitting measurements and refraction studies, allows many of the features of both the radial and transverse receiver functions to be explained. A relatively highly anisotropic layer about 7 km thick at the base of the crust with an axis of symmetry dipping at 20° - 40° may be caused by metamorphosed oceanic crust.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB00795

Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand

SUMMARY

We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.

/pdf/ambient-noise-rayleigh-wave-tomography-of-new-zealand-zeuyxgqumb.pdf

Ambient noise Rayleigh wave tomography of New Zealand

SUMMARY We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green’s functions. We filter these Green’s functions to compute Rayleigh wave group dispersion curves at periods of 5‐50 s, using a phase-matched filter, frequency‐time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.

/pdf/ambient-noise-rayleigh-wave-tomography-of-new-zealand-34o0minmyc.pdf

Martha K. Savage

Papers

Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?

The Interpretation of Shear‐Wave Splitting Parameters In the Presence of Two Anisotropic Layers

Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand

Ambient noise Rayleigh wave tomography of New Zealand

Ambient Noise Rayleigh Wave Tomography of New Zealand