Optimizing data usage in regional body wave tomography by using asynchronous network data and relative sensitivity kernels: an example from Patagonia

Several recent studies have demonstrated the importance of crustal corrections when inverting surface wave data to model lateral variations in mantle radial anisotropy. It has also been shown that the choice of the prior crustal model to correct the data can strongly influence the anisotropy model and potentially lead to different geodynamic interpretations. In comparing tomographic models of radial anisotropy obtained from different crustal corrections, these studies did not, however, determine quantitative model uncertainties. Nevertheless, mantle models resulting from different prior crustal corrections are statistically different only if the posterior model errors stemming from the non-uniqueness of the inverse problem are smaller than the effect of the crustal correction itself. Here, we applied a model space search approach to global fundamental and higher mode Rayleigh and Love wave phase velocity maps to determine reliable, quantitative model uncertainties on seismic velocities and radial anisotropy. The technique employed enabled us to describe the model space with a posterior probability density function, and therefore to test whether models obtained from different crustal corrections are statistically different. We thus assessed the significance of the choice of the crustal model by comparing the posterior model errors to the differences in mantle structure resulting from different crustal corrections. We tested prior crustal models CRUST2.0, CRUST1.0, and 3SMAC. Our study shows that the use of prior crustal corrections from different crustal models yields significant discrepancies in mantle velocities around 50 km depth and in radial anisotropy down to 100 km. The impact of the crustal correction on radial anisotropy can extend down to 250 km in some locations.We found that choosing 3SMAC instead of the other crustal models has a stronger influence on the mantle model, but that CRUST1.0 and CRUST2.0 yield statistically identical anisotropy models at all depths, except at a few grid cells. Importantly, the effect of the crustal model is most significant in continental regions and not so much beneath oceans, which has important consequences for determining the depth of continental roots. Our results therefore suggest that improving constraints on crustal structure in continents is essential for our understanding of continent formation. Our work also demonstrates that the prior crustal model does not significantly affect radial anisotropy and velocities at depths greater than 100 km. This implies that if geodynamic interpretations of radial anisotropy below 100 km depth were to account for tomographic model uncertainties, they would not depend on the choice of the prior crustal model. It is therefore important for geodynamicists and seismologists to work in concert and to put effort into determining quantitative tomographic model uncertainties before interpreting the results. Our results also caution against the use of 3SMAC to correct surface wave data for studies of the continental lithosphere, and suggest that the solid Earth community would benefit from putting some efforts toward building a revised 3SMAC. The discrepancies between mantle models built based on 3SMAC crustal corrections and those based on CRUST1.0 or CRUST2.0 should also help shed light on the validity of the geodynamical assumptions made in the construction of models like 3SMAC.

Accurately inferring the radially anisotropic structure of the mantle using seismic waveforms requires correcting for the effects of crustal structure on waveforms. Recent studies have quantified the importance of accurate crustal corrections when mapping upper mantle structure using surface waves and overtones. Here, we explore the effects of crustal corrections on the retrieval of deep mantle velocity and radial anisotropy structure. We apply a new method of nonlinear crustal corrections to a three‐component surface and body waveform data set and invert for a suite of models of radially anisotropic shear velocity. We then compare the retrieved models against each other and a model derived from an identical data set but using a different nonlinear crustal correction scheme. While retrieval of isotropic structure in the deep mantle appears to be robust with respect to changes in crustal corrections, we find large differences in anisotropic structure that result from the use of different crustal corrections, particularly at transition zone and greater depths. Furthermore, anisotropic structure in the lower mantle, including the depth‐averaged signature in the core‐mantle boundary region, appears to be quite sensitive to choices of crustal correction. Our new preferred model, SAW642ANb, shows improvement in data fit and reduction in apparent crustal artifacts. We argue that the accuracy of crustal corrections may currently be a limiting factor for improved resolution and agreement between models of mantle anisotropy.

SUMMARY The CALIXTO (Carpathian Arc Lithosphere X-Tomography) experiment offers a dense, high-quality data set to study the lithospheric/asthenospheric system underneath SE Romania, an earthquake-prone region in SE Europe. To increase the image resolution of structures in the uppermost mantle, the application of crustal traveltime corrections by a priori information before the teleseismic traveltime inversion has become a well-accepted procedure. For such a correction we present a regional 3-D crustal seismic velocity model that serves as the basis for a high-resolution teleseismic tomography (forthcoming paper by Martin et al.). Our 3-D crustal model is based on recent research in the region. We collect new results from two long-range seismic refraction lines, 3-D refraction tomography and teleseismic Ps conversions. Adding previously published models of the sediment distribution, Conrad and Moho depths, as well as crustal seismic P-wave velocities, we compile a 3-D crustal model for SE Romania. This 3-D model does not contain shallow small-scale heterogeneities (<10 km), but it reflects the large-scale structures such as variations in sediment thickness, average seismic velocities and 3-D Moho depth. It is well suited for the correction of teleseismic traveltime residuals, a prerequisite for a high-resolution teleseismic tomography study: for example, traveltime delays of up to 1.3 s are caused by the almost 20-km thick layer of sediments in the Focsani Basin. Such delays are comparable to or larger in size than the expected upper mantle traveltime residuals. We study the significance of 3-D crustal traveltime corrections relative to 1-D station corrections and show that the complex basin structures in SE Romania require a 3-D approach to reduce the smearing of crustal anomalies into the mantle. By modelling synthetic mantle structures with a slab, as it is expected for SE Romania, we also investigate how to adapt the inversion strategy, if crustal corrections are applied. Significant improvements are found by including the already corrected crustal layers in the inversion procedure, thereby enabling the inversion algorithm to project still remaining uncertainties in the less-resolved upper crustal layers. However, the fixing of the upper layers during the inversion due to the a priori knowledge of the crustal velocity anomalies clearly leads to smearing of uncorrected anomalies that are possibly located close to the crust–mantle boundary.

Patagonia is one of the key places to study the interaction of plate tectonics and mantle flow patterns with geological processes. This part of the continent is shaped by the northward migration of the Chile Triple Junction, currently marked by subduction of the Chilean spreading ridge at latitude 46oS, opening a slab window beneath Southern Patagonia. The idea of slab window was hypothesized to explain the volcanic gap between north Patagonia and the southern part of the peninsula. The analysis of volcanic rock composition shows the transition between a domain with the signature of slab melt (metasomatized MORB) and a domain with no slab signature (OIB source mantle). Along the Pacific coast, other slab windows were suggested in Central America, California and North Cordillera. The analysis of uplifted terranes and seismic imaging tried to constrain the geometry of these slab windows and map the mantle flow pattern that controls the present-day surface expression (topography, volcanism distribution). From a limited seismic coverage, early studies mapped the Patagonian slab window from body wave tomography and shear wave splitting. The recent deployment of a temporary seismic array from 2018 to 2021 and the Chilean seismic networks fills the data gap between the seismically active northern part of Patagonia and the more poorly studied southern part. This presentation will show the results of our recent seismic studies in Patagonia and help constrain the geodynamical processes associated with the slab window. From the analysis of SKS and similar core phases, we determine the pattern of azimuthal seismic anisotropy resulting from the mantle flow pattern beneath South America. Fast splitting directions are generally NE-SW throughout most of Southern Patagonia, similar to the pattern of large-scale azimuthal seismic anisotropy from global and regional surface wave models. However, between 46oS - 48oS, we observe large splitting values and an E-W direction showing the effect of the slab edge. This is consistent with models of rapid upper mantle flow from the Pacific around the southern edge of the Nazca slab. Seismic imaging using receiver functions and Rayleigh waves from earthquakes and ambient noise show very low upper mantle velocities and an absence of mantle lithosphere in this region, suggesting the lithosphere has been thermally eroded by the dynamics of the slab window. We will also show and discuss preliminary results of a body wave tomographic analysis of the same seismic station dataset.

P-wave travel time and amplitude anomalies from 59 teleseismic earthquakes are analyzed for 17 stations of a seismic network in Hokkaido region, Japan. Relative travel time and log amplitude variations within the network are parameterized by an azimuth-independent term (average), and first and second azimuthal terms. For the travel time, the azimuth-independent terms, which represent shallow near-station velocity anomalies, show an obvious geographical variation and a significant correlation with gravity (Bouguer) anomalies. The azimuth-independent terms vary from - 0.73 to 0.58 s. The slow directions of the first azimuthal terms for stations surrounding the Hidaka Mountains point consistently toward an area near the axis of the mountains, suggesting the existence of a low-velocity zone in the uppermost mantle beneath the Hidaka Mountains. For the amplitude, the azimuth-independent terms have large positive values (amplification) at stations where the geological setting is young, i.e., Quaternary. The first azimuthal terms for stations surrounding the Hidaka Mountains show some indication of large amplitudes for the incidence azimuth in the direction of the mountains. If the azimuth-independent terms are subtracted from the observed values, the travel time residuals and log amplitudes show some correlation, suggesting the focusing and defocusing of rays by the heterogeneity in the uppermost mantle beneath the network.

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittier Narrows earthquake in California (ML=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittier Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short‐period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three‐dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low‐pass filtered, and thePandSwave cross‐correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross‐correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well‐defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with aML=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.

: This report consists of two parts. The first consists of a surface wave regionalization and tomographic analysis of China. The second involves modeling of regional body waveforms from earthquakes in western China. Love and Rayleigh waves recorded at CDSN stations for earthquakes within China and on its periphery are used to determine dispersion along more than 400 paths in China and its immediate vicinity. These data are used to determine the dispersion characteristics of 17 regions, which may then be inverted for velocity structure. We have also attempted to determine the anisotropic nature of the crust and upper mantle of this area. However, at present, the data is not sufficient to resolve the additional parameters. Regional body waves have been collected and modeled from a profile of earthquakes located southwest of CDSN station WMQ. The profile is compared to a profile of synthetic seismograms computed using a frequency-wavenumber integration technique with an assumed velocity structure model. Since the depths of the different earthquakes varies, we also compare individual P waveforms with synthetics computed for 10, 20 and 30 km depth. The variable moveout of different phases within the P sub n-P sub g wavetrain enables a fairly accurate determination of source depth. This illustrates that the interference of phases that contribute to the regional P sub n-P sub g waveforms can serve as a discriminant.

Abstract. We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.

The paper deals with the timetabling problem of a single-track railway line. To solve the timetabling problem, we propose a three-stage approach combining several optimization criteria. Initially and mainly, the maximum relative travel time (ratio of travel time to minimum possible travel time) is minimized subject to a set of constraints, including departure time, train speed, minimum and maximum dwell time, and headway at track segments and stations. Since this problem has many solutions, the process is repeated for other trains, keeping the relative travel times of the critical train fixed, until all trains have been assigned their optimal relative travel times. In the second stage, the prompt allocation of trains is a secondary objective, and finally, in the third stage, the one minimizing the sum of the station dwell times of all trains, keeping the relative travel times constant, is selected to reduce fuel consumption, as a tertiary objective. To consider the user preferences in the optimization problems, the user preference departure time is used instead of the actual planned departure times. In order to guarantee that the exact or a very good approximate global optimum is attained, an algorithm based on the bisection rule is used. This method allows the computation time to be reduced in at least one order of magnitude for 42 trains. The problem of sensitivity analysis is also discussed, and closed form formulas for the sensitivities in terms of the dual variables are given. Several examples of applications are presented to illustrate the goodness of the proposed method. The results show that an adequate selection of intermediate stations and of the departure times are crucial in the good performance of the line and that inadequate spacings between consecutive trains can block the line. In addition, it is shown that, in order to improve performance, regional trains must be scheduled just ahead of or following the long distance trains, rather than having independent schedules. The sensitivities are shown to be very useful in identifying critical trains, segments, stations, departure times, and headways and in suggesting line infrastructure changes.

Abstract. We present a data-processing routine to compute relative finite-frequency travel time residuals using a combination of the Iterative Cross-Correlation and Stack (ICCS) algorithm and the Multi-Channel Cross-Correlation method (MCCC). The routine has been tailored for robust measurement of P- and S-wave travel times in several frequency bands and for avoiding cycle-skipping problems at the shortest periods. We also investigate the adequacy of ray theory to calculate crustal corrections for finite-frequency regional tomography in normal continental settings with non-thinned crust. We find that ray theory is valid for both P and S waves at all relevant frequencies as long as the crust does not contain low-velocity layers associated with sediments at the surface. Reverberations in the sediments perturb the arrival times of the S waves and the long-period P waves significantly, and need to be accounted for in crustal corrections. The data-processing routine and crustal corrections are illustrated using data from a~network in southwestern Scandinavia.

Three‐dimensional modeling of P wave travel time residuals was performed to investigate the deep slab‐related velocity structure within the Kurile subduction zone. Travel times of P waves from earthquakes at all depths within the Kurile subduction zone to World‐Wide Standard Seismograph Network and Canadian stations were measured. Instead of analyzing the travel time residuals of a single event, relative travel time residuals of P waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals. The effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference Earth model have been significantly reduced by analyzing relative travel time residuals. The effect of uncertainties in station corrections are eliminated by relative travel time residuals. Our measured P wave travel time data clearly indicate that high‐velocity anomalies exist in the lower mantle beneath the Kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle. The travel time modeling was accomplished using a fully three‐dimensional finite difference technique for wave propagation within the source region. The modeling results show that high‐velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated. A large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed P wave travel time data. The slab‐related velocity anomalies in the upper mantle of the Kurile subduction zone are generally consistent with the seismicity. The high‐velocity anomalies in the lower mantle below the Kurile subduction zone are broad and geometrically change from the southern to the northern Kuriles. Below the southern Kuriles, the high‐velocity anomalies appear to be subhorizontal. However, beneath the central Kuriles, there are broad, about 600‐km‐wide, high‐velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle. The high‐velocity anomalies found below the northern Kuriles are broad and on both the continental and the ocean side of the slab, and extend several hundred kilometers into the lower mantle.

P and SH velocity structure in the upper mantle beneath Northeast China: Evidence for a stagnant slab in hydrous mantle transition zone

SUMMARY We compare 3-D upper mantle anisotropic structures beneath the North American continent obtained using standard and improved crustal corrections in the framework of Non-linear Asymptotic Coupling Theory (NACT) applied to long period three component fundamental and higher mode surface waveform data. Our improved approach to correct for crustal structure in high-resolution regional waveform tomographic models goes beyond the linear perturbation approximation, and is therefore more accurate in accounting for large variations in Moho topography within short distances as observed, for instance, at ocean‐continent margins. This improved methodology decomposes the shallow-layer correction into a linear and non-linear part and makes use of 1-D sensitivity kernels defined according to local tectonic structure, both for the forward computation and for the computation of sensitivity kernels for inversion. The comparison of the 3-D upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected. However, significant variations are observed in the retrieved 3-D perturbations. The largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle, as would be expected. We suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow-layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity.

Crust and Lithospheric Structure – Natural Source Portable Array Studies of Continental Lithosphere

1.14 - Crust and Lithospheric Structure – Natural Source Portable Array Studies of Continental Lithosphere