A 3-D shear velocity model of the crust and uppermost mantle beneath the United States from ambient seismic noise (original) (raw)
Related papers
2021
We present new estimates of lithospheric shear velocities for the intraplate seismic zones and the Illinois Basin in the U.S. midcontinent by analyzing teleseismic Rayleigh waves. We find that relatively high crustal shear velocities (V S) characterize the southern Illinois Basin, while relatively low crustal velocities characterize the middle and lower crust of the central and northern Illinois Basin. The observed high crustal velocities may correspond to high-density mafic intrusions emplaced into the crust during the development of the Reelfoot Rift, which may have contributed to the subsidence of the Illinois Basin. The low crustal V S beneath the central and northern basin follow the La Salle deformation belt. We also observe relatively low velocities in the mantle beneath the New Madrid seismic zone where V S decreases by about 7% compared to those outside of the rift. The low V S in the upper mantle also extends beneath the Wabash Valley and Ste. Genevieve seismic zones. Testing expected V S reductions based on plausible thermal heterogeneities for the midcontinent indicates that the 7% velocity reduction would not result from elevated temperatures alone. Instead this scale of anomaly requires a contribution from some combination of increased iron and water content. Both rifting and interaction with a mantle plume could introduce these compositional heterogeneities. Similar orientations for the NE-SW low-velocity zone and the Reelfoot Rift suggest a rift origin to the reduced velocities. The low V S upper mantle represents a weak region and the intraplate seismic zones would correspond to concentrated crustal deformation above weak mantle.
Complex and variable crustal and uppermost mantle seismic anisotropy in the western United States
Nature Geoscience, 2011
The orientation and depth of deformation in the Earth is characterized by seismic anisotropy 1 -variations in the speed of passing waves caused by the alignment of minerals under strain into a preferred orientation. Seismic anisotropy in the western US has been well studied 2-11 and anisotropy in the asthenosphere is thought to be controlled by plate motions and subduction 6-9 . However, anisotropy within the crust and upper mantle and the variation of anisotropy with depth are poorly constrained. Here, we present a three-dimensional model of crustal and upper mantle anisotropy based on new observations of ambient noise 12 and earthquake 13 data that reconciles surface wave and body wave 9 data sets. We confirm that anisotropy in the asthenosphere reflects a mantle flow field controlled by a combination of North American plate motion and the subduction of the Juan de Fuca and Farallon slab systems 6-9 . We also find that seismic anisotropy in the upper mantle and crust are largely uncorrelated: patterns of anisotropy in the crust correlate with geological provinces, whereas anisotropy in the upper mantle is controlled by temperature variations. We conclude that any coupling between anisotropy in the crust and mantle must be extremely complex and variable.
Journal of Geophysical Research, 2010
1] Surface wave dispersion measurements from ambient seismic noise and array-based measurements from teleseismic earthquakes observed with the EarthScope/USArray Transportable Array (TA) are inverted using a Monte Carlo method for a 3-D V S model of the crust and uppermost mantle beneath the western United States. The combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period, which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km. The high lateral resolution produced by the TA network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3-D model, particularly for the crustal part of the model. We show that a relatively simple model in which V S increases monotonically with depth in the crust can fit the data well across more than 90% of the study region, except in eight discrete areas where greater crustal complexity apparently exists. The regions of greater crustal complexity are the Olympic Peninsula, the MendocinoTriple Junction, the Yakima Fold Belt, the southern Cascadia back arc, the Great Central Valley of California, the Salton Trough, the Snake River Plain, and the Wasatch Mountains. We also show that a strong Rayleigh-Love discrepancy exists across much of the western United States, which can be resolved by introducing radial anisotropy in both the mantle and notably the crust. We focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by Voigt averaging. Model uncertainties from the Monte Carlo inversion are used to identify robust isotropic features in the model. The uppermost mantle beneath the western United States is principally composed of four large-scale shear wave velocity features, but lower crustal velocity structure exhibits far greater heterogeneity. We argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle, including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle. Upper and middle crustal wave speeds are generally correlated, and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions.
Forward modeling of the development of seismic anisotropy in the upper mantle
Earth and Planetary Science Letters, 1998
Development of seismic anisotropy in response to upper mantle flow is approached through an integrated numerical model. This model allows to predict the splitting parameters for a shear wave propagating across an upper mantle which deformed in response to a given geodynamic process. It consists of (1) thermo-mechanical modeling of the finite strain field, (2) modeling olivine lattice-preferred orientation (LPO) generated by this strain field, (3) calculation of the 3-D elastic properties associated with this LPO, and (4) estimation of the shear-wave splitting parameters: the time lag between the fast and slow split shear wave arrivals .δt/ and the polarization azimuth of the fast wave. /. Modeled olivine LPO are constrained relative to LPO measured in naturally and experimentally deformed peridotites. Comparison of predicted shear-wave splitting parameters with seismological data allows us to quantify the possible contribution of the modeled upper mantle flow to the measured splitting values and, hence, to constrain the interpretation of shear-wave splitting data in terms of upper mantle flow. We use this forward model to investigate the seismic anisotropy generated in ocean basins by a velocity gradient between the plate and the deep mantle. Fast-shear wave polarizations calculated assuming a constant plate motion are in good agreement with both the SKS polarization and the fast propagation direction for P and Rayleigh waves observed in the Pacific and Indian oceans, suggesting that, away from mid-ocean ridges, seismic anisotropy in oceanic basins primarily results from asthenospheric deformation by resistive drag beneath the plate. Delay times are, however, overestimated. This may be ascribed to a stronger strain localization in nature or to partial erosion of the anisotropic layer by hotspots. Indeed, hotspot activity may explain the short length scale variations of δt in the southern Pacific. Finally, twolayer models that simulate a change in Pacific plate motion as suggested by the bend in the Hawaii-Emperor chain fail to reproduce the observed shear-wave splitting. This is consistent with previous suggestions that the Emperor chain track may not faithfully record the Pacific plate absolute motion before 43 Ma.
Variable Azimuthal Anisotropy in Earth's Lowermost Mantle
Science, 2004
A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the Dµ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20-are consistent with the data. Small-scale convection cells within the mantle above the Dµ layer may cause the observed variations by inducing laterally variable crystallographic or shapepreferred orientation in minerals in the Dµ layer.
Mapping P-wave azimuthal anisotropy in the crust and upper mantle beneath the United States
Physics of the Earth and Planetary Interiors, 2013
Much progress has been made on revealing seismic structure and mantle dynamics beneath the United States (US) with the EarthScope/USArray project. Seismic anisotropy revealed by shear-wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images, but the shear-wave splitting observations have poor vertical resolution and so their interpretations are often not unique. In this work we used a large number of arrival-time data from local and distant earthquakes recorded by the USArray to determine the first P-wave azimuthal anisotropy tomography of the crust and upper mantle beneath the US. Our results show that fast velocity directions (FVDs) in the lithosphere under the tectonically active areas correlate well with the surface tectonic features, suggesting that the P-wave anisotropy mainly reflects the present deformation. A circular pattern of the FVDs centered in the Great Basin is revealed, which is well consistent with the specific circular shear-wave splitting observations there, suggesting that the anisotropy occurs in the crust and uppermost mantle. In contrast, beneath the stable cratonic region, the FVDs revealed by this study differ from the shear-wave splitting observations but consistent with the features of gravity and magnetic anomalies, indicating that the P-wave FVDs mainly reflect the fossil anisotropy in the lithosphere, whereas the Swave splitting observations mainly reflect the significant anisotropy in the asthenosphere. The present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the US.
Ubiquitous lower-mantle anisotropy beneath subduction zones
Nature Geoscience, 2019
The Earth's upper and lower mantle have quite distinct physical properties, with the characteristics of material exchange between them being a long-debated issue. Progress in global seismic tomography in the 1990s 1,2 showed that the upper and lower mantle interact mainly via subducting slabs and mantle plumes, albeit subject to the presence of strong resistance along the upper-lower mantle boundary at ~660 km depth. More recently, enhanced tomography images showed that among the slabs that penetrate into the lower mantle, many of them stagnate down to about ~1,000 km depth 3. Conversely, mantle plumes rising from the deep lower mantle seem to deflect laterally when they reach this region 4. However, the uppermost lower mantle, located at depths of ~660-1,000 km, remains an enigmatic part of the Earth. It has been suggested that compositional layering 5,6 or a viscosity increase 7,8 may cause flow stagnation in this region, but its rheology and role in mantle convection are poorly understood. The stagnation of subducting slabs at ~660 km depth and their penetration into the lower mantle lead to intense strain and deformation around the slabs, which in turn can align mineral aggregates. As the most abundant lower-mantle mineral (bridgmanite) is anisotro-pic, observable seismic anisotropy should develop when considering a dislocation creep deformation mechanism 9-11. However, apart from the D" region in the lowermost mantle 12 , the presence of seismic anisotropy in the lower mantle is uncertain and debated 13-15 , with most previous seismological models suggesting that the bulk of the uppermost lower mantle is radially isotropic in shear wavespeed 16. To resolve this paradox, it has been proposed that the dominant deformation mechanisms in the lower mantle, such as superplastic flow 17 or a pure climb creep mechanism 18 , may not produce anisotropy. Observations of anisotropy in the uppermost lower mantle Some recent regional shear-wave splitting studies suggest the presence of anisotropy in the transition zone and uppermost lower mantle near some subduction zones 19-21. However, the limited depth resolution and azimuthal coverage in regional studies, together with the difficulty in isolating lower-mantle anisotropy from upper-mantle effects, can restrict the interpretation of these studies. While illuminating mostly large-scale features, global anisotropy tomography overcomes these issues by mapping the whole mantle, which is key to interpreting large-scale processes and global mantle flow in a unified way 22,23. Nevertheless, several issues such as the use of different data and modelling approaches, notably when handling crustal effects 24-26 , led to poor agreement between past global mantle anisotropy models. SGLOBE-rani is a recent whole-mantle shear-wave radially anisotropic model that is based on a large seismic dataset of over 43 million seismic measurements with complementary sensitivity to the entire Earth's mantle. It simultaneously models crustal thickness and mantle structure to reduce artefacts in the retrieved anisotropic structure 27,28. The use of a huge set of over 10 million surface-wave overtone measurements, which have sensitivity down to ~1,000 km depth (Supplementary Fig. 1), enables good data coverage in the transition zone (Supplementary Figs. 2-4). Below that, a large set of body-wave travel-time measurements assures good data coverage in the remainder of the lower mantle (Supplementary Fig. 3). However, the poor balance between SV-and SH-sensitive travel-time data in existing body-wave datasets leads to poorly resolved lowermost-mantle anisotropy and leakage effects 28 , in agreement with the findings from other previous whole-mantle anisotropy studies 29,30. Thus, we take the conservative approach of not interpreting any anisotropic structures below ~1,400 km depth. Chang et al. 28 compared SGLOBE-rani with other recent global anisotropy models and, as expected, found better correlations between the iso-tropic part of the models than between the anisotropic structure. Yet, a correlation of about 0.5 was found between the anisotropic structure in SGLOBE-rani and in the recent model Savani 31 , which Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use three-dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660 km seismic discontinuity slabs are associated with fast SV anomalies up to about 3%, in the lower mantle fast SH anomalies of about 2% persist near slabs down to about 1,000-1,200 km. These observations are consistent with 3D numerical models of deformation from subducting slabs and the associated lattice-preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth's lower mantle, providing new constraints on the debated nature of deformation in this key, but inaccessible, component of the deep Earth.
Geophysical Journal International, 2007
Seismic anisotropy provides insight into palaeo and recent deformation processes and, therefore, mantle dynamics. In a first step towards a model for the North American upper mantle with anisotropy characterized by a symmetry axis of arbitrary orientation, aimed at filling the gap between global tomography and SKS splitting studies, we inverted long period waveform data simultaneously for perturbations in the isotropic S-velocity structure and the anisotropic parameter , in the framework of normal mode asymptotic coupling theory (NACT). The resulting 2-D broad-band sensitivity kernels allow us to exploit the information contained in long period seismograms for fundamental and higher mode surface waves at the same time. To ensure high quality of the retrieved regional upper-mantle structure, accurate crustal corrections are essential. Here, we follow an approach which goes beyond the linear perturbation approximation and split the correction into a linear and non-linear part. The inverted data set consists of more than 40 000 high quality three component fundamental and overtone surface waveforms, recorded at broad-band seismic stations in North America from teleseismic events and provides a fairly homogeneous path and azimuthal coverage. The isotropic part of our tomographic model shares the large-scale features of previous regional studies for North America. We confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central/eastern stable shield, as well as the presence of subducted material (Juan de Fuca and Farallon Plate) at transition zone depths. The new regional 3-D radial anisotropic model indicates the presence of two distinct anisotropic layers beneath the cratonic part of the North American continent: a deep asthenospheric layer, consistent with present day mantle flow, and a shallower lithospheric layer, possibly a record of ancient tectonic events.
Crustal structure of the Western U.S. from Rayleigh and Love wave amplification data
Journal of Geophysical Research: Solid Earth, 2023
Seismic imaging plays a crucial role in probing the structure and composition of the Earth's crust, especially when combined with laboratory measurements of crustal rocks (e.g., Christensen & Mooney, 1995; Rudnick & Gao, 2014). Seismic images of the Earth's crust are also useful for seismic hazard assessment (e.g., by providing key input information for accurate ground motion simulations) and are crucial for accurate earthquake source modeling (e.g., Frietsch et al., 2021). Moreover, removing the effects of the heterogeneous crust on seismic measurements can help constrain mantle structure (e.g., Ferreira et al., 2010; Schaeffer & Lebedev, 2015). There are several global models of the crust, including CRUST1.0 (Laske et al., 2012), LITHO1.0 (Pasyanos et al., 2014) and the more recent model of Szwillus et al. (2019). These models constrain crustal seismic velocities on a 1° × 1° grid scale and are mainly based on compilations of existing seismic and geophysical information, as well as on the modeling of seismic data. However, higher resolution models can be achieved on a regional scale. The dense network of EarthScope's USArray, which ended in 2021 (http://www.usarray.org/), provides an opportunity to image the local crust in finer detail across the continental U.S. (e.g., Porter et al., 2016; Schmandt & Humphreys, 2011). In particular, the western U.S. is an interesting study region due to its complex geological history and its wide range of tectonic provinces.
Geophysical Journal International, 2011
Using a combination of long period seismic waveforms and SKS splitting measurements, we have developed a 3-D upper-mantle model (SAWum_NA2) of North America that includes isotropic shear velocity, with a lateral resolution of ∼250 km, as well as radial and azimuthal anisotropy, with a lateral resolution of ∼500 km. Combining these results, we infer several key features of lithosphere and asthenosphere structure.