ALBERT

Description: We argue for and present a reformulation of the seismic surface-wave inverse problem in terms of a thermal model of the upper mantle and apply the method to estimate lithospheric structure across much of the Canadian Shield. The reformulation is based on a steady-state temperature model, which we show to be justified for the studied region. The inverse problem is cast in terms of three thermal parameters: temperature in the uppermost mantle directly beneath Moho, mantle temperature gradient, and the potential temperature of the sublithospheric convecting mantle. In addition to the steady-state constraint, prior physical information on these model parameters is based on surface heat flow and heat production measurements, the condition that melting temperatures were not reached in the crust in Proterozoic times and other theoretical considerations. We present the results of a Monte Carlo inversion of surface-wave data with this thermal parameterization' subject to the physical constraints for upper mantle shear velocity and temperature, from which we also estimate lithospheric thickness and mantle heat flux. The Monte Carlo inversion gives an ensemble of models that fit the data, providing estimates of uncertainties in model parameters. We also estimate the effect of uncertainties in the interconversion between temperature and seismic velocity. Variations in lithospheric temperature and shear velocity are not well correlated with geological province or surface tectonic history. Mantle heat flow and lithospheric thickness are anti-correlated and vary across the studied region, from 11 mW/m2 and nearly 400 km in the northwest to about 24 mW/m2 and less than 150km in the southeast. The relation between lithospheric thickness and mantle heat flow is consistent with a power law relation similar to that proposed by Jaupart et al. (1998), who argued that the lithosphere and asthenosphere beneath the Canadian Shield are in thermal equilibrium and heat flux into the deep lithosphere is governed by small-scale sublithospheric convection.

Description: Using data from more than 2000 seismic stations from multiple networks arrayed throughout China (CEArray, China Array, NECESS, PASSCAL, GSN) and surrounding regions (Korean Seismic Network, F-Net, KNET), we perform ambient noise Rayleigh wave tomography across the entire region and earthquake tomography across parts of South China and Northeast China. We produce isotropic Rayleigh wave group and phase speed maps with uncertainty estimates from 8 to 50 s period across the entire region of study, and extend them to 70 s period where earthquake tomography is performed. Maps of azimuthal anisotropy are estimated simultaneously to minimize anisotropic bias in the isotropic maps, but are not discussed here. The 3D model is produced using a Bayesian Monte Carlo formalism covering all of China, extending eastwards through the Korean Peninsula, into the marginal seas, to Japan. We define the final model as the mean and standard deviation of the posterior distribution at each location on a 0.5° x 0.5° grid from the surface to 150 km depth. Surface wave dispersion data do not strongly constrain internal interfaces, but shear wave speeds between the discontinuities in the crystalline crust and uppermost mantle are well determined. We design the resulting model as a reference model, which is intended to be useful to other researchers as a starting model, to predict seismic wave fields and observables and to predict other types of data (e.g. topography, gravity). The model and the data on which it is based are available for download. In addition, the model displays a great variety and considerable richness of geological and tectonic features in the crust and in the uppermost mantle deserving of further focus and continued interpretation.

Description: Using data predominantly from the NECESS Array, but also incorporating surface wave data from surrounding networks, we present the results of a Bayesian Monte Carlo inversion of receiver functions, Rayleigh wave ellipticity (H/V ratio) and Rayleigh wave group and phase speeds from 8 to 80 s period for the 3-D shear velocity structure of the crust and uppermost mantle beneath Northeast China. We define the final model as the mean and standard deviation of the posterior distribution at each location on a 0.5° x 0.5° grid from the surface to 150 km depth. The primary scientific motivation is to investigate the expression of intracontinental volcanism across the region. The model lithosphere displays prominent features (middle and lower crustal velocity, Moho depth, lithospheric thickness) across the study area that coincide with the location of volcanoes, which are predominantly situated in two distinct volcanic regions, which we call the ‘Northeast China Lineated Quaternary Volcanic Zone’, found near the eastern margin of the Songliao Basin and extending to Changbaishan Volcano, and the ‘Northern and Southern Greater Xing'an Range Pleistocene Volcanic Zones’. There is a strong similarity between the lateral distribution of depth-integrated mantle velocity anomalies in our model with the teleseismic body wave model of Tang et al ., although the vertical distribution of anomalies differ.

Description: Radial and azimuthal anisotropy in seismic wave speeds have long been observed using surface waves and are believed to be controlled by deformation within the Earth's crust and uppermost mantle. Although radial and azimuthal anisotropy reflect important aspects of anisotropic media, few studies have tried to interpret them jointly. We describe a method of inversion that interprets simultaneous observations of radial and azimuthal anisotropy under the assumption of a hexagonally symmetric elastic tensor with a tilted symmetry axis defined by dip and strike angles. We show that observations of radial anisotropy and the 2 component of azimuthal anisotropy for Rayleigh waves obtained using USArray data in the western United States can be fit well under this assumption. Our inferences occur within the framework of a Bayesian Monte Carlo inversion, which yields a posterior distribution that reflects both variances of and covariances between all model variables, and divide into theoretical and observational results. Principal theoretical results include the following: (1) There are two distinct groups of models (Group 1, Group 2) in the posterior distribution in which the strike angle of anisotropy in the crust (defined by the intersection of the foliation plane with Earth's surface) is approximately orthogonal between the two sets. (2) The Rayleigh wave fast axis directions are orthogonal to the strike angle in the geologically preferred group of models in which anisotropy is strongly non-elliptical. (3) The estimated dip angle may be interpreted in two ways: as a measure of the actual dip of the foliation of anisotropic material within the crust, or as a proxy for another non-geometric variable, most likely a measure of the deviation from hexagonal symmetry of the medium. The principal observational results include the following: (1) Inherent S -wave anisotropy ( ) is fairly homogeneous vertically across the crust, on average, and spatially across the western United States. (2) Averaging over the region of study and in depth, in the crust is approximately 4.1 ± 2 per cent. in the crust is approximately the same in the two groups of models. (3) Dip angles in the two groups of models show similar spatial variability and display geological coherence. (4) Tilting the symmetry axis of an anisotropic medium produces apparent radial and apparent azimuthal anisotropies that are both smaller in amplitude than the inherent anisotropy of the medium, which means that most previous studies have probably underestimated the strength of anisotropy.

Description: A non-linear Bayesian Monte-Carlo method is presented to estimate a Vsv model beneath stations by jointly interpreting Rayleigh wave dispersion and receiver functions and associated uncertainties. The method is designed for automated application to large arrays of broad-band seismometers. As a testbed for the method, 185 stations from the USArray Transportable Array are used in the Intermountain West, a region that is geologically diverse and structurally complex. Ambient noise and earthquake tomography are updated by applying eikonal and Helmholtz tomography, respectively, to construct Rayleigh wave dispersion maps from 8 to 80 s across the study region with attendant uncertainty estimates. A method referred to as ‘harmonic stripping method’ is described and applied as a basis for quality control and to generate backazimuth independent receiver functions for a horizontally layered, isotropic effective medium with uncertainty estimates for each station. A smooth parametrization between (as well as above and below) discontinuities at the base of the sediments and crust suffices to fit most features of both data types jointly across most of the study region. The effect of introducing receiver functions to surface wave dispersion data is quantified through improvements in the posterior marginal distribution of model variables. Assimilation of receiver functions quantitatively improves the accuracy of estimates of Moho depth, improves the determination of the Vsv contrast across Moho, and improves uppermost mantle structure because of the ability to relax a priori constraints. The method presented here is robust and can be applied systematically to construct a 3-D model of the crust and uppermost mantle across the large networks of seismometers that are developing globally, but also provides a framework for further refinements in the method.

Description: Recently constructed models of crustal structure across Tibet based on surface wave data display a prominent mid-crustal low velocity zone (LVZ) but are vertically smooth in the crust. Using six months of broad-band seismic data recorded at 22 stations arrayed approximately linearly over a 440 km observation profile across northeastern Tibet (from the Songpan–Ganzi block, through the Qaidam block, into the Qilian block), we perform a Bayesian Monte Carlo joint inversion of receiver function data with surface wave dispersion to address whether crustal layering is needed to fit both data sets simultaneously. On some intervals a vertically smooth crust is consistent with both data sets, but across most of the observation profile two types of layering are required: a discrete LVZ or high velocity zone (HVZ) formed by two discontinuities in the middle crust and a doublet Moho formed by two discontinuities from 45–50 km to 60–65 km depth connected by a linear velocity gradient in the lowermost crust. The final model possesses (1) a mid-crustal LVZ that extends from the Songpan–Ganzi block through the Kunlun suture into the Qaidam block consistent with partial melt and ductile flow and (2) a mid-crustal HVZ bracketing the south Qilian suture coincident with ultrahigh pressure metamorphic rocks at the surface. (3) Additionally, the model possesses a doublet Moho extending from the Qaidam to the Qilian blocks which probably reflects increased mafic content with depth in the lowermost crust perhaps caused by a vertical gradient of ecologitization. (4) Crustal thickness is consistent with a step-Moho that jumps discontinuously by 6 km from 63.8 km (±1.8 km) south of 35° to 57.8 km (±1.4 km) north of this point coincident with the northern terminus of the mid-crustal LVZ. These results are presented as a guide to future joint inversions across a much larger region of Tibet.

Description: 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.