S42A-01
Lateral Variations of 1 Hz Crustal Lg Q In Eastern Eurasia Mapped Using A Two-step Tomographic Procedure
More than 6,000 Lg spectra have been collected from 162 broad band seismic stations in Eastern Eurasia to map lateral variations of Q. To obtain Lg Q maps with high resolution while minimizing effects of the parameter trade-off with source spectra, we proposed a two-step tomographic procedure in January, 2003. In the first step, path-variable Q0 and eta (Lg Q at 1 Hz and its frequency dependence) are measured using the standard and reversed two-station methods. Reliable lower resolution maps of Q0 and eta are obtained in this step. In the second step of the procedure, spectra from individual events are used to simultaneously solve for the source spectrum and path-variable Q with a Bayesian method. Maps of Q0 and eta obtained in step 1 are used as a priori knowledge to constrain the source spectra. At the end of step 2 all path-variable Q measurements are brought together to obtain a tomographic map with a higher resolution. The first step of the inversion using the 6,000 plus spectra has been completed, resulting in long-wavelength maps of Q0 and eta (Xie et al., GRL 2006). We have since been conducting the second step of the inversion. We find that a reliable inversion of eta is difficult with our currently imprecise theoretical modeling of Lg spectra and the limited frequency range of the available data. Therefore we have focused on the inversion of Q0 only. Various numerical procedures have been implemented or fine-tuned for this inversion. The new high resolution Q0 map for eastern Eurasia, and its correlation with regional tectonics, will be presented.
S42A-02
Joint inversion of receiver functions and Rayleigh wave dispersion for crustal and upper mantle structures: a comparison of a nonlinear genetic approach and a linear inversion
To reduce the non-uniqueness in receiver function inversion, surface wave dispersion data have been inverted simultaneously for crustal and upper mantle structure using either a linear iterative approach or nonlinear global search algorithms. The former, however, has been implemented with a fixed Vp/Vs ratio while the latter was exercised only in determining crustal structure. Here we developed a parallelized inversion scheme utilizing the niching genetic algorithm to jointly invert receiver function and Rayleigh wave group velocity dispersion data to determine crustal and upper mantle structure down to approximately 200 km depth. Layer thickness and Vp/Vs ratio were treated as free and fixed parameters for the crustal and upper mantle layers, respectively. To better constrain Moho depth and Vp/Vs ratio in the crust, we included the H-κ stacking amplitude of high quality receiver functions in the object function. We applied the inversion to 7 CDSN stations in northeast China. For each station, the PREM model with a total of 40 layers (9 crustal layers + 31 upper mantle layers) was used as the initial model. We have searched a model space that consists of 500 generations that are made by a total of 500,000 individual models. Our preliminary results reveal a low velocity layer at 60-80 km depth beneath 3 stations located in the Songliao Basin which are not seen from the uplifted surrounding areas. We also perform a linear iterative inversion with the same data set. In general, the two inversions yield very similar velocity structure, especially at shallow depth such as crustal velocity and the crust mantle transition. There are, however, some significant differences between velocity models derived from the two methods, for example, the linear inversion always shows a low velocity zone at approximately 160-180 km depth, which are not seen by the nonlinear inversion models. Our synthetic tests indicate that such a lower velocity layer is likely an artifact from fitting the dispersion curve in the linear inversion.
S42A-03
The Crust and upper mantle structure beneath Yunnan from joint inversion of receiver functions and Rayleigh wave dispersion data
We apply joint inversion of receiver function and Rayleigh wave dispersion (phase and group) velocities data to estimate a shear-wave velocity structure for the Yunnan and its adjacent region. Our joint inversion models indicate strong lateral variations in crustal thickness in this area, which is gradually increase from 32 km in SE to 56km in NW of Yunnan. The gradual change in crustal thickness matches with the gradual topography change over this region. Our models also show a uniform crust upper mantle S velocity structure across the region, with a 8 ~ 22 km thick upper crust (Vs <3.4 ~ 3.5 km/s), overlying a gradational middle to lower crust with shear wave velocities up to 3.5 ~ 4.1 km/s at Moho depth. The thickness of middle to lower crust is correlated with the Moho depth beneath the seismic stations. We see evidence of crustal flow in middle to lower crust beneath the Yunnan region, especially in the western Yunnan. It is possible that the thick crust in this study area is mainly caused by a more ductile flow in the middle to lower crust. The upper mantle S velocities (Sn) vary from 4.15 to 4.75 km/s across the Yunnan and its adjacent region. The Sn velocities are slightly higher beneath the southern Yangtze block and Southeast Yunnan fold belt than the other block. This low Sn velocity and LVZ in middle to lower crust beneath western Yunnan and Panxi rift may imply that there has been thermal disturbance to the uppermost mantle away from deep structures in this region.
S42A-04
Mapping The Variations Of Moho Depth And Poisson's Ratio In China With Receiver Function Analyses
We collected and processed a large amount of high-quality broadband teleseismic waveforms recorded at all 48 stations in the Chinese National Digital Seismic Network (CNDSN) to estimate the lateral variations of Moho depth and crustal Vp/Vs ratio (hence Poisson's ratio) in China by receiver function analyses. A cross-correlation based method was used to select mutually coherent receiver functions, which yielded over 200 traces for most of the stations. Because multiple maxima often present within a thin band in the H-k domain due to the depth- velocity trade-off, we stabilized this method by weighing each H-k grid using the cross-correlation between Ps converted phase and other Moho multiples. An nth-root stacking method was also applied to reduce uncorrelated noise relative to the linear stack. These modifications successfully ruled out any unrealistic results from H-k search. Relatively reliable crustal thickness and Poisson's ratio were consistently obtained from both the RZ (radial and vertical components) and SP (components concentrate SV- and P- wave energy in a skew coordinate system) based receiver function data. Although we utilized average crustal P- wave velocities obtained from Pn/Sn tomographic studies in projecting time to depth, the crustal thickness and Poisson's ratio obtained from receiver functions still show significant discrepancies with those inferred from Pn and Sn waves. For the stations along the east coast of China, the crustal thickness varies from 29km to 37km and the Vp/Vs ratio is about 1.70 on average. While for the stations at the middle section of China across the Sino-Korean platform and the Yangtze platform, the crust turns to be 39km on average, and the Vp/Vs ratios are higher than those to the east coast. The results presented beneath the stations in the west of China well illustrated the complicated and active tectonic complexion in this region. Along the Tianshan fold system, the Moho is at about 53 km on average, whereas in the Tibetan plateau, the Moho could extend to over 80 km. The average Poisson's ratios are a little bit higher than the sub-regions to the east, but are rather lower than that inferred from Pn tomography. The azimuthal variations, as well as complicated crustal and upper mantle structures such as sediment and LVZ beneath some stations have also been observed from receiver functions. These results will assist to constitute initial models in linear and non-linear inversions to obtain a much finer 1-D crustal and upper mantle velocity model beneath each station.
S42A-05
Velocity Structure and Poisson's Ratio of the Crust and Upper Mantle Beneath Canada and Their Tectonic Implications
We apply a global-search waveform modeling method to fit S, Sp, SsPmP, and shear-coupled PL phases recorded from 132 earthquakes at eleven permanent broadband seismic stations in Canada. Our objective is to determine the one-dimensional azimuthally dependent crust and uppermost mantle velocity structure and Poisson's ratio beneath individual stations for the purpose of tectonic interpretations. Our technique uses the reflectivity method to compute synthetic seismograms and applies a global optimization algorithm called Very Fast Simulated Annealing (VFSA) to obtain the best possible velocity models. We also use built-in statistical tools such as the Posterior Probability Density (PPD) distributions and parameter correlation matrices to evaluate the reliability of resulting parameter estimates. Our method complements traditional receiver function techniques in that it models a different part of the seismogram and obtains P- and S-wave velocities directly. The Canadian seismic stations each recorded between 3 and 25 earthquakes suitable for analysis. The stations sample major tectonic provinces within the Canadian landmass such as the Cordilleran orogen, western plains and Slave province in western Canada, Grenville province and Appalachian orogen in eastern Canada, and the Canadian Arctic. Preliminary results from our study are consistent with the results from earlier studies using receiver functions and active source data collected through LITHOPROBE, and regional tectonics. Crustal thicknesses beneath stations in the northern Cordilleran orogen, western plains, and Slave province range between 35 and 37 km, whereas their average Poisson's ratio range between 0.21 and 0.24. The Moho appears to be slightly shallower (31-35 km) beneath stations in the southern Cordilleran orogen. However, the average crustal Poisson's ratio is similar (0.20–0.24). In eastern Canada, the crust beneath stations of the Grenville province and Appalachian orogen is generally thick (approximately 44 km) with the exception of that beneath station DRLN, in the northeast, where it is approximately 33 km. The average crustal Poisson's ratio is also higher (0.24-0.29). Moho depths beneath stations in the Canadian Arctic range between approximately 30 and 41 km, whereas the average crustal Poisson's ratio range between 0.22-0.26. Most of the crustal Poisson's ratio values beneath Canada are below the global average for continental crust (approximately 0.265). The higher Poisson's ratio values that are well-constrained tend to be associated with crust of older age, typically Archaean. The spectrum of Poisson's ratio values is suggestive of a more felsic crustal composition in the western parts of Canada and a more mafic composition in the eastern parts. We also observe low-velocity zones (LVZ) in the crust and uppermost mantle in the vicinity of a few stations, however the constraints on these LVZs are poor.
S42A-06
Upper mantle shear wave velocity structure of the Arabian Shield from teleseismic surface wave tomography
We investigate the seismic shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield by inverting Rayleigh wave phase velocity measurements between 44 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 50 s from Pasyanos. The Rayleigh wave phase velocity measurements between 44 and 140 s have been made using teleseismic earthquake data mainly from the Saudi Arabia National Digital Seismic Network (SANDSN) or Saudi Arabian PASSCAL experiment. A model of upper mantle structure beneath the Arabian Shield obtained from Rayleigh wave phase velocity tomography shows a broad low velocity region in the lithospheric mantle across the Shield and a low velocity region at depths > 150 km localized along the Red Sea coast and MMN volcanic line. The estimated thickness of the lithosphere from the model is ~90-100 km beneath the southern part of the Shield, ~70 km beneath the northern Shield under the MMN volcanic line, and ~100 km near the Platform boundary. These finding, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities beneath the southern part of the Arabian Shield, body wave tomographic models, and a S-wave polarization analysis, the model presented in this study can be used to support geodynamic interpretations invoking a single large plume or a Superplume to explain the Cenozoic hotspot tectonism on the Arabian Shield.
S42A-07
Shear Wave Splitting Observations Beneath the Iranian Plateau
Recordings of teleseismic events from 69 three-component stations deployed over the Iranian Plateau were used for shear wave splitting studies. No significant splitting was measured in Zagros, Kopet-Dagh and NW Iran suggesting absence of any coherent anisotropic fabric or presence of complexities in the upper mantle beneath these tectonic regions leading to the null splitting for shear-waves. Intricate splitting patterns with varying fast directions were observed in Central Alborz implying presence of complex anisotropic fabrics. On the other hand, time lags greater than 0.8 sec with a general NW-SE fast polarization direction were observed in a narrow zone in Central Iran. Inhomogeneous splitting pattern over the Iranian Plateau excludes mantle shearing induced by present-day absolute motion of the Plateau as being the cause of the observed shear wave splitting. Rather, we suggest that the observed splitting pattern in Central Iran is related to the mantle flow associated with the relative motion of Arabian lithosphere with respect to the Central Iranian blocks.
S42A-08
Shear Wave Splitting Beneath the New Madrid Seismic Zone and Adjacent Areas
Teleseismic shear-wave splitting parameters are determined at 15 permanent and portable broadband stations within and around the New Madrid seismic zone (NMSZ) in order to map the direction and strength of mantle fabrics and to explore the origin of seismic anisotropy. Both the splitting times and fast polarization directions of the fast shear-wave show significant spatial variations. The observed splitting times range from 0.7 to 1.7s with a mean value of 1.0s which is the same as the global average. The resulting fast directions range from 34 to 118 degrees from north with a mean of 65 degrees which is consistent with the motion direction of the North American plate in a hot-spot frame. Fast directions with ray-piercing points in the NMSZ are oblique to the rift axis. In the vicinity of the Ozarks Plateau, the split times range from 0.7s to 1.1s with a mean of 0.9s. The observed fast directions show a striking clockwise rotating pattern in which these change systematically from nearly N-S in the St. Francois Mountains to approximately NE-SW further north to be concordant to that of North American Craton. The area with anomalous fast directions has recently been suggested to be a downward asthenospheric flow as a result of the sinking of the Farallon slab in the lower mantle (Forte et al 2007). The observed anisotropy will be discussed in relation to the lower mantle flow, and the recently-proposed two-layer model of Marone and Romanowicz (2007).