A Bayesian Nonlinear Inversion of Seismic Body-Wave Attenuation Factors

It is a well-known fact that the uncertainties in measuring relative attenuation factors within a local or regional seismic network are usually high, due to noise of different kinds and unrealistic assumptions. Numerical experiments using nine synthetic seismograms, created using t* values ranging from 0.1 to 0.9 sec, reveal that the commonly used spectral ratio method is strongly affected by the selection of data processing parameters such as width of the spectral smoothing window, reference station, and so on. The numerical experiments demonstrate that a Bayesian nonlinear inversion approach that directly matches the spectra is better at finding the correct parameters used to generate the synthetic seismograms. The Bayesian inversion approach uses a priori information to simultaneously search for the t* values, the common spectrum for all the records from an event, and the near-receiver amplification factors by using all the recordings from an event. When z, the ratio of Gaussian noise to signal, ≦ 0.1, the spectral ratio and Bayesian methods yield similar results with mean t* measurement errors \u3c 0.05 sec. For 0.1 \u3c z ≦ 0.8, the mean errors of the spectral ratio method are larger than 0.1 sec and in some cases as large as 0.6 sec, while those of the Bayesian method are less than 0.08 sec. Frequency-independent t* and near-receiver amplification factors are assumed. A multi-step procedure is proposed to reject records with a large misfit

Temporal Variation of Seismic B-Values beneath Northeastern Japan Island Arc

Analysis of a high quality seismic catalog reveals that the average of seismic b-values in the crust beneath most part of northeastern Japan island arc decreased from 0.86 between 1984 and 1990, to 0.73 between 1991 and 1995. The two areas with the largest decrease are found to be in the same areas where the coupling between the North American and the Pacific plates is the highest, as suggested by a recent geodetic study. In the same time period, the annual seismic moment release increased by 10 times. In addition, there seems to be a corresponding increase in volcanic activities in the same area. One of the most likely interpretations for the observations is an increase in the subduction rate starting from 1991. The timing of this possible increase in subduction rate is consistent with an apparent increase in global seismic activity

Estimation of the Depth of Anisotropy using Spatial Coherency of Shear-Wave Splitting Parameters

Shear-wave splitting (SWS) analyses are essential in understanding the structure and dynamics of the Earth\u27s deep interior. While splitting measurements have excellent horizontal resolution relative to other anisotropy-measuring techniques, their vertical resolution is low due to the steep incidence angle of the seismic phases used by the analyses. Here, using synthetic and real data, we present and test a simple approach to estimate the optimal depth of anisotropy by measuring the spatial coherency of the splitting parameters. The approach searches for the optimal depth by computing a spatial variation factor. Tests using synthetic SWS data produced with varying number of events, number of stations, and levels of noise suggest that the approach can satisfactorily find the depth of the source of anisotropy. Successful application of the depth-estimation procedure requires well-defined splitting parameters obtained from a multistation network and multiple events from a decent back-azimuthal range. It also requires significant and smooth spatial variations of anisotropy with horizontal axis of symmetry within a single layer of anisotropy. We applied the approach to 448 pairs of splitting measurements obtained at about 50 stations on the Ethiopian Plateau and found an optimal depth of anisotropy of about 300 km, suggesting an asthenospheric origin of the observed anisotropy

Mantle Transition Zone Discontinuities beneath the Baikal Rift and Adjacent Areas

Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is presumably underlain by a hot and partially molten mantle, which has a reduced seismic velocity relative to surrounding areas. Recent seismic tomography studies, however, gave conflicting results about the depth extent and even the existence of the low-velocity anomaly beneath the BRZ, suggesting that additional constraints are needed. Here we present results from stacking of about 1700 radial P-to-S receiver functions from a single long-running seismic station, TLY, located at the SW tip of Lake Baikal. A clear uplift of the 410 km discontinuity (d410) with a magnitude of about 47 km relative to the south margin of the Siberian platform is observed beneath the rift. Currently available seismic results suggest that the uplift is unlikely to be caused by addition of water to mantle transition zone (MTZ) silicates but is the result of a 550°C reduction in temperature in the vicinity of the d410. In addition, the 660 km discontinuity (d660) shows a downward trend toward the rift from the south, suggesting that the entire MTZ might have a low temperature beneath the rift. The thickening of the MTZ suggests a high-velocity anomaly of about 2% in the MTZ, and rules out the possibility that the rifting is caused by a mantle plume originated in or beneath the mantle transition zone

Significant Seismic Anisotropy beneath the Southern Lhasa Terrane, Tibetan Plateau

Shear wave splitting measurements using teleseismic PKS, SKKS, and SKS phases recorded by station LSA on the southern part of the Lhasa Terrane of the Tibetan Plateau reveal significant azimuthal anisotropy with a splitting time of up to 1.5 s, a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy. In addition, systematic variations of the splitting parameters (fast polarization direction and splitting time) with the arriving azimuth of the seismic ray path are observed, suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry. The measurements are consistent with a model with two layers of anisotropy. The top layer has a NE-SW fast direction, which is consistent with GPS-revealed direction of surface movement, and can be associated with lattice preferred orientation of middle-lower crustal minerals such as amphibole. The lower layer has a nearly E-W fast direction and can be the consequence of either the N-S directed compressional stress originated from the India-Eurasia collision or flow in the asthenosphere related to the absolute motion of Eurasia. This study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

Spatial Variations of Crustal Characteristics beneath the Hoggar Swell, Algeria, Revealed by Systematic Analyses of Receiver Functions from a Single Seismic Station

The Hoggar swell in Algeria is one of the significant massifs of northwest Africa. The paucity of high-resolution geophysical studies of the crust and mantle beneath the massifs is mostly responsible for the heated debates about the depth of the source region of the Cenozoic volcanism and the closely related uncertainty about the mechanism that formed and maintains the high elevation of the swells. Here we report results from a systematic study of 1386 high-quality receiver functions (RFs) recorded by station TAM, the only permanent broadband seismic station on the Hoggar swell. The resulting crustal thickness is about 34 km and the Vp/Vs is 1.77 when all the RFs from the station are stacked. Our study reveals a sharp contrast in the amplitude of the P-to-S converted phases between the volcanic, highly-fractured Tefedest terrane and the non-volcanic, less fractured Laouni terrane. The former has a stacking amplitude that is comparable to typical cratonic areas, and the latter has an amplitude that is only about 25% as large. Spatially consistent crustal thickness and an intermediate-mafic crust are inferred on the Tefedest terrane, while spatially variable crustal thickness and a felsic crust is inferred beneath the Laouni terrane. The observations can be best explained by a mantle-derived underplated magmatic layer beneath the mechanically-stronger Laouni terrane, and magmatic diking and resultant volcanism associated with the mechanically weaker Tefedest crust. The study demonstrated the significance of a long-running station in the investigation of spatial variations of crustal characteristics

Making Reliable Shear-Wave Splitting Measurements

Shear-wave splitting (SWS) analysis using SKS, SKKS, and PKS (hereafter collectively called XKS) phases is one of the most commonly used techniques in structural seismology. In spite of the apparent simplicity in performing SWS measurements, large discrepancies in published SWS parameters (fast direction and splitting time) suggest that a significant portion of splitting parameters has been incorrectly determined. Here, based on the popularly used minimization of transverse energy technique, we present a procedure that combines automatic data processing and careful manual screening, which includes adjusting the XKS window used for splitting analysis, modifying band-pass filtering corner frequencies, and verifying and (if necessary) changing the quality ranking of the measurements. Using real and synthetic data, we discuss causes and diagnostics of a number of common problems in performing SWS analysis, and suggest possible remedies. Those problems include noise in the XKS window being mistaken as signal, non-XKS seismic arrivals in the XKS window, excessive use of null ranking, measurements from misoriented sensors and from sensors with mechanical problems, and inappropriate dismissal of usable measurements

Shear Wave Splitting and Mantle Flow Associated with the Deflected Pacific Slab beneath Northeast Asia

A total of 361 SKS and five local S wave splitting measurements obtained at global and regional seismic network stations in NE China and Mongolia are used to infer the characteristics of mantle fabrics beneath northeast Asia. Fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features. The dominant fast directions at the eastern part, beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone (MTZ), are about 100° from north, which are almost exactly the same as the motion direction of the Eurasian plate relative to the Pacific plate, and are independent of the direction of local geological features. The splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3% anisotropy. The shear wave splitting observations, complemented by the well-established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the Paleozoic era, can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected Pacific slab in the MTZ, or by back-arc asthenospheric flow in the mantle wedge above the slab

Response versus Chain Length of Alkanethiol-Capped Au Nanoparticle Chemiresistive Chemical Vapor Sensors

Au nanoparticles capped with a homologous series of straight chain alkanethiols (containing 4−11 carbons in length) have been investigated as chemiresistive organic vapor sensors. The series of alkanethiols was used to elucidate the mechanisms of vapor detection by such capped nanoparticle chemiresistive films and to highlight the molecular design principles that govern enhanced detection. The thiolated Au nanoparticle chemiresistors demonstrated rapid and reversible responses to a set of test vapors (n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate, methanol, ethanol, isopropanol, and 1-butanol) that possessed a variety of analyte physicochemical properties. The resistance sensitivity to nonpolar and aprotic polar vapors systematically increased as the chain length of the capping reagent increased. Decreases in the nanoparticle film resistances, which produced negative values of the differential resistance response, were observed upon exposure of the sensor films to alcohol vapors. The response signals became more negative with higher alcohol vapor concentrations, producing negative values of the sensor sensitivity. Sorption data measured on Au nanoparticle chemiresistor films using a quartz crystal microbalance allowed for the measurement of the partition coefficients of test vapors in the Au nanoparticle films. This measurement assumed that analyte sorption only occurred at the organic interface and not the surface of the Au core. Such an assumption produced partition coefficient values that were independent of the length of the ligand. Furthermore, the value of the partition coefficient was used to obtain the particle-to-particle interfacial effective dielectric constant of films upon exposure to analyte vapors. The values of the dielectric constant upon exposure to alcohol vapors suggested that the observed resistance response changes observed were not significantly influenced by this dielectric change, but rather were primarily influenced by morphological changes and by changes in the interparticle spacing

A Systematic Comparison of the Transverse Energy Minimization and Splitting Intensity Techniques for Measuring Shear-Wave Splitting Parameters

Over the past several decades, shear-wave splitting (SWS) analyses have been increasingly utilized to delineate mantle structure and probe mantle dynamics. However, the reported splitting parameters (fast polarization orientations and splitting times) are frequently inconsistent among different studies, partially due to the different techniques used to estimate the splitting parameters. Here, we report results from a systematic comparison of the transverse minimization (TM) and the splitting intensity (SI) techniques. The study was motivated by the fact that recent comparative studies led to conflicting conclusions, which include the suggestion that TM, which is arguably the most widely used SWS-measuring technique, performs significantly poorly relative to SI under most circumstances in terms of stability and reliability of the resulting splitting parameters. We use both synthetic and real seismograms to evaluate the performance of the techniques for noise resistance, dominant period dependence, and complex anisotropy recognition. For one-layer anisotropy models with a horizontal axis of symmetry, our results show the two techniques can provide measurements with similar reliability. The testing confirms conclusions from previous studies that, although SI cannot distinguish between simple and complex anisotropy models with a horizontal axis of symmetry, TM can serve as a powerful tool in recognizing the existence of complex anisotropy, which is characterized by a systematic dependence of the splitting parameters on the back azimuth of the events. Therefore, when the existence of complex anisotropy beneath a study area is unknown, TM is a better choice. Ⓔ A FORTRAN program for the calculation of Wiener-filtered wavelet and splitting intensity using SI technique is provided as an electronic supplement to this article