Xiaoning Yang

A Numerical Investigation of Lg Geometrical Spreading

This study is a comprehensive numerical investigation of the L g-wave geometrical spreading for vertically inhomogeneous media. I modeled a suite of source and path parameters and measured different L g amplitudes including root-mean-square (rms), peak-to-peak, third-peak (Nuttli, 1980; Patton, 2001), envelope-peak, and spectral amplitudes for analysis. The main result of this investigation is that the estimation of L g spreading rates from the rms amplitudes and from the spectral amplitudes yielded the most reliable estimates that are basically independent of almost all the source and path variables within the parameter ranges that I simulated. I obtained a spreading rate of Δ-1.0 for the rms amplitude and a rate of Δ-0.5 for the spectral amplitude across the frequency band of the data. The relationship between the two spreading-rate estimates is consistent with Parsevals theorem. Spreading-rate estimates from other amplitude measurements show larger variations with the variation of source and path parameters. These variations suggest that the behavior L g peak amplitudes might be more complex than that of a single-mode Airy phase. The decay of the third-peak amplitudes appears to be less rapid than the decay of the peak-to-peak and envelope-peak amplitudes. There are no systematic changes in the spreading rates due to the variation of variables such as velocity model, source depth, and source mechanism. The introduction of a gradient zone at the crust-mantle boundary and the earth-flattening transformation did not change the spreading rates significantly, at least not beyond certain source-receiver distances. The most important variable affecting the L g spreading appears to be the closeness of the source to major velocity discontinuities in the velocity models. Manuscript received 24 January 2001.

Geometric Spreading of Pn and Sn in a Spherical Earth Model

Geometric spreading of Pn and Sn waves in a spherical Earth model is different than that of classical headwaves and is frequency dependent. The behavior cannot be fully represented by a frequency-independent power-law model, as is com- monly assumed. The lack of an accurate representation of Pn andSn geometric spread- ing in a spherical Earth model impedes our ability to characterize Earth properties including anelasticity. We conduct numerical simulations to quantify Pn and Sn geometric spreading in a spherical Earth model with constant mantle-lid velocities. Based on our simulation results, we present new empirical Pn and Sn geometric- spreading models in the form Gr;f ��� 10 n3� f� =r0�� r0=rn1� flogr0=r�� n2� fand nif �� ni1� logf=f0�� 2 � ni2 logf=f0 �� ni3, where i � 1 ,2 , or 3;r is epicentral distance; f is frequency; r0 � 1 km; and f0 � 1 Hz. We derive values of coefficients nij by fitting the model to computed Pn and Sn amplitudes for a spherical Earth model having a 40-km-thick crust, generic values of P and S velocities, and a constant-ve- locity uppermost mantle. We apply the new spreading model to observed data in Eur- asia to estimate average Pn attenuation, obtaining more reasonable results compared to using a standard power-law model. Our new Pn and Sn geometric-spreading models provide generally applicable reference behavior for spherical Earth models with con- stant uppermost-mantle velocities.

Bayesian Lg Attenuation Tomography Applied to Eastern Asia

Bayesian attenuation tomography (Tarantola, 1987) is being used to refine existing Lg attenuation models in eastern Asia. The advantages of a Bayesian approach to tomography are that large-scale and high-resolution tomographic models available from other well-accepted studies can be used as prior background models. The resulting refined tomographic model will blend into these prior background models. Moreover, the uncertainties are well established in a Bayesian framework. We assume a general linear Gaussian (least squares) model, where the covariance matrix is partitioned into data and prior model components. Uncertain data are naturally down weighted and model components with small errors will be subject to little change. Amplitude tomography provides only an approximation to the propagation effects a seismic wave may experience. For example, phase blockage can occur over relatively short distances in the absence of any anelastic effects. Station-centric kriged amplitude correction surfaces on top of the tomographic models may assist in identifying blocked paths. Because a signal-to-noise criterion is used to select amplitudes for the inversion, the data are left-censored and the resulting Q 0 models will be biased high. We examine the utility of a maximum-likelihood data augmentation method to the left-censored data problem (Schafer, 1997). In this case, we can only measure an upper bound to measured amplitudes on the basis of prephase noise. Data augmentation is used to impute the missing data values with their conditional expectation based on the relationship of amplitudes with other completely observed variables. We have initially chosen a bivariate Gaussian model for filling in missing amplitudes based on the relationship between amplitude and prephase noise. We apply the technique to eastern Asia for Lg signals at 1 Hz using data from 1651 earthquakes recorded at 12 stations. Tomographic patterns correlate well with those expected from geophysical considerations (e.g., high attenuation in Tibet and low attenuation up into the stable regions of Kazakhstan). Many of the large cratonic basins surrounding the Tibet Plateau (e.g., Tarim, Junggar) show reduced attenuation consistent with the hypothesis that they represent regions of stronger lithosphere. Data augmentation tends to increase attenuation in Tibet (particularly western Tibet) and the cratonic basins to the North of but not to the East of Tibet. In addition, the resulting models are much smoother than models that do not account for censoring.

Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals

Abstract — This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10 Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5 Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined.

Effects of 2D Random Velocity Heterogeneities in the Mantle Lid and Moho Topography on Pn Geometric Spreading

Pn-wave energy refracts through the uppermost mantle, with the first seismic wave arrival at distances of ∼200 to ∼1500 km from crustal sources. The Pn phase provides important constraints on source type, location, and magnitude, but its propagation is complicated by frequency-dependent sensitivity to the Earths sphericity and lithospheric velocity structure. Converging on an acceptable Pn geometric spreading correction and specifying its uncertainties, a requirement for accurately determining frequency-dependent attenuation models for Pn, depends on improved understanding of the behavior of Pn geometric spreading for various heterogeneous models. We investigate the effects of radial mantle lid velocity gradi- ents, mantle lid random volumetric velocity heterogeneities, and Moho topography on Pn geometric spreading using reflectivity and two-dimensional (2D) finite-difference 1-Hz wave propagation calculations for elastic Earth models. Mantle lid velocity gra- dients systematically modify the frequency-dependent geometric spreading from that found for models with constant velocity but retain the same overall functional form. Pn amplitudes are also sensitive to the presence of modest 2D random lateral velocity heterogeneities within the uppermost mantle, with geometric spreading approaching a power-law behavior as the root mean square strength of heterogeneity increases. 2D Moho topography introduces scatter into the amplitude of Pn, but the overall behavior remains compatible with that for a laterally homogeneous model. Given the lack of knowledge of specific small-scale structure for any particular Pn path, the preferred geometric spreading parameterization is the frequency-dependent model for a constant mantle lid velocity structure unless Pn travel-time branch curvature can constrain the radial gradient in the mantle lid.

Characteristics of Chemical Explosive Sources from Time-Dependent Moment Tensors

Using a frequency-domain linear inversion technique and near-source broadband data, we inverted for the time-dependent source moment tensors of eight chemical explosions detonated in an open-pit coal mine during the Source Phenomenology Experiments (SPE) conducted by a consortium of U.S. research institutions to investigate a suite of explosive-source related problems. The moment tensors of the explosions from the inversion are dominated by their isotropic components regardless of variations between explosions in source size, confinement condition, and whether the explosion was on a bench and collapsed the vertical face of the bench. The percentage of isotropic moment-tensor component ranges from 96% to 98% for largest part of the source-time histories. Source-configuration variations result in differences that are most apparent in long-period moment-tensor spectra reflecting possible secondary source effects such as cylindrical source shape, spall, and compensated linear vector dipole (CLVD). Unconfined explosions show more oscillatory diagonal moment-tensor component time histories than confined and partially confined explosions possibly due to stronger free-surface effects such as material cast. Compared with pit explosions, deviatoric components of moment tensors of the two bench explosions are of higher amplitudes. There is a discernible long-period (<5 Hz) signal on one of the off-diagonal components, which could be related to the presence of the bench face in the source region and the horizontal material cast by the explosions. Although off-diagonal moment-tensor components comprise a small portion of the moment tensor, they are capable of generating a disproportionally large amount of shear waves.

A Pn Spreading Model Constrained with Observed Amplitudes in Asia

Abstract By modeling synthetic Pn amplitudes, I and my colleagues in 2007 proposed a Pn geometric-spreading model (Y2007) that takes into account the spherical shape of the Earth. In this study, I used a set of observed Pn amplitudes from the tectonically active regions of Asia to evaluate the performance of Y2007 and to develop new, observation-based Pn spreading models. Even though Y2007 provides improved geometric-spreading correction of Pn amplitudes over the traditional power-law model, the corrected amplitudes exhibit undesirable decay rate variations. To address this issue, I used a procedure to develop Pn spreading models based on observed data. I first correct the Pn amplitudes for attenuation using an average quality factor Q estimated from Y2007-corrected Pn amplitudes. I then develop a spreading model, which is a simplified version of Y2007, by fitting the corrected amplitudes. Compared with Y2007, the new spreading model significantly reduces amplitude variations, particularly at short distances. To more accurately model the complex data behavior, I also developed a segmented spreading model in which separate sets of model parameters are derived for amplitudes in different distance ranges. The spreading models developed in this study account for radially symmetric elastic and other effects, such as velocity gradient, forwarding scattering, and potential depth-dependent attenuation variation, as well as wavefront expansion and the spherical shape of the Earth. Using the new model for spreading correction results in better attenuation isolation and allows amplitudes in a broader distance range to be used in the accurate mapping of lateral attenuation variations. The method I employed in this study could be used as a general procedure to develop observation-based Pn spreading models for other regions.

SOURCE PHENOMENOLOGY EXPERIMENTS IN ARIZONA

The Arizona Source Phenomenology Experiments (SPE) have resulted in an important dataset for the nuclear monitoring community. The 19 dedicated single-fired explosions and multiple delay-fired mining explosions were recorded by one of the most densely instrumented accelerometer and seismometer arrays ever fielded, and the data have already proven useful in quantifying confinement and excitation effects for the sources. It is very interesting to note that we have observed differences in the phenomenology of these two series of explosions resulting from the differences between the relatively slow (limestone) and fast (granodiorite) media. We observed differences at the two SPE sites in the way the rock failed during the explosions, how the S-waves were generated, and the amplitude behavior as a function of confinement. Our consortiums goal is to use the synergy of the multiple datasets collected during this experiment to unravel the phenomenological differences between the two emplacement media. The data suggest that the main difference between single-fired chemical and delay-fired mining explosion seismograms at regional distances is the increased surface wave energy for the latter source type. The effect of the delay-firing is to decrease the high-frequency P-wave amplitudes while increasing the surface wave energy because of the longer source duration and spall components. The results suggest that the single-fired explosions are surrogates for nuclear explosions in higher frequency bands (e.g., 6-8 Hz Pg/Lg discriminants). We have shown that the SPE shots, together with the mining explosions, are efficient sources of S-wave energy, and our next research stage is to postulate the possible sources contributing to the shear-wave energy.

The Frequency-Domain Moment-Tensor Inversion: Retrieving the Complete Source Moment-Tensor Spectra and Time Histories

When the wavelength of a seismic signal of interest is much longer than the dimension of the internal seismic source that generates the signal, whether it is an earthquake, an underground explosion or an underground mine collapse, the seismic source may be represented by a symmetric second-order moment tensor.

The lateral variation of Pn velocity gradient under Eurasia

Mantle lid P wave velocity gradient, or Pn velocity gradient, reflects the depth and lateral variations of thermal and rheological state of the uppermost mantle. Mapping the Pn velocity gradient and its lateral variation helps us gain insight into the temperature, composition, and dynamics of the uppermost mantle. In addition, because Pn velocity gradient has profound influence on Pn propagation behavior, an accurate mapping of Pn velocity gradient also improves the modeling and prediction of Pn travel times and amplitudes. In this study, I used measured Pn travel times to derive path-specific Pn velocity gradients. I then inverted these velocity gradients for two-dimensional (2-D) Pn velocity-gradient models for Eurasia based on the assumption that a path-specific Pn velocity gradient is the mean of laterally varying Pn velocity gradients along the Pn path. Result from a Monte Carlo simulation indicates that the assumption is appropriate. The 2-D velocity-gradient models show that most of Eurasia has positive velocity gradients. High velocity gradients exist mainly in tectonically active regions. Most tectonically stable regions show low and more uniform velocity gradients. Strong velocity-gradient variations occur largely along convergent plate boundaries, particularly under overriding plates.