Two approaches to travel-time computations in laterally inhomogeneous anisotropic media are presented. The first method is based on ray tracing in an anisotropic inhomogeneous medium, the second on the linearization procedure. The linearization procedure, which can be applied to inhomogeneous, slightly anisotropic media, does not require ray tracing in an anisotropic medium. Applications of linearized equations to the solutions of direct and inverse kinematic problems are discussed. A program package to perform the linearized computations for rather general 2-D laterally inhomogeneous layered structures is described and a numerical example is presented. IntroductionThe most important peculiarities of seismic body waves now used for investigating the anisotropic properties of the Earth's crust and the upper mantle are as follows:(1) The azimuthal dependence of the travel-time field as observed along special observational systems on the Earth's surface. This azimuthal dependence has been used mainly to investigate the anisotropy of velocities of the quasi-compressional waves, but can ltio be used for quasi-shear waves.(2) The time delay between two quasi-shear waves (shear waves splitting).(3) The polarization effects and polarization anomalies of S-waves.In most cases the anisotropic effects are relatively weak, hidden in, or at least combined Firh effects due to the lateral variability of the Earth's structure. It is natural to assume that for laterally inhomogeneous media the anisotropy may also vary in all directions. Not only the relative amount of anisotropy, but also the symmetry-system and orientation of the preferred axes change from place to place. Trying to investigate the anisotropic properties or' the Earth's interior under the simplified assumption that the real medium is homogeneous may lead to serious errors. A more thorough approach must consider both these effects rqether, in laterally inhomogeneous anisotropic medium.
Systematic calibration of a network of seismographs could meet the seismic monitoring needs of the United Nations' comprehensive nuclear test ban treaty, a feasibility study suggests. To calibrate the network, known as the International Monitoring System (IMS), the three‐dimensional seismic structure of the Earth must be taken into account. Deep seismic sounding (DSS) profiles and global three‐dimensional seismic velocity inversions play prominent roles. A verifiable test ban treaty is an important societal and technical goal. An essential ingredient is the ability to accurately locate seismic events, probably within a circle with a radius of 18 km, which is about 1000 km2. This would be for events with magnitude greater than 4 in continental areas.
Ð Seismic event locations based on regional 1-D velocity-depth sections can have bias errors caused by travel-time variations within dierent tectonic provinces and due to ray-paths crossing boundaries between tectonic provinces with dierent crustal and upper mantle velocity structures. Seismic event locations based on 3-D velocity models have the potential to overcome these limitations. This paper summarizes preliminary results for calibration of IMS for North America using 3-D velocity model. A 3-D modeling software was used to compute Source-Station Speci®c Corrections (SSSCs(3-D)) for Pn travel times utilizing 3-D crustal and upper mantle velocity model for the region. This research was performed within the framework of the United States/Russian Federation Joint Program of Seismic Calibration of the International Monitoring System (IMS) in Northern Eurasia and North America.An initial 3-D velocity model for North America was derived by combining and interpolating 1-D velocity-depth sections for dierent tectonic units. In areas where no information on 1-D velocity-depth sections was available, tectonic regionalization was used to extrapolate or interpolate. A Moho depth map was integrated. This approach combines the information obtained from refraction pro®les with information derived from local and regional network data. The initial 3-D velocity model was tested against maps of Pn travel-time residuals for eight calibration explosions; corrections to the 3-D model were made to ®t the observed residuals. Our goal was to ®nd a 3-D crustal and upper mantle velocity model capable predicting Pn travel times with an accuracy of 1.0±1.5 seconds (r.m.s.).The 3-D velocity model for North America that gave the best ®t to the observed travel times, was used to produce maps of SSSCs(3-D) for seismic stations. The computed SSSCs(3-D) vary approximately from +5 seconds to )5 seconds for the western USA and the Pre-Cambrian platform, respectively. These SSSCs(3-D) along with estimated modeling and measurement errors were used to relocate, using regional data, an independent set of large chemical explosions (with known locations and origin times) detonated within various tectonic provinces of North America. Utilization of the 3-D velocity model through application of the computed SSSCs(3-D) resulted in a substantial improvement in seismic event location accuracy and in a signi®cant decrease of error ellipse area for all events analyzed in comparison both with locations based on the IASPEI91 travel times and locations based on 1-D regional velocity models.
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