Based on the finite difference inversion and ray inversion method, the crust and upper mantle structure along the Zhefang–Binchuan and Menglian–Malong wide–angle seismic profiles, both of which are located in Yunnan province, are imaged using the geophysical data of travel‐time, amplitude ratio and Bouguer gravity anomalies. Thus some new recognitions about the geodynamics and the seismotectonic environment are derived. The crust thickness along the Zhefang–Binchuan profile is 35~46km, and that of the Menglian–Malong profile is 33~44.5km. The geometry of the Moho interface looks like that given by Hu H X et al. The P wave velocity on the top of the mantle beneath some places is relatively low and the variation range of the velocity is very large, which may indicates that the Yunnan region is a typical area of active tectonics. Moreover, there exits a prominent consistency between the shallow and deep structures, which implies that the activity of shallow material always has a deep background. The velocity structure of Zhefang–Binchuan profile indicates that there exists a large low velocity anomaly which penetrates the whole crust to the east of the Nujiang fault exposed on the ground, and this possibly associated with the upwelling of the deep material . The characteristic of the large faults which are the borders between some first order tectonic units can also be derived from the both velocity structure profiles: the Red River fault is a hyper‐fault that cuts the lithosphere, the Nujiang fault cuts deeply in the crust and even stretches downward into the top of the upper mantle, yet the Changning–Shuangjiang fault has a listric shape with a small dip angle, which may suggest that its incision depth is not very deep. The large earthquakes took place in Yunnan province always have a relationship with the large and deep faults that stretches into the upper mantle. Some earthquakes with shallow focal depths are generally located at the converge positions of different faults within the upper‐middle crust or the places where the velocity contours on the belt between the high and low velocity anomaly blocks are crooked. It is estimated that these locations are favorable to the accumulation of energy and regional stress.
Focal loci are often required in earthquake location. However, it is extremely difficult to analytically express them when the earthquake lies in a complex model. Therefore, the calculation of focal loci is usually limited to simple media. In this paper, we present a method for calculating focal loci in complex media by means of a minimum traveltime tree algorithm for ray tracing. The focal loci are constrained with observed arrival time differences at seismic stations so that the problem of origin time is evaded. From all the model nodes, we select a small part with smaller absolute residuals between observed and calculated traveltime differences (or double differences) as reference points of the focal locus. The reference point with minimum double difference is assigned as an initial point. The ray paths from the initial point to the other selected reference points in the doubledifference field actually represent the focal locus, which are traced with a minimum traveltime tree algorithm. When the obtained focal locus is rather rough due to the excessive amount of selected reference points, it can be improved by removing some ray paths to the reference points that there are less rays go through. Additionally, the stop condition of the minimum traveltime tree algorithm is modified to reduce computational time. The results of numerical tests including velocity perturbations and noisy arrival data in a laterally heterogeneous model indicate that the presented method is feasible.
In this paper, we simulated the trapped waves generated by explosions in the Kunlun fault zone, using the three‐dimensional staggered‐grid finite difference algorithm. In order to improve the reliability of final fault‐zone model, we use the data of three components. The simulations indicate that the region above the depth of 1.0 km in the Kunlun fault zone produces main effects on the characteristics of the trapped waves. The S‐wave velocities and the width of fault zone have more effects on arrival times, waveforms, amplitudes and phases of the trapped waves. By simulation, the detailed structure parameters of the Kunlun fault zone are as follows: the width of shallow fault zone is 300m, and 250m in deep; The S‐wave velocity inside the fault zone is 0.98 km/s in the layers above the depth of 400 m, and that of surrounding rock is 1.70 km/s, and the Q value is 13.8. The S‐wave velocities and Q increase with depth. Beneath the depth of 1000 m, the S‐wave velocity inside the fault zone is 2.80 km/s and that of surrounding rocks is 3.3 km/s.
Two shear wave vibration modes can be produced in laboratories: the shear vibration and the torsion vibration. The propagation characteristics of the torsion shear wave in anisotropic medium are observed using the torsion vibration transducers. The observations indicate that when the torsion waves without the polarization direction propagate in anisotropic media the shear wave splitting can be observed, and the velocities of the fast and slow shear waves are consistent with those of the fast and slow shear vibration waves. When the fast and slow torsion waves are received by torsion wave receivers, their waveforms are not affected by the anisotropic azimuth. Through observation with two homogeneous anisotropic samples of different origins, we preliminarily uncovered some propagation characteristics of the torsion wave in anisotropic medium.
Pseudo‐spectrum method is used to simulate the crust effect on the analysis of the upper mantle anisotropy. A model of two‐layer anisotropic media is used to simulate the crust and the upper mantle respectively and the elastic wave fields are calculated for four coupling conditions between the two layers. The characteristics of split shear waves at various depths are analyzed in detail. The results show when the crust is isotropic or its anisotropy orientations are parallel or perpendicular to that of the upper mantle, the fast or slow direction observed in the crust keeps the same as that in the upper mantle, the time delay increases when they are parallel to each other or decreases when they are perpendicular to each other. But in other conditions the motions of particles in the crust becomes complicated and the fast direction of the upper mantle is difficult to be directly picked out and the time delay is also changed due to the effects of the crust anisotropy. Furthermore, two popular approaches are used to inverse the anisotropy parameters from simulated results. It is found that when the anisotropy orientations of the crust and the upper mantle are neither parallel nor perpendicular, the inversed fast direction of the upper mantle may be incorrect. It is suggested that in such cases one can use several different inversion techniques together and refer to the contours of residual or cross‐correlation for further determination of anisotropy parameters of the upper mantle.
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