Showing papers on "Mohorovičić discontinuity published in 1982"

The seismic reflection character of the continental mohorovicic discontinuity

Abstract: The continental Mohorovicic discontinuity is most often interpreted as a step-function velocity boundary. However, on deep crustal seismic reflection profiles, reflections, at depths where refraction data places the Moho, have laminated character and are laterally discontinuous. These observations on continental reflection data point to a model of the Moho that has thin layered rather than block structure. Previous workers, who have synthetically generated seismic responses from various crust-mantle boundary models, have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed Moho arrivals. Geologic interpretations of the thin layering include relatively undeformed metasediments, cumulate layering, tectonic banding, and lenses of partial melt. To obtain more direct geologic evidence, a synthetic seismogram is generated from geologic cross-sections of the Ivrea-Verbano Zone, an exposed section through the lower crust and upper mantle. The synthetic shows laminations similar to those observed on reflection data. Without more direct evidence, no single geologic interpretation of the Moho is reasonable. Indeed, the Moho may be a laterally variable boundary, its composition and structure dependent upon the geologic history of the overlying crust.

Interpretation of refraction experiments in the North Sea

Abstract: Before 1977, only three long-range seismic refraction profiles had been shot in the North Sea Basin, with somewhat inconsistent results in terms of a description of the Mohorovicic discontinuity (Moho). In the summer of 1977, the Marine Group of Cambridge University fired three refraction lines with the use of pull-up, shallow-water seismometers, the longest reversed profile extending for 400 km along the 0.5° E meridian from east of the Shetland Islands to the latitude of Dundee. Although records varied in quality, a time term interpretation of the results was made with the use of detailed velocity-depth information supplied by the oil industry. This interpretation indicates that there is an anticlinal form to the Moho beneath the deepest sediments of the Moray Firth Basin, as demanded by gravity data from the area. It is further suggested that the seismic evidence supports an extensional model to explain the origin of the North Sea Basin.

Structure of the upper mantle and low velocity layer of the mantle under the hsingtai earthquake region on the north china plain

Abstract: Under the Hsingtai earthquake region of the North China Plain and beneath the Mohorovicic Discontinuity, there are four layers respectively of longitudinal velocities 8.1, 7.6, 7.8 and 8.1 km/sec. Further below, a low velocity layer of velocity 7.2-7.4 kin/sec exists in the mantle, the upper boundary of which lies at a depth of about 83 kilometers.In this paper, comparison of the low velocity layer of the Hsingtai region with that of the other parts of the world seems to show that the low velocity values of the tec-tonically, active regions are usually less than the normal value for regions such as oceans and continents.

A preliminary survey of the crustal velocity structure beneath the beijing-tianjin region and its environs

Abstract: In recent years, in the region around the cities of Beijing and Tianjin, work of seismic deep sounding has been carried out. The purpose is principally for studying the structure of the crust and upper mantle which involves the earthquake source regions of this seismically active portion of the North China Plain. By observing wide angle reflected waves, the reflection PM from the Mohorovicic Discontinuity and PM from the intermediate layer within the crust are recorded. Eefracted waves from the Mohorovicic Discontinuity have been observed. Assuming a horizontally homogeneous layered structural model, computations of the velocity structure by the damped least square method of inversion were done. Eesults show that apparently, in this entire region velocity varies with depth, as a layered earth. On the other hand, horizontal inhomogenuity actually exists.