华北地区上地幔速度结构

本文选用了北京台网记录的来自不同方位并通过华北地区的140个地震,使用平滑法和dT/d△法,得到P波慢度曲线;采取走时和慢度结合使用的方法,得到走时曲线;从Gerver-Markushevich公式出发,导出一个易于计算的简捷形式,并由此反演初始速度模型;通过模型理论走时曲线的计算,选出与实测数据符合得较好的模型。平均模型中,低速层出现的深度较浅,约60公里;在360-420公里和660-680公里深处,存在两个速度急剧增加的梯度层;500公里处可能还有一个较小的梯度层。华北的东南和西南方向上,相当于660公里梯度层的深度的差异还较为显著。

P-WAVE VELOCITY STRUCTURE OF THE UPPER MANTLE IN NORTHERN CHINA

In this paper, the data of 140 earthquakes recorded at different azimuths from the Beijing network are used. By using both the smoothing method and dT/d△ method, the slowness curves of P-wave are obtained. By incorporating the travel-time and slowness data, continuous travel-time curves can be obtained. A simpler formcla than Gerver-Marku-shevish's formcla is derived and used to invert the intial veloctity models. Through computing theoretical travel-time curves of th initial velocity models, the final velocity models which agree better with the observational data are obtained. The average velocity model shows that: a shallow low velocity zone begins near 60 km in depth: two abroupt increases in velocity from 360 to 420 km and from 660 to 680 km are also obtained; a possible minor zone of high-velocity gradient near 500 km is detected. Differences in depth of the zone of 600 km between the south-east and south-west directions in Northern China can clearly be seen.