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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。
In this paper, we select 140 earthquakes recorded by Beijing Network from different directions and passing through the North China region, and use the smoothing method and the dT / d △ method to get the P-wave slowness curve. Taking the combination of walking time and slowness, Based on the Gerver-Markushevich formula, an easy-to-calculate simple form is deduced and the initial velocity model is derived. Based on the model theoretical traveltime curve calculation, the model which is in good agreement with the measured data is selected. In the average model, the low velocity layer appears shallow, about 60 km; there are two steeply increasing gradient layers at 360-420 km and 660-680 km; there may be a smaller gradient at 500 km Floor. In the southeast and southwest of North China, the difference between the depths corresponding to the 660 km gradient layer is still significant.