夏垫断裂夏垫段浅部构造特征地震探测

Seismic Detecting for the Shallow Tectonic Features of the Xiadian Fault

采用高分辨率浅层地震勘探技术对夏垫断裂夏垫段进行了探测,获得高质量、高分辨率的地震勘探剖面图。划分出9条断裂构造,通过地震时-深转换剖面对比钻孔地层资料确定出断裂的上断点埋深,从而确定了断裂的活动时代。探测结果表明:夏垫断裂是由四条断裂组成,其中的主断裂至今仍在活动;在断裂的上盘还探测到三条活动时代较新的断裂构造。

The Xiadian fault is a Holocene active fault in the north China plain.It exists from Pinggu in Beijing,to Mafang and Pangezhuang in the southwest,and it ends in Yongledian in the south.It is oriented towards NE 45°and the total length is 45 km.Many scholars have studied the characteristics of the quaternary period activity on this fault,and of a magnitude 8 earthquake which happened in 1679.But the Xiadian fault is a complicated structure and the understanding of the shallow structural features of the fault are not entirely accurate.In this paper,the Xiandian fault is detected with high resolution shallow seismic exploration.The observation system contains 72 records,a 4 m interval,an 8 m shot interval,18 CMP stacking,and a unilateral trigger.A combination of a 60 Hz vertical geophone and an 80 kg rammer artificial lamping source are used,and each shot point was hammered on above 25 times.The data acquisition parameters are：a sampling interval of 0.25 ms,a recording time of 1 000 ms,a 15 Hz low cutoff frequency,and a 500 Hz high cutoff frequency.Vista2D / 3D seismic processing software is applied for data processing.The following aspects are focuses of this study：careful removal of the primary wave,ensurence of the shallow reflection wave,consistent compensation processing of amplitude and surface,suppression of interference,improved signal-to-noise ratio,consistent deconvolution of the surface,wavelet processing （in order to improve the resolution and fidelity）,velocity analysis,residual static correction and good post-stack migration processing,and so on.Rich strata of the interface appear in the period of less than 700 ms reflection wave group sections,according to wave group characteristics,and the profile can be roughly divided into two parts of the south and north.The strong reflections on the southern section are greater the reflection power is stronger with better continuity-while in the north the weak reflection energy leads to poor continuity.According to the characteristics of the rupture and the distortion in the seismic section,the entire section is divided into nine fracture structures by a breaking F5 fault.The sections in the south and north display obvious differences：the southern section of the reflection in the phase axis is continuous and the layer is clear,while the north section of the reflection phase axis is messy.According to the characteristics of the seismic profile,nine tectonic faults were confirmed.From the time-depth conversion profile comparison with the borehole data,the depths of faults’ upbreakpoints were determined and the faults’ activity periods were also determined.The detection results show that the fracture is composed of four faults,one of the main faults is still active and three new active faults were detected in the hanging wall of the fracture.According to the characteristics of Xiadian fault,it is a high dip angle fault to near the surface and the quaternary active fault zone.The results show that the Xiadian fracture structure is complex.