芦山震中区上地壳剖面二维波速结构模型反演

Inversion of 2-D velocity structure model of upper crust section in the Lushan epicentral region

CT技术已经广泛应用到科学和工程领域,特别是在地球物理学和土木工程质量无损检测方面.本文用精密可控的地震波雷达在芦山震中区持续300 s向地壳主动发射低能量的线性调频弹性波,通过高灵敏度的地震仪器接收信号,实现对上地壳剖面波速结构进行无破坏性探测.用短时傅里叶变换分析记录到的线性调频信号的时间和频率特征,并用能量累积法提取信号的走时形成走时曲线图,识别震相后用射线追踪方法进行P波走时反演,得到了芦山地震震中区上地壳剖面二维波速结构.分析结果表明地震波雷达发出的信号符合控制要求,重复性高达99.7%;用能量累积法得到的地震波走时曲线易于识别震相.从芦山地震震中区上地壳剖面二维波速结构中可以看出,在芦山震中区附近有明显的波速差异,并且出现了向上凸起、向下急剧下降,再缓慢上升,呈"铲形"特征,存在低速层,分析出芦山地震是一次逆冲型地震,并且距震中区近的检测点波速比偏低,距震中区远的检测点波速比偏高.通过与其他方法研究结果对比,证明了用地震波雷达信号走时反演地壳波速结构是可行的.

Computerized Tomography（ CT） technology has been widely applied to science and engineering,especially in geophysics and non-destructive testing（ NDT） for civil engineering. In this paper,in order to realize nondestructive detection for the upper crustal velocity structure,we actively launched low-energy linear frequency-modulated signal（ LFM） to the crust of the Lushan epicentral region with precisely controllable Seismic Wave Radar（ SWR） in 300 s duration,and received the signal with highly sensitive seismographs. By analyzing time and frequency characteristics of the received LFM signals with Short-Time Fourier Transform（ STFT）,extracting travel-time of the signals with energy accumulation method, and creating the travel-time curves, we finally obtained the upper crustal section 2-D velocity structure of the Lushan epicentral region after identifying the seismic phases and using the ray-tracing method to invert the P seismic wave phases＇ travel-time. The results show that the SWR signals meet the requirements,and the repeatability can reach up to 99. 7%. In addition,we can easily identify phases from the travel-time curves which are acquired by energy accumulation method. From the upper crustal section 2-D velocity structure of the Lushan epicentral region,we can find out that wave velocity differences within the Lushan epicentral region are obvious,with rising up firstly,then declining down sharply,and finally slowing rising. The results show that the fault has a listric feature and there is a low-velocity layer.Therefore,we can assume that the Lushan earthquake is a thrust earthquake. Furthermore,we can find out that the wave velocity ratios of the detecting sites close to the epicenter are relatively low,while those far from the epicenter are higher. Comparing with results of other methods,it is proved that inverting crustal velocity structure with the travel-time of SWR LFM is reliable.