【摘要】 2006年7月4日在首都圈数字地震台网的中南部发生了河北文安MS5.1地震.此次地震为该数字地震台网2002年投入运行以来发生的最强地震事件.本文利用多台和达法,分析处理了首都圈107个数字地震台站2002—2007年判读的大量震相数据.为提高波速比计算的精度和稳定性,对波速比计算环境和条件进行了多方面限定.例如,地震发生位置相对集中,台站选取范围有限,参与计算到时数据在5个以上,以及线性拟合精度高、误差低等.经这些限定条件后的研究发现,文安地震前,在文安和唐山附近地区地震波速比出现正常—降低—恢复—发震的异常演变过程.震前1年,出现波速比较明显降低过程,异常持续时间1年左右;同期邢台、京西北、京西南、京津和北京地区波速比的变化则基本平稳.文安地震发生后,文安和唐山地区的波速比恢复正常的平稳变化状态.通过波速比计算结果的误差和稳定性分析认为,文安地震前在文安和唐山附近地区地震波速比异常是可信的.更多还原

【Abstract】 On July 4, 2006, an earthquake of MS5.1 took place in Wen’an, Hebei Province, just in the south part of China’s Capital area digital seismograph network. It is the strongest event recorded ever since the network went into operation in 2002. We processed the large amount of phase data yielded by the 107 digital seismic stations during 2002—2007 using the Wadati method. In order to improve the precision and stability of shear and compressional wave velocities (vP/vS) estimation, we imposed a number of restrictions on the computation condition, e.g., the earthquakes are densely distributed, selected stations are in limited area, the number of stations involved in the computation is more than 5, and the linear fitting featuresa high precision. Under these restrictions, the study shows that vP/vS in and around Wen’an and Tangshan underwent a normal-low-normal variation process before the Wen’an earthquake: one year before the quake, vP/vS became obviously low and the low ratio lasted for about one year, meanwhile, little variation of vP/vS was seen in Xingtai, northwest of Beijing, southwest of Beijing, Beijing-Tianjin and Beijing; after the quake, the vP/vS returned to normal in Wen’an and Tangshan. Error and stability analysis of the calculated vP/vS shows that the vP/vS anomalous variation appeared in and around Wen’an and Tangshan before the Wen’an earthquake is convincible.更多还原