大同地震窗P波频散衰减的异常变化特征

Variation Characteristics of the Q Value Deduced from P Wave Dispersion Analysis in Datong Window

通过测量介质品质因子来开展大同窗附近测震台站记录P波衰减频散特征的研究,并分析大同窗近场区和外围区震例前后的变化。对山自皂台记录的2001年以来大同窗地震序列的研究结果表明,近场区3次和外围区6次地震前,介质品质因子显示超出1倍均方差的高值变化特征,震后逐渐恢复到背景起伏状态;与大同窗传统的频次指标相比,介质品质因子的映震效果更好;阳原台的结果表明,仅近场区1次和外围区2次地震前,介质品质因子显示超出1倍均方差的高值变化特征,且有虚报、漏报。所处地质构造位置不同、传播路径介质差异可能是引起上述两个台站附近介质品质因子的地震对应效果明显不同的原因。

This paper describes the dispersion characteristics of first-period P waves recorded by seismic stations near Datong window by measuring the medium quality factor （Q,n） and analyzing Qm variations before and after the occurrence of earthquakes near and far from Datong window. By applying this method to the SZZ record of the Datong window earthquake sequence from 2001, we obtained the following results： before 3 earthquakes near the Datong window and 6 earth quakes far from the Datong window, the Qm was the high values over 1 times mean square error; but Q,, returned gradually the background after these earthquakes. However, after eliminating the influences of epieentral distance and depth, large anomalies were still observed and small a nomalies disappeared before the earthquakes. Furthermore, the corresponding rate of earthquake decreased from 0.9 to 0.5. Compared with the frequency of Datong window, the Qm could betterreflect earthquake probability. The results of YAY showed that before 1 earthquake near the Da- tong window and 2 earthquakes far from the Datong window, the Qm were the high values over 1 times mean square error, and false and omitted cases were observed. Minimal changes to the curve were observed after eliminating the influences of epicentral distance and depth. Preliminary analysis suggested that the short-term high-value anomalies of Qm before earthquakes recorded by the SZZ and YAY stations could directly reflect the seismogenic process. However, the Qm recor- ded by the two stations obviously different before the same earthquakes. Two reasons may ex- plain this phenomenon. The small earthquakes measured by Qm differed between the two stations as these stations were located in different geological and tectonic positions. The SZZ station was located inside the Datong basin, while the YAY station was located in its eastern border. This difference could lead to the different deep medium and broken medium through by seismic wave ray paths and, therefore, marked variations in Qn, values. The length and width of Datong win- dow were 60 and 5 km, respectively, so high-value anomalies were limited in this area. From the results of Qm, for the moderate earthquakes which are not only near the Datong window but also far form Datong window, the amplitude of anomaly was almost the same and the distribute range had little change. As such, determining the variation of deep anomalies near or far from Datong window by using the spatial anomaly distribution is difficult. The Datong window showed high- value anomalies before the far-field earthquake of the Shanxi seismic belt because this window is located to the north of the same continental rift. However, the reason why it had high-value anomalies before the earthquakes of Zhangjiakou-Bohai seismic belt which were a few hundred kilometers away, it was possible that the ＂Y＂ type of low wave velocity anomaly from Datong volcanoes to below Bohai had played an important role in transmission.