德国大陆超深钻井注水诱发地震的精确定位

THE ACURATE LOCATION OF THE INJECTION-INDUCED MICROEARTHQUAKES IN GERMAN CONTINENTAL DEEP DRILLING PROGRAM

2000年8月21日～10月20日,德国大陆超深钻井(KTB)进行了为期60天的新一轮注水诱发地震实验.在对KTB台网所记录到的2 700多次地震的原始波形进行分析和处理的基础上,对其中的237次地震进行了精确定位.定位时,先采用和达法剔除了误识的非KTB诱发地震事件;然后,使用大折刀法求得了台站偏差的最佳值,以及中误差和平均误差;最后应用Geiger法进行修定,从而保证了237个事件定位的中误差达到0.1 km的高精度.定位结果显示,在大于9.3 km(几乎是主井底的深度)没有发生诱发地震.这一现象表明,在这个深度,应力可能低于摩擦强度,以至于由注水引起的孔隙压力的变化不足以诱发地震;在这样的深度不存在可渗透的、取向恰当的断层.这个深度也可能是这一区域内的地震活动性的最大深度,即在这一相对稳定地区的板块内部,主井底已接近脆-延性转换带.并用实验室的结果和超深钻井中所观测到的热流值资料解释了这一现象.

From August 21, 2000 to October 20, 2000 a fluid injection-induced seismicity experiment has been carried out in the KTB (German Continental Deep Drilling Program). More than 2 700 events were recorded by the KTB seismic network. Among them 237 events were of high signal-to-noise ratio, and were processed and acurately located. When the events were located, non KTB events were weeded out by Watatis method. The standard deviation, mean and median were obtained by Jackknifes technique, and finally the events were accurately located by Geigers method. So that the mean error is about 0.1 km. No earthquakes with focal depth greater than 9.3 km, which is nearly at the bottom of the hole, were detected. One of the explantion is that at such depths the stress levels may not close to the rocks frictional strength so that failure could not be induced by the relatively small perturbation in pore pressure. Or at these depths there may be no permeable, well-oriented faults. This depth may be in close proximity to the bottom of the hole to the brittle-ductile transition, even in this relatively stable interior of the interaplate. This phenomenon is explained by the experimental results and geothermal data from the superdeep borehole.