Study of prediction of volcanic eruptions at Taal, Philippine

塔尔火山是一座直喷式火山，由一个15*22平方公里的南北走向的火山口湖和一个面积约5*5平方公里的火山岛组成，位于火山湖的中心，被称为菲律宾最活跃的火山之一，尽管自1977年以来一直没有喷发过。我们进行了两次地震爆炸调查，第一次是在菲律宾的塔尔火山。1993年3月，在火山口湖西岸发生了两次爆炸，安装在东岸扇形测线沿线的30个数字事件记录仪记录了从火山下面通过的地震波。随后，在1994年2月，东南海岸发生了一次爆炸，西岸至北岸的27个事件记录器记录了波信号。我们注意到在主陨石坑下方传播的地震波与在陨石坑外传播的地震波之间存在显著差异。前一种波比后一种波衰减更强烈，传播更慢，这是一种…更多的前一波之后是幅度较大的反射相位，其视速度比初始P波的到达速度快。P波速度结构是由穿过主坑外的直接P波和折射P波用时间项方法得到的。结果表明：表层为1.8公里/S，厚度为0.1~1公里，第二层为3.6公里/S，厚度为0.3~2公里，第三层为5.6公里/S，P波到达主陨石坑下方的延迟为0.2~0.4秒，这不能用台下时间来解释。这些结果表明，在深度约2公里处存在低速低Q区，该区域位于主坑以东约5公里的2*2公里的水平范围内。正常时差改正结果表明，上述反射震相来自主陨石坑以东约5公里、深度约7公里处。这种反射面可能暗示岩浆房的顶面，在这种情况下，由于反射相幅度大，低速、低Q带只应限制在第三层的较浅部分。较少

Taal Volcano is strato-volcano which is composed of a 15*22km^2north-south trending caldera lake and a volcanic island with extent of about 5*5km^2 centered in the lake and known as one of the most active volcanoes in Philippines, though no eruption has occured since 1977. We carried out seismic explosion surveys two times, for the first time in Philippines, at Taal Volcano. In March, 1993, two explosions were executed on the west shore of the caldera lake and seismic waves that passed beneath the volcano were recorded by 30 digital event recorders installed on the east shore along the fan-shooting survey line. Next, in February, 1994, one explosion was made on southeast shore and wave signals were recorded by 27 event recorders on the west to north shore. We noticed significant differences between the seismic waves that passed just below the main crater and those traveled outside of the crater. The former waves attenuate more strongly and propagate more slowly than the latter waves, a … More nd the former waves are followed by reflected phases with large amplitude which show faster apparent velocity than initial P arrivals. The P velocity structure is derived from the direct and refracted P waves that passed outside of the main crater by using the time-term method. The result is as follows ; the surface layr, 1.8 km/s with thickness of 0.1 to 1km, the second layr, 3.6km/s with thickness of 0.3 to 2km, and the third layr, 5.6km/s. The delay of P arrivals that passed beneath the main crater is 0.2 to 0.4sec, which cannot be explained in terms of the time-terms beneath the stations. These results show the existence of low-velocity and low-Q region at about 2 km depth and the region is located within the horizontal extent of 2*2km^2 and about 5 km east of the main crater by the two surveys. The result of normal-moveout-correction shows that the reflection phases mentioned above come from about 7 km depth and 5 km east of the main crater. This reflector may suggest the top surface of magma chamber, and in this case, the low-velocity and low-Q zone should be restricted only shallower part of the third layr because of large amplitude of the reflected phases. Less