Study of prediction of volcanic eruptions at Taal, Philippine

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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