vni, a, 4 Saderra Maso and Smith: Seismic Disturbances 213 



the eruptive period — January 27 to February 7 — 995 shocks 

 were recorded in the Manila Observatory, all of them between 

 I and V of the Rossi-Forel scale. Some of the principal shocks 

 were perceptible at a radius of from 120 to 200 kilometers. 

 The meizoseismic area of the earthquakes was a prolonged 

 zone which took in not only the volcano itself, but also the 

 south-southwestern part of Laguna de Bay and the bordering 

 territory as far as the sea, some 20 kilometers away (Plate II) . 



The decrease of perceptible intensity of the seismic action out- 

 side the epicentral area was, according to many comparisons 

 made on the spot, 1 degree of the Rossi-Forel scale for every 15 

 kilometers. The same result was also deduced from the fact 

 that the maximum intensities of several of the earthquakes as felt 

 in Manila, a distance of 63 kilometers, were from IV to V of the 

 scale, while in the epicenter, judging from the effects in the 

 ground and on buildings, they were from VIII to IX. These 

 same earthquakes, which had an intensity VIII to IX in the 

 epicenter and IV to V at a distance of 63 kilometers, were also 

 registered at Taihoku (Formosa) some 1,000 kilometers away, 

 but not in any more distant observatory. 



The reports of previous eruptions of Taal also make mention of 

 numerous volcanic earthquakes, which no doubt possessed the 

 same characteristics as those that occurred recently. A curious 

 fact is noted in these reports; namely, that while in the recent 

 eruption the meizoseismic area extended 20 kilometers to the 

 south-southwest, in the previous eruptions it extended to the 

 north-northeast for about 30 kilometers as far as Laguna de Bay. 

 From this it is easy to recognize the direction of the rift on which 

 the volcano originated. 



In 1871 a series of volcanic earthquakes occurred on the 

 volcanic Island of Camiguin, north of Mindanao. These earth- 

 quakes culminated in the opening of a small crater whose 

 activity lasted four or five years. They were first felt in 

 February, and went on increasing in intensity and frequency 

 until the morning of April 30, when the volcano exploded and 

 the earthquakes suddenly ceased. The greater number of these 

 shocks were perceptible on the island only, although many were 

 also heavily felt on the neighboring coasts of northern Min- 

 danao and southern Bohol, while only four or five, whose in- 

 tensity in the epicenter was between VII and X, were noticed at 

 a distance of 250 kilometers. 



The following table gives some idea of the number and in- 

 tensity of the shocks preceding the eruption of Camiguin. 



