vm, a, 4 Saderra Maso and Smith: Seismic Disturbances 215 



comprehends the greater number of earthquakes felt in all parts 

 of the world and in particular all those of greater intensity 

 and extent. To it belong practically all the great Philippine 

 earthquakes enumerated by Saderra Maso 20 and especially those 

 which occur in eastern Mindanao, in the Valley of the Agusan, 

 and on the Pacific coast, regions which appear to be closely 

 related to the great submarine trough, "the Philippine deep" 

 of 9,780 meters, already referred to. Further on we shall 

 discuss some of the tectonic earthquakes of greater importance. 



The rockfall type embraces the earthquakes of small extent, 

 which, having their focus or seat of origin at a slight depth, 

 are brought about by the fall of rock in caves and underground 

 passages, by landslides, and in certain cases by the settling of 

 superficial rock masses displaced by tectonic seismic motions. It 

 appears that this is the predominant type of shocks felt in several 

 nonvolcanic regions of the Philippines, but earthquakes of this 

 type are very often extremely difficult to recognize on account 

 of the distance between the seismic stations and the large extent 

 of the uninhabited mountainous and forest districts in which 

 they occur, so that it is impossible to fix the limits of the area 

 where many of the earthquakes are perceptible. We consider 

 as typical instances of this class of earthquakes those of 1881 

 in Nueva Vizcaya. From January to October of that year there 

 was a continuous series of earthquakes, the maximum of intensity 

 and frequency occurring in September. To get an idea of this 

 seismic period, the well-known catalogue of the missionary 

 Xavert should be consulted. This catalogue takes in sixty-three 

 days between January and October, and contains the record of 

 196 earthquakes with the times at which they occurred. Phrases 

 such as "almost continuous," "many more," "the whole day," 

 "the whole night" appear twenty-five times in the list, thus 

 indicating that the smaller earthquakes were not counted. 



Abella, who examined the effects of these earthquakes in the 

 field, found that the meizoseismic area was of very small extent 

 and that its center coincided with the town of Bambang. The 

 great majority of the shocks were only perceptible within an 

 area of 60 kilometers, so that only 5 of intensity VII to IX 

 exerted any influence beyond the province. Much of the data 

 supplied by Abella fully confirms his statement which is further 

 strengthened by an examination of the records of the hourly 



" Catalogue of violent and destructive earthquakes in the Philippines, 

 1599-1909. Manila (1910). 



118365 2 



