vra. a, 4 Saderra Maso and Smith: Seismic Disturbances 219 



meridian 122° east — one stretching from the volcanic Island of 

 Camiguin to the northeast coast of Luzon and the other not far 

 from the Batanes Islands. 



In Luzon, therefore, no province is free from the effects of 

 earthquakes, for although it is true that in some, such as Tar- 

 lac, La Union, Isabela, Cavite, and Pampanga, no epicenter seems 

 to exist, still they are affected by the movements which originate 

 in other provinces. 



Seismotectonic lines. — The map, Plate II, gives the location 

 of the principal seismic areas (red ellipses) of the Islands 

 (the stars indicating the principal epicenters in these areas) 

 and the general direction of certain principal mountain chains. 

 Each figure includes one or more epicenters, and shows the gen- 

 eral shape and extent of the meizoseismic areas corresponding to 

 the greatest earthquakes which have occurred within the re- 

 gion limited by the same. The figures 3, 5, 7, 8, 9, 10, and 20 

 bear a heavy line to indicate that the earthquakes originating 

 near the same place have been too many to be specified. Twenty- 

 five different areas are recorded, and 5 of these (12, 13, 15, 

 17, and 18) are undoubtedly due in large part to rockfall and 

 volcanic activity, the other 20, in our opinion, being due to 

 tectonic causes. This is probably quite contrary to the general 

 belief regarding the earthquakes in the Philippines. The tec- 

 tonic areas are 1 to 11, 14, 16, and 19 to 26. The areas of 

 greatest seismicity are 2, 3, k, and 21, where at the present time 

 there is no known volcanic activity and where probably there 

 has been none since the end of the Tertiary period. 



Following the methods of other students of seismology, we 

 have connected the various epicenters shown on the map by lines 

 and have also added a few more of the latter where no epicenters 

 are indicated. There is a remarkable coincidence between these 

 lines and the principal lineaments in the Philippines. The va- 

 rious lines are denoted A — A, B — B, etc., on Plate II so that 

 they can be easily referred to. 



Line A — A, which is drawn through many epicenters, passes through 

 the northwest corner of the Province of Ilocos Norte, follows approximately 

 the coast west of the city of Vigan, and then cuts across the northwest 

 portion of Pangasinan Peninsula. It is impossible definitely to state 

 whether this line marks a fault line which lies along the coast, or is due 

 to a contact between the recent sediments and the older rocks which here 

 lie close to the coast, but we are of the opinion from geological studies 

 in the Province of Ilocos Norte that the latter is true. 



There is a very small development of coastal plain in this region and 

 also rocks of doubtful age which may possibly belong to the Jurassic against 

 which these very recent sediments abut. There is also a considerable 



