224 The Philippine Journal of Science 1913 



northern course of the Rio Grande Valley. This line seems to be necessary 

 to explain the anomalous course of this river. Line and line M seem to 

 explain this satisfactorily. 



Numerous other lines might be suggested, but we have included 

 all for which we have any geological basis and a few others as 

 suggestions. 



DISCUSSION OF IMPORTANT EARTHQUAKES 



In discussing the various important earthquakes, little will 

 be said about those previous to 1870, save the greatest which 

 occurred in 1645, but we shall consider the entire series 

 represented by the charts published in La Seismologia en Fili- 

 pinas, and discuss most of them separately. 



Earthquake of 1645, Luzon (3-9) . 30 This earthquake com- 

 pares favorably in magnitude with the greatest mentioned in the 

 history of the world; its meizoseismic or epicentral area was 

 not less than 490 kilometers from north to south; that is, from 

 the southern coast of Batangas and Tayabas to the northern 

 part of Cagayan. On the western coast it seems to have been 

 of less intensity, at least the chronicles of the time are silent 

 about its effects in these parts, while they deal very much 

 with the destruction caused in Manila and neighboring provinces 

 of the south, east, and north, and the tremendous effects produced 

 as far north as Lalloc. in the Cagayan Valley, and in the east- 

 ern part of the Central Cordillera; that is, in the Mountain 

 Province. That such an earthquake was due to tectonic move- 

 ments, there cannot be the slightest doubt; furthermore, it is 

 very certain that its origin was along a north and south line 

 and that this line was within the Island of Luzon and not 

 beyond its eastern periphery; it may be reasonably supposed 

 that it was along the seismotectonic line D — D. The question 

 arises whether the dislocations which then occurred along that 

 line or fault were of such proportions as to be responsible for 

 the many singular topographic features now existing along it 

 in the Provinces of Nueva Ecija, Nueva Vizcaya, and Isabela. 

 Moreover, as the origin of that earthquake seems to have been 

 very close to the city of Manila, our opinion is that some 

 dislocations occurred at the same time along the fracture 

 represented by the line H — H. Similar occurrences of lines 

 crossing each other are frequent in severe earthquakes. For 

 example, the earthquake which occurred in 1870, having its epi- 

 center in the northwestern portion of Luzon, apparently was 



30 Figures refer to Plate II and indicate the location. 



