vm, a, 4 Saderra Maso and Smith: Seismic Disturbances 231 



and the alluvial formations. Plate III shows the isoseismals 

 of this earthquake and the geology of the region. 32 



The Sorsogon epicenter lies on the secondary seismotectonic 

 line N' — N' which future events may prove to be, as it has been 

 stated elsewhere, 33 the continuation or a branch of the Camarines 

 line N—N. 



It has been suggested by Mr. Wallace E. Pratt that this earth- 

 quake was due to differential movement between the igneous 

 and the sedimentary block to the eastward and northward as 

 shown on the map. This seems to be highly probable. 



PRACTICAL CONSIDERATIONS 



1. The fact of the instability of the earth's crust has been 

 proved time and again both by tremendous catastrophes and by 

 laboratory experiments. It has been demonstrated that many 

 of these devastating earth movements take place along definite 

 lines of weakness in the crust. The location and extent of 

 these lines can usually be fairly accurately determined by a 

 geological examination. 



2. The points of intersection of such lines are dangerous as 

 can be shown by an examination of the Province of Calabria 

 in Italy. 34 



3. Volcanoes are only incidental phenomena, and are results 

 rather than causes. They are usually found to be lined up along 

 some rift line. 



4. Points of danger in the Archipelago are : 



a. Along the Taal rift line from the town of Lemery to Los Bafios 



on Laguna de Bay, and possibly farther to the northeast. 



b. All "made" land and recent alluvium. The California Earthquake 



Commission (1908) reported that the intensity of the shocks and 

 the destruction were greatest and the amplitude of the waves 

 longest in the "made" ground. 35 



c. The Agusan Valley, Mindanao. 



d. The Straits of San Juanico. 



e. The district northeast of Manila near the east coast and northwest 



of the Island of Polillo. Three prominent lines intersect in two 

 places close together in this region. 

 /. That part of Batangas Peninsula which ends in Cape Santiago. 



32 Adams, G. I., and Pratt, W. E., This Journal, Sec. A (1911), 6, 449. 

 : ' Bull. P. I. Weather Bur. (1912), 447. 



" Hobbs, W. H., Some principles of seismic geology, Beitragen zur 

 Geophysik (1907), 8, 224. 



"Report of California Earthquake Commission. Washington (1910). 



118365 8 



