218 The Philippine Journal of Science ms 



was very probably in the past the scene of many volcanic 

 earthquakes, * although the data we possess are very deficient. 

 Volcanic formations more recent than the Pliocene do not occur 

 in Cebu. 



The principal seismic regions of southeastern Luzon and 

 adjacent region are those of Camarines, Albay, and Masbate 

 with three principal and well-defined epicenters — the first along 

 the central depression of Ambos Camarines, the second to the 

 north of San Bernardino Strait, and the third in the Island of 

 Masbate or near its northern and southern coasts. Between 

 Sorsogon Bay and the Gulf of Albay there is also an epicenter 

 of small extent. 



The southern part of Luzon constitutes the second seismic 

 region of greater importance in the Archipelago. Four epi- 

 centers may be distinguished in it ; namely, one in the east near 

 the coast which appears to stretch from the north of the Bay 

 of Lamon to the south in the sea between Mindoro and Marin- 

 duque; a second between Mindoro and Luzon; a third in the 

 China Sea along the coasts of Cavite and Zambales; and the 

 fourth, which may be called the Manila epicenter, is situated 

 in the Eastern Cordillera and its spurs, between Laguna de Bay 

 and the Gulf of Baler. The volcanic epicenters of Taal and 

 other volcanoes are not reckoned. 



Northern Luzon from parallel 16° northward contains 4 

 extensive seismic regions: That of Pangasinan whose long axis 

 appears to cross the island approximately from east to west from 

 Casiguran Bay to Lingayen Gulf, following the edge of the 

 great central plain of the island. The Nueva Vizcaya epicenter 

 may be considered as belonging to this central seismic region. 

 The second region includes the various epicenters of Ilocos 

 Sur and Norte, some of them in the sea close to the coast, 

 others probably at the extreme east of the plain or coastal 

 belt which borders these provinces. The third region, which 

 is an important one, is situated along the central mountain chain 

 of the Mountain Province, and extends as far as the Babuyanes 

 Islands. Within the confines of the extensive Cagayan Valley 

 there are very important tectonic lines, but they have not, since 

 1645, given rise to any very great earthquake. However, there 

 are frequent earthquakes of slight extent and intensity, those 

 occurring in the north being probably of volcanic origin, while 

 those in the south are due to epicenters whose influence seems 

 to decrease. 



In the extreme northern part of the Archipelago and outside 

 the limits of Luzon there are at least two epicenters close to the 



