THE GREAT FAULT TROUGHS OF THE ANTILLES 105 



as along the north coast the land has been rising. The seismicity 

 of the region immediately north of Mona Passage is high. 



Many earthquakes have been recorded in Porto Rico and the 

 Virgin Islands.^ Most of them have had a low intensity and have 

 been reported from only one or two places, so that it is impossible 

 to determine accurately their epicenters, but it is usually possible 

 to locate, approximately at least, the epicenters of the stronger 

 shocks. If consideration is limited to earthquakes which have had 

 a probable maximum epicentral intensity of above VI in the 

 Rossi-Forel scale, it is found that, with very few exceptions, they 

 have originated along the steep slopes descending into the Brownson 

 or the Anegada troughs. 



The earthquake of April 16, 1844, which damaged buildings at 

 Isabela on the north coast of Porto Rico, probably had its origin 

 a short distance north or possibly northwest of the island. The 

 shock of November 28, 1846, was most strongly felt in the north- 

 western part of Porto Rico where some buildings were injured, the 

 distribution of the intensity indicating an origin off the northwest 

 coast. The earthquake of October 11, 191 5, which was felt over 

 most of Porto Rico and as far west as Puerto Plata, Santo 

 Domingo, probably originated a short distance north of Mona 

 Passage. 



The destructive earthquake of October 11, 1918, with its 

 accompanying sea-wave, and the strong aftershocks of October 

 18 and 24 and November 12, as well as a host of weaker shocks felt 

 during 1918-19, all originated a few kilometers west of Point 

 Borinquen on the northwest coast of Porto Rico. 



Other earthquakes have originated at points farther east along 

 the southern scarp of the Brownson Trough. On the night of 

 December 8, 1875, an earthquake, which probably had its epicenter 

 a short distance north of the coast, damaged buildings in Arecibo. 

 The earthquake of September 27, 1906, having an epicentral 



I A catalogue of earthquakes felt in Porto Rico and the Virgin Islands from 1772 

 to 1918 was given in "The Porto Rico Earthquake of 1918 with Descriptions of Earlier 

 Earthquakes. Report of the Earthquake Investigation Commission," by Harry 

 Fielding Reid and Stephen Taber, Document No. 269, U.S. House of Representatives, 

 66th Congress, ist Sess. (1919), pp. 53-66. 



