THE GREAT FAULT TROUGHS OF THE ANTILLES 109 



Near Ponce on the south coast of Porto Rico, Berkey has 

 traced an east-west fault which he thinks is of very late Tertiary 

 age/ It brings the older rocks of the pre-Tertiary into contact 

 with the younger series forming the present coastal margin. This 

 fault may be one of a series by which the descent to the floor of 

 the Caribbean Basin is accomplished. There is, however, no topo- 

 graphic evidence of recent displacements along it ; and the Seismic 

 history of the region indicates that, if the Caribbean scarp be due 

 to faulting, it is now relatively stable. The epicenter of no earth- 

 quake can be definitely assigned to this scarp, although some of 

 the shocks reported only from the south coasts of Porto Rico and 

 Haiti may have originated there. Certainly no disastrous earth- 

 quakes and comparatively few light shocks have occurred there in 

 several centuries. 



On August 4 and 13, 1908, moderately strong shocks were felt 

 at Ponce and neighboring points. The intensity was about 

 VI R.-F., and some buildings were slightly damaged. The ori- 

 gin was probably not far from Ponce. 



Earthquakes of low intensity have originated at many places 

 in Porto Rico, but in contrast with the north coast none has had 

 an intensity of more than VI or VII. A shock on August 30, 1865, 

 which slightly damaged churches at Manati and Ponce, probably 

 had its origin near the center of the island. On October 23, i860, 

 an earthquake with intensity of VI-VII caused some damage at 

 Mayagiiez on the west coast, and the epicenter was probably not 

 far away. Several light shocks have originated in the eastern part 

 of Porto Rico near Cidra, the strongest, on October 22, 1901, having 

 an intensity of about VI. 



RELATION OF FAULT ZONES TO EARLIER TECTONIC TRENDS 



The fault zones described in this paper collectively form' a 

 great fault system which extends from the eastern end of the 

 Brownson Trough westward to the Gulf of Honduras, a distance 

 of 2,500 km. The two fault zones inclosing the Bartlett Trough 

 are apparently in alignment with the Matogua Valley and the 



^ C. p. Berkey, "Geological Reconnoissance of Porto Rico," Ann. N.Y. Acad. 

 ScL, Vol. XXVI (1915), p. 40. 



