<P> Following the 2004 tsunami that affected more than forty countries in the Indian Ocean, many more people now fear the consequences that such an event would bring to the Caribbean . Local governments have begun emergency planning . In the case of Puerto Rico and the U.S. Virgin Islands, the United States government has been studying the problem for years . It is increasing its seismic investigations and developing tsunami warning systems . </P> <P> On 11 October 1918, the western coast of the island was hit by a major earthquake which caused a tsunami . The 1918 earthquake was caused by an old left - lateral strike - slip fault near the Mona Passage . In 1953, Santo Domingo, Dominican Republic, was affected by the Santo Domingo earthquake . The actual subduction zone (Puerto Rico Trench) has not ruptured in over 200 years, which is a major concern to geophysicists, as they believe it may be due for a major event . </P> <P> Puerto Rico has always been an area of concern to earthquake experts because, apart from the 1918 episode, there are frequent tremors in and around the island, indicating activity . A 1981 tremor was felt across the island, while another in 1985 was felt in the towns of Cayey and Salinas . </P> <P> "The January 13, 2014 M 6.4 earthquake north of Puerto Rico occurred as a result of oblique - thrust faulting . Preliminary faulting mechanisms for the event indicate it ruptured either a structure dipping shallowly to the south and striking approximately east - west, or a near - vertical structure striking northwest - southeast . At the location of this earthquake, the North America plate moves west - southwest with respect to the Caribbean plate at a velocity of approximately 20 mm / yr, and subducts beneath the Caribbean plate at the Puerto Rico Trench . The location, depth and mechanism of the earthquake are consistent with the event occurring on this subduction zone interface ." </P>

Which of the following phrases describes the shape of the puerto rico trench