TSUNAMI POTENTIAL OFFSHORE NORTHWEST PUERTO RICO I: MAPPING ACTIVE FAULTS AND MASS WASTING FEATURES IN THE MONA RIFT

Four deadly, historical earthquakes and tsunamis have occurred since 1492 in the zone of oblique convergence between Hispaniola on the Caribbean plate and the southeastern Bahaman platform on the North American plate. The Mona Rift, which marks the eastern boundary of this collision zone, is opening at 5 mm/yr due to the differential eastward motion of Puerto Rico relative to the collided area of Hispaniola to the west. In 1918, a M7.3 earthquake occurred ~40 km off the northwest coast of Puerto Rico in the Mona Rift. The earthquake and associated tsunami resulted in over 116 deaths and $50 million (current US dollars) in damage to coastal communities of western Puerto Rico. However, because of the lack of an integrated marine geophysical data set, the location and source mechanism of the 1918 tsunami remains poorly understood. We address this problem by correlating multibeam bathymetry, sidescan sonar imagery, and 2150 km of seismic reflection profiles with earthquake data from the local seismic network. These data reveal that the Mona Rift includes five major fault zones: 1) 15 km off the coast of Aguadilla, the upper rift is controlled by active northwest-trending normal faults. A 6-km-wide amphitheater-shaped scarp marks the headwall of a 15 km³ debris avalanche; 2) 25 km to the northwest of Aguadilla, the rift splits into a Y-shaped fault pattern. The smaller southwest-trending south limb is a region of down-dropped blocks bounded by west and northwest-trending normal faults; 3) The larger north-trending central Mona Rift displays a half-graben structure controlled by the Mona Canyon master fault on the steep (45°) eastern wall. At the base of the eastern wall, large, unsedimented blocks form a 15 km³ debris avalanche deposit. The western hanging wall (10° dip) has a 10 km x 20 km rotational slump that is capped by a 3 km³ mudflow deposit; 4) The Mona Block is a 50 km x 30 km seismically active region of forearc uplift bounded by the transpressional East Septentrional fault zone on the south side, and down-dropped blocks on the north side, exposing a Cretaceous blueschist belt; 5) The northwest-trending lower Mona Rift is dominated by northwest-trending normal faults, and a 10 km³ debris avalanche deposit. The dimensions and locations of active faults and submarine landslides will be used in numerical simulations of the 1918 tsunami.