EARTHQUAKE POTENTIAL OF THE MUERTOS TROUGH: ONSHORE SLEUTHING FOR TSUNAMI DEPOSITS

Sedimentary deposits and large overwash fans found at two lagoons in the Bahia de Ocoa may record tsunami inundation along the south coast of Dominican Republic during the Late Holocene. The western section of the Muertos Trough is located approximately 60 km offshore from Bahia de Ocoa and is currently characterized by minor earthquakes. The Muertos Trough is thought to be the source of the October 18, 1751 moment magnitude (Mw) 8.0 earthquake. According to historic accounts, the strong earthquake destroyed the town of Azua and spawned a tsunami that inundated the town. If a similar event occurred today, it could prove deadly for densely populated, low-lying areas along the southern coasts of Dominican Republic and Puerto Rico. In order to investigate the earthquake and tsunami potential of the Muertos Trough, we performed reconnaissance for tsunami deposits in two shallow lagoons along the south coast. Using a gouge corer, sediment cores were collected along transects across both lagoons. The location of each core was measured with a hand-held GPS. Each core was described and photographed and samples were collected for sedimentological and radiocarbon dating analyses. The eastern lagoon is protected by a storm berm of lithic cobbles. We found at least two large overwash fans covered by mangroves. Two cores several meters apart in the westernmost side of the lagoon contain at least 6 overwash deposits. The core farthest from shore yielded three deposits between 100 and 120 cm depth characterized by coarse lithic sand, similar in composition to beach sand on the shoreface. The core halfway across the lagoon contained three sandy deposits within the upper meter. The western lagoon is surrounded by mangroves and does not have a storm berm. An overwash fan covered with mangroves extends from the seaward border to the middle of the lagoon. All the cores show at least three overwash deposits characterized by clasts of organic material. The core closest to the coast contains two sandy layers that are not observed in the other cores. Future­ dating of organic samples will provide age constraints of the lagoonal deposits. Additional field work will involve more thorough sampling at both lagoons to help establish the origin of large overwash fans at both lagoons, and a visit to a third lagoon that hopefully will contain a longer sediment record.