Towards improvement of northeastern Caribbean tsunami hazard assessments: Coring of coastal ponds yields seven events in a 5,000 year-old period

Abstract

Current tsunami coastal hazard assessments in the northeastern Caribbean, limited by an uncertain interplate coupling and a short historic and paleoseismologic record, would benefit from ground truthing by means of detailed studies of the coastal sediment record. The northeastern Caribbean, surrounded by active faults, has experienced seven M 7.5 events in the last 500 years (1690, 1843, 1867, 1943, 1946, 1946, and 1974), of which five (1690, 1843, 1867, 1946, and 1946) generated tsunamis. Investigating the coastal sediment record requires differentiation between storm and tsunami overwash deposits. A post-storm survey of 2010 hurricane Earl described the effects on the coast of Anegada, British Virgin Islands, the island closest to the Puerto Rico Trench (PRT), as limited to wrack deposition along shores, deposition of an extensive microbial detritus coating from interior salt ponds and spillover fan deposits on the south shore that extended inland a few tens of meters. These effects contrasted significantly with two previously inferred pre-historic tsunami events (A.D. 1200-1450 and A.D. 1650-1800) characterized by underlying sand and shell sheet that extends 1.5 km southward from the north shore, large erosional features such as breached ridges and southward-strewn limestone boulders as far as 1 km inland. At St. Thomas, U.S. Virgin Islands, located southwest of Anegada and northwest of the 1867 tsunami source, seven tsunami deposits, from coastal ponds along the north and south coasts, are inferred from a 5,000 years old sediment record. Of these, one correlates to the AD 1650-1800 event, another to the AD 1200-1450, two occurred between 18 B.C.-A.D. 1151 and 351 B.C.-A.D. 129, another occurred between 1628-168 B.C. with the oldest recorded event exceeding 2901 B.C. Of these, two were found at three ponds and could be related to an event similar to that from A.D. 1200- 1450. Most of these events were coincident with drastic environmental changes marked by sharp contacts, rip-up clasts, and lithological and ecological changes. These findings expand the tsunami record for the northeastern Caribbean to seven events within a 5,000-year time span. This work would benefit from additional dating of events to better constrain the time frame these events occurred and calculate a more precise recurrence rate for the region.

Evaluaciones sobre el peligro costero en el Caribe noreste, limitadas a un acoplamiento de placas impreciso, y un registro histórico y paleosismológico cortos, se beneficiaría de la comprobación de datos por medio de estudios detallados del registro sedimentario costero. El Caribe noreste, rodeado por fallas activas, ha experimentado siete eventos de magnitud 7.5 en los últimos 500 años, (1690, 1843, 1867, 1943, 1946, 1946, y 1974) de los cuales cinco (1690, 1843, 1867, 1946, y 1946) han generado tsunamis. Investigar el registro sedimentario requiere poder diferenciar entre depósitos de tormenta y de tsunami. Un estudio de reconocimiento luego del huracán Earl 2010, describe los efectos en la costa de Anegada, Islas Vírgenes Británicas, como limitados a una línea de escombros depositados en las orillas de las playas, capa extensa de material residual de mats microbianos existentes en lagunas salitrales interiores y depósitos de arena en la costa sur que se extendían decenas de metros tierra adentro. Estos efectos contrastan significativamente con dos eventos de tsunami previamente inferidos, AD 1200-1450 y AD 1650-1800 caracterizados por capas subyacentes de arena y conchas que se extienden 1.5 km hacia el sur, crestas de playa erosionadas y peñascos de caliza movidos hasta 1 km de distancia tierra adentro. En St. Thomas, Islas Vírgenes Americanas, localizada al suroeste de Anegada y al noroeste de la fuente del tsunami del 1867, siete depósitos de tsunami son inferidos de un registro de sedimento obtenido de 5,000 años de antiguedad. De éstos, uno correlaciona con el evento del A.D. 1650-1800, otro con el del AD 1200- 1450, dos ocurrieron entre 18 B.C.-A.D. 1151 y 351 B.C.-A.D. 129, otro ocurrió entre 1628-168 B.C. y el mas antiguo excede 2901 B.C. La mayoría de estos eventos coincide con cambios ambientales drásticos marcados por contactos bruscos, clastos desgarrados de otras unidades, y cambios ecológicos y litológicos. Estos hallazgos expanden el registro de tsunamis en el Caribe noreste a siete eventos dentro de un periódo de tiempo de 5,000 años. Éste trabajo se beneficiaría de dataciones de eventos adicionales para mejorar el marco de tiempo en el que estos eventos ocurrieron y así calcular una razón de recurrencia mas precisa para la región.