Soft-sediment deformation structures in Holocene coastal gravel deposits reveal two 1.8–2.0 ka old Mw > 7.0 earthquakes in southern-central Hispaniola

Abstract

Abstract An exceptionally well-preserved outcrop of Holocene liquefaction structures in the Tortuguero Beach of southern-central Hispaniola has been investigated. We present a new high-resolution orthoimage mosaic, combined with fieldwork, sedimentary logging, structural analyses, and rock sampling for granulometric, grain-shape, and geochronological analysis to improve our understanding of the seismic hazard and the magnitude of the cyclic paleo-earthquakes occurred in this high seismically active region. Our results revealed three sedimentary sequences of deformed layers separated by undeformed sections. These metric-scale, episodic liquefaction structures resulted in an unusual negative density gradient in a coarsening upward stratified succession. Deformed layers form NNW-trending dome and basin elongated structures controlled by the present-day NE-directed regional shortening. Radiocarbon dating of the lower, intermediate, and upper sequences yielded ages (1σ) of 2332—2008, 1982—1803, and 1770—1530 cal BP, respectively. Liquefaction structures were triggered by M w  > 7 earthquakes likely occurring every 200 years. Seismic hazard modeling establishes that the primary sources of earthquakes are the large-scale, strike-slip fault zones that accommodate the collision of the Beata Ridge with southern-central Hispaniola. These fault zones probably generated the 1751 M w 7.5 Azua earthquake, and given the recurrence of such seismic events in southern Hispaniola, they could trigger future destructive earthquakes.