Late holocene evidence of coseismic subsidence on the San Juan delta, Pacific coast of Colombia

The San Juan delta of Colombia formed under a complex physical setting. Amo ng the unusual factors that controlled the delta formation are: its locatio n in an area with one of the highest precipitation rate anywhere in the wor ld, highly variable river discharge and high sea levels of short duration, both associated with El Nino-La Nina climatic disturbance, and high mesotid al range combined with a medium to low wave energy. Of all the aspects that make the physical setting complex, the high seismicity produced by the sub duction of the Nazca plate under the South American plate, has perhaps, bee n the most important controlling factor in the Late Holocene development of the delta. Goring on its SW island retrieved soils buried, 1.2-1.5 m by in tertidal deposits; the sharp contact between the soils and the mantling dep osits, implies large sudden submergence, which is most simply explained as being coseismic, associated with the high seismicity of the area. Timing of the earthquake that caused subsidence, is placed around 500 years BP. Evid ence from recent earthquakes along the South central Pacific coast of Colom bia indicates that-subsidence > 1 m requires a M 7.9 or greater earthquake.