PALEOSEISMIC INVESTIGATIONS OF THE MIDDLE AMERICA TRENCH ON THE NICOYA PENINSULA, COSTA RICA: A FEASIBILITY STUDY OF THE TAMARINDO ESTUARY

The Middle America Trench offshore of the Nicoya Peninsula, Costa Rica, is an ideal setting for investigating problems in magathrust seismogenesis, including the inherent variability of rupture mode and the influence of boundary conditions on megathrust mechanics. Despite extensive recent studies of this megathrust, geologic constraints have not yet been obtained for its prehistoric rupture history. The historical record indicates that large (M>7) earthquakes occur with typical recurrence of ~50 years, but it is unknown whether less frequent, larger ruptures are possible. We have begun evaluating potential paleoseismic sites in this area. The Tamarindo Estuary is located along the northern Nicoya coast and is a ria-formed, tide-dominated estuary at the mouth of the moderate sized (73 sq km) Rio Matapalo basin. The zoned ecology of tidal flats and gradational mangrove successions imply that recent relative sea level change would have been recorded in the physical characteristics and micropaleontology of shallow sediments. Reconnaissance gouge and Russian-type coring reveal a complex stratigraphy dominated by tidal mud and sand in the upper meter lying above several meters of peat. Radiocarbon ages indicate that the upper 1.5 m was deposited slowly (~0.2-0.3 mm/yr) over the past ~5 Ka. Our working model is that the estuary has been close to sea level since the mid-Holocene, experienced slow subsidence and modification due to lateral migration of tidal channels and mangrove infilling, but does not clearly record oscillation or sudden elevation change related to the seismic cycle. Although ongoing palynological and sedimentary analysis will further test this, these initial findings suggest this site is not ideal for paleoseismology. This may be due to the small size of the basin (i.e. slow subsidence), potentially small seismic uplift and subsidence along the edge of the Nicoya segment, or the inherent complexity of mangrove depositional environments.