SORTING AND TRANSPORT OF SAND BY THE 1992 NICARAGUA TSUNAMI

Quantifying sorting as a function of settling velocity in tsunami deposits may provide a means by which tsunami sedimentation models can be developed and refined. Onshore sand deposits commonly record large tsunamis, so understanding their sedimentology can be important in hazard mitigation and neotectonics.

As a test case we used deposits of the tsunami from the Mw 7.2 1992 Nicaragua earthquake. This tsunami, as a single large wave, deposited several centimeters of sand onshore for several hundred meters at Playa de Popoyo (11.43 N, 86.07 W). Flat topography and a beach ridge limited backwash that might have otherwise reworked the deposit.

We used sediment transfer functions to relate sorting to settling velocity at Playa de Popoyo. Sediment transfer functions use one settling velocity or grain size distribution in order to normalize another related distribution (McLaren and Bowles, J. Sed. Pet. 55, 457-470, 1985). This normalization quantifies sorting as a function of settling velocity and reduces the confounding effects of size distribution in source sediment, thus emphasizing effects of sedimentation process.

For the Playa de Popoyo tsunami deposits the average transfer function has a step shape that suggests grouping the sediment into subsets with fast, intermediate, and slow settling velocities. Neither the fast nor slow subset shows much sorting, though the two subsets are sorted relative to one another. The intermediate subset by contrast shows good sorting. The minimal sorting in the fast-settling subset suggests transport energy high enough to homogenize the sand. Thus the sorting of intermediate-settling-velocity grains must be relegated to a volume of water separated from transport.

These observations are consistent with transport in the high-energy breaking head of the tsunami bore and deposition in lower energy water behind. This distinction accords with patterns of brick transport at Playa de Popoyo and patterns of sediment transport in swash bores (Puleo, Beach, Holman, and Allen, JGR 105 (C7), 17021-17044, 2000.) Sharp transitions between the intermediate and fast subsets may allow quantification of tsunami height.