UNUSUAL PRESERVATION OF A TSUNAMI DEPOSIT ASSOCIATED WITH THE 1771 MEIWA TSUNAMI, OKINAWA, JAPAN

A layer of coral sand thins and fines landward in stream valleys on the steep eastern flank of Ishigakijima, an island where ~10,000 people died during a large tsunami in April, 1771. This tsunami also moved coral boulders hundreds of meters inland; the sand layer is associated with these better-known boulders, and probably resulted from the same event. The sands are preserved under natural channel levees, suggesting that even in overall erosive coastlines such as Ishigaki, local preservation may occur along stream channels.

In two unnamed river valleys near Inoda, the sheet consists of 30 cm of moderately sorted carbonate sand overlying siliciclastic cobble gravel similar to the modern stream bed. The deposit appears discontinuously in stream banks for ~700 m inland, thinning to less than 5 cm before becoming indistinguishable from other streambank sediments. In one location ~200 m from shore, a modern drainage ditch cut perpendicular to the bank allows for access to the sand sheet. Here, the sheet is continuously exposed for 50 m, and thins from 30 cm thick near the stream to 0 cm 50 m from the stream. The deposit is mantled by more than 50 cm of mud near the stream channel, but the mantle thins away from the channel so that by 50 m, the sand sheet is exposed at the surface.

Because there are no other carbonate sand sheets in the sections we studied, nor is there a source of carbonate material upstream, we suspect that these sheets are associated with the 1771 Meiwa tsunami. Preservation occurs only where the sheets have been draped by mud along stream channels—elsewhere the steep topography of Ishigaki does not favor preservation. The mud drape likely results from renewed erosion following extensive agriculture on Ishigaki starting in the late 1800’s.

The search for prehistoric tsunami deposits on tectonically active coastlines has been hampered, in part, because of the relatively few depositional environments on overall erosional coastlines. Tidal marshes represent one such environment, but in tropical climates where tidal marshes are few, other locations will have to provide the “recorder” needed to establish a prehistoric tsunami history. On Ishigaki, a local depositional environment was created along stream banks when intensive agriculture increased upland erosion, causing natural levees to preserve stream bank sediments.