RECURRENCE OF GIANT EARTHQUAKES INFERRED FROM TSUNAMI SAND SHEETS AND SUBSIDED SOILS IN SOUTH-CENTRAL CHILE

At the Río Maullín estuary, in the region of the giant 1960 earthquake, tsunamis and post-earthquake tides deposited sand sheets on coseismically subsided soils. The two sand-sheet facies can be distinguished by their thickness, intertidal erosion and bioturbation of the underlying soil. A sequence of sand sheets on buried soils suggests a history of four giant earthquakes in the past 1000 years.

The estuary, at 41.5°S, has a tide range of 1.5 m, low turbidity, and ample sources of sand. Pastures at Chuyaquén subsided 1.5 m during the 1960 Chile earthquake and were overrun by the ensuing tsunami. The lowest of the subsided pastures became tidal flats while higher ones became tidal marshes.

We studied 31 backhoe pits along a 1-km transect. The pits exposed sand sheets hundreds of meters in horizontal extent, not only from 1960 but also from earlier events.

The sand sheet from the 1960 tsunami differs from post-1960 tidal-flat sand in the preservation of the underlying 1960 soil and in the thickness of the sand itself. Erosion on post-1960 tidal flats stripped the 1960 soil to levels below the rhizomes of plants. Exposure on post-earthquake tidal flats also allowed worms and crabs to riddle the soil with burrows. By contrast, beneath post-earthquake tidal marshes, the 1960 soil retains a continuous upper surface beneath tsunami deposits. These deposits protect the soil and locally surround upright stems and leaves of rushes (Juncus balticus and J. procerus) from the 1959 growing season. The 1960 tsunami deposits rarely exceed 0.1 m in thickness. The post-1960 deposits of intertidal bars commonly thicken to 0.5 m.

These criteria aid in interpreting a sequence of four sand sheets interbedded with buried soils. The oldest sheet is probably a tsunami deposit. It formed AD 990-1160, the 2-sigma age range of upright J. procerus culms that the sand surrounded. Above lie three younger sand sheets: an undated tidal-flat deposit, a tsunami deposit slightly younger than AD 1410-1630, and the 1960 tsunami deposit. The sequence thus implies four earthquakes in 800-970 years. If these four represent all the area’s giant earthquakes in that time, these events occurred less often than did the historical earthquakes of 1575, 1737, 1837, and 1960.

Work supported by Fondecyt 1020224 and 7020224