2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 4
Presentation Time: 2:15 PM


CISTERNAS, Marco, Agronomy Faculty, Catholic University of Valparaiso, San Francisco s/n, La Palma, Quillota, ATWATER, Brian, U.S. Geological Survey at Department of Earth and Space Sciences, Univ of Washington, Seattle, WA 98195-1310, Seattle, WA, MACHUCA, Gonzalo, Department of Historical & Social Sciences, Univ of Concepcion, Barrio Universitario s/n, Concepcion and LAGOS, Marcelo, Institute of Geography, Catholic Univ of Chile, Av. Vicuña Mackenna 4860, Santiago, Chile, marco.cisternas@ucv.cl

Midway along its 1000-km rupture in south-central Chile, the 1960 earthquake produced a bigger tsunami and more subsidence than did its predecessors in 1837 and 1737. The region’s earliest historical earthquake, of 1575, had the size of the giant 1960 event.

These preliminary interpretations come from the Río Maullín estuary, which indents the Pacific coast at 41.5º S. The 1960 earthquake tectonically lowered the landscape by ~1.5 m, and the ensuing tsunami spread sand across lowland soils of Chuyaquén, 10 km upriver. The subsidence killed forests and changed pastures into tidal flats. Guided by these 1960 analogs, we inferred tsunami and earthquake history from buried soils, old maps, and tree rings.

At Chuyaquén, the penultimate buried soil underlies a sand sheet that resembles the tsunami deposit from 1960. This soil contains, 1 cm below its top, a brushfire layer. Carbonized twigs at two sites gave four ages, all in the 2-sigma range AD 1410-1630. If the overlying sand sheet shortly postdates these twigs, the penultimate large tsunami predates 1837 and 1737 and probably dates to 1575.

Compared with the 1960 earthquake, the 1837 and 1737 earthquakes did little harm to the estuary’s trees and islands. On islands of Misquihué, 30 km up the estuary, brackish tidal water admitted by the 1960 subsidence killed or mostly defoliated tens of thousands of trees. Their bare trunks now stand ghostly gray above new growth. Many of the victims contain over 200 annual rings and some, 300. Eyewitnesses and airphotos confirm that these trees stood green up to 1960. Describing the same islands in 1874, surveyor Francisco Vidal and botanist Carlos Juliet noted nothing unusual. On his nautical chart of the estuary, moreover, Vidal depicted as vegetated several islands that the 1960 earthquake would lower into tidewater. These islands remain submerged today, 43 years after they subsided. Yet Vidal found these same islands emergent 36 years after 1837; the islands probably failed to subside in 1837 by as much as they did in 1960.

Implications: Strain not released in the Chilean earthquakes of 1737 and 1837 contributed to the enormity of the 1960 event. By analogy with this sequence of dissimilar Chilean earthquakes, successive great earthquakes may routinely differ in size at the Cascadia subduction zone.

Research supported by Fondecyt 1020224