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Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile

Geologic Hazards Team, U.S. Geological Survey, MS 966, P.O. Box 25046, Denver, Colorado 80225 anelson{at}usgs.gov

Kaoru Kashima

Department of Earth and Planetary Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan

Lee-Ann Bradley

Geologic Hazards Team, U.S. Geological Survey, MS 966, P.O. Box 25046, Denver, Colorado 80225

Online Material: Diatom species data from the Las Coloradas site, Valdivia estuary.

A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9.5 earthquake caused 1–2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic subsidence. In most of the 150 hand-driven cores that were examined, a distinct unconformity separates 0.5–1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying middle Holocene subtidal mud and sand. At the Las Coloradas site, where stratigraphy is best preserved, two A horizons of marsh and meadow soils abruptly overlain by sand and mud probably record coseismic subsidence shortly followed by tsunamis. The amount of subsidence during the earthquakes proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum ages on macrofossils from the two A horizons at the Las Coloradas site of 1.7–1.3 ka and 2.7–1.7 ka allow correlation of the younger horizon with either of two of six -dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullín, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Tidal records of prehistoric coseismic subsidence at Valdivia are scarce because of a sea-level fall of 3–8 m over the past 6000 years, erosion of marsh and meadow soils during subsidence-induced flooding of the estuary, and largely complete land-level recovery during cycles of coseismic subsidence and postseismic uplift.