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

Abstract

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 sub- sidence. In most of the 150 hand-driven cores that were examined, a distinct uncon- formity 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 fol- lowed 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 14 C 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 14 C-dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullin, 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. Online Material: Diatom species data from the Las Coloradas site, Valdivia estuary.