FRAGMENTARY EVIDENCE OF PREHISTORIC GREAT-EARTHQUAKE SUBSIDENCE IN THE REGION OF THE M9.5 CHILEAN EARTHQUAKE OF 1960

As we approach the half-century anniversary of the largest instrumentally recorded megathrust earthquake, the M 9.5 mainshock of May 1960 in south central Chile, the effects of its ground shaking, subsidence, and tsunamis remain a baseline for the application of subduction-zone paleoseismology worldwide. But only a fragmentary record of the land-level changes that must have accompanied earlier great earthquakes near Valdivia, south central Chile, has been found. Despite a climatic, vegetative, and coastal-geomorphic setting much like that of the Cascadia subduction zone, where stacks of peaty, muddy, and sandy sediment beneath tidal wetlands at tens of estuaries archive a 5000-yr history of magnitude 8 to 9 plate-boundary earthquakes and tsunamis, in south central Chile records of prehistoric coseismic subsidence are sparse and well documented only at a single site.

Here I describe fragmentary evidence of coseismic subsidence uncovered during an early search for evidence of prehistoric land-level changes beneath the fringing marshes of the Valdivia estuary, where the 1960 earthquake caused 1-2 m of regional coseismic subsidence. In most of 150 hand-driven cores examined, a distinct unconformity separates 0.5-1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying mid-Holocene subtidal mud and sand. At the site where stratigraphy is best preserved, two A horizons of marsh and meadow soils each abruptly overlain by sand and mud probably record coseismic subsidence shortly followed by tsunamis. The amount of subsidence during the earthquake proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum ¹⁴C ages on macrofossils from the two A horizons (1.7-1.3 ka and 2.7-1.7 ka) allow correlation of the younger horizon with one of six ¹⁴C-dated A horizons buried by sand 200 km to the south at Maullín, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Records of prehistoric coseismic subsidence at Valdivia are scarce because of erosion and non-deposition of tidal sediment during 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.