PRESENT AND PAST LAND-LEVEL CHANGES AT GUAFO ISLAND, CHILE (43.5°S): TRANSIENT ACCUMULATION OF PERMANENT STRAIN OVER THE SEISMIC CYCLE

We investigate the seismic cycle of megathrust earthquakes at Guafo Island, located 70 km landward of the south Chile trench, and which was uplifted 3.6-4.0 m by the M9.5 1960 earthquake. Coastal geology and historical aerial images show rapid subsidence after 1960 that increases from 5-10 mm/a (1975-1998) to 20-25 mm/a (1998-2009), and compares with a GPS rate of subsidence of ~12 mm/a (1994-2009). Ongoing subsidence is leading to erosion, by wave action, of a soil that formed on an uplifted bedrock platform in the years immediately following the 1960 event. In contrast, emerged wave-cut notches and mid-Holocene radiocarbon ages of estuarine deposits suggest long-term uplift at 0.2-0.3 mm/a. To bridge the gap in time scales, we excavated 15 pits in a forested coastal plain on the NW of the island. Two buried soils were found in most pits. The lower peaty soil, which commonly lies above a bedrock platform, retains growth position Juncus rhizomes from AD 411-534 (3 samples). The top of the soil is eroded and overlain by intertidal marine sand. This apparent sea-level rise is attributed to interseismic subsidence as observed at present in the post-1960 soil. The upper buried soil formed around AD 679-773 (2 samples). It grades from marsh to forest character and contains layers of gravel, sand, shells, or bedrock fragments, which may have been deposited by storms or possibly tsunamis. The absence of marine sand overlying the upper soil in all pits and its marsh-to-forest transitional signature suggest higher uplift during the AD ∼700 event, or lower subsequent interseismic subsidence. The two buried soils at Guafo may be tentatively correlated to events F and E of the Maullín record (Cisternas et al., 2005), 250 km northeast, where co-seismic subsidence has been inferred for the last 2 ky. There, soil E is edaphically distinct and associated with a geomorphic break, as is its counterpart at Guafo. Our preliminary results suggest that long-term coastal uplift in southern Chile is modulated by variable transient processes associated with individual earthquakes. Apparent acceleration of the ongoing subsidence may be related to an increase in fault locking resulting from healing since 1960. Further insights will be provided by permanent GPS stations, numerical modeling, and more 14C ages.