WESTWARD DECREASE IN POSTSEISMIC UPLIFT IN THE REGION OF THE 1960 CHILE EARTHQUAKE

A town near southern Chile’s Pacific coast shows little if any uplift since 1960, despite conspicuous emergence of shorelines 50-75 km farther inland. The inland emergence, reported previously from Puerto Montt and nearby shores, amounted to 1 m between 1960 and 1990. Postseismic uplift on this scale is unknown at Maullín, 55 km west of Puerto Montt and 10 km east of the Pacific coast. Maullín sank tectonically 1.5 m during the 1960 earthquake. Uplift has yet to undo this subsidence, as judged from tree stumps, pasture soils, and building foundations that remain covered by ordinary high tides.

To check for inconspicuous, partial rebound at Maullín, we use two series of measurements made during summer months on temporary tide staffs in the town marina. One series, from 1989, contains 11 high and 12 low tides; the other, from 2003, has 10 highs and 11 lows. The staffs were read to the nearest 5 cm at 30-minute intervals clustered near times of predicted high and low tides. We referred the staffs from the two years to a common datum.

To refer both series to a datum outside the region of the 1960 earthquake, we compare tide levels at Maullín with those gauged at Valparaíso, 1100 km to the north. We make the simplifying assumption that the Valparaíso gauge is stable with respect to land-level changes at Maullín. Monthly mean sea levels at Valparaíso were 5-7 cm higher in 2003 than in 1989. ENSO may explain this difference because 1989 shortly preceded a La Niña (sea surface low), while 2003 coincided with a moderate El Niño (high). Such oceanographic effects, probably shared by Maullín, overwhelm any postseismic land-level change from the 1985 Valparaíso earthquake (Mw 8.0).

We compared the height of each tide measured at Maullín against that tide’s crest or trough height from hourly readings at Valparaíso. This comparison between 1989 and 2003 shows no statistically significant land-level change at Maullín relative to the Valparaíso gauge that we assume stable. The mean uplift is 19 cm, the standard deviation 17 cm, and the two series have a six-in-ten chance of coming from the same population.

Research supported by Fondecyt 1020224. Valparaíso data from SHOA, Armada de Chile. Maullín staff read by Rebecca Oyarzo and Eduardo Maldonado.