Unexpected coseismic surface uplift at Tirúa-Mocha Island area of south Chile before and during the Mw 8.8 Maule 2010 earthquake: a possible upper plate splay fault

The Tirua-Mocha Island area (38.2°-38.4° S) in southern Chile has been affected by two megaearthquakes in only 50 years: the 1960 Mw=9.5 Valdivia earthquake and 2010 Mw=8.8 Maule earthquake. We studied in the field the vertical ground movements occurred during the interseismic period between both earthquakes and the coseismic period of 2010 Maule earthquake and 2011 Mw=7.1 Araucania earthquake. During the 1960 earthquake, vertical coseismic ground movements are typical of subduction related earthquakes with Mocha Island, located close to the trench, experienced bigger ground uplift (150 cm) than that occurred in Tirua (-20 cm), place located in the continental margin at the latitude of Mocha Island. Then during the 1960-2010 interseismic period, the 1960 coseismic uplift remained at Mocha Island unlike the normal interseismic subsidence that occurred northward at Arauco Peninsula and Santa Maria Island. Also Tirua experienced the biggest interseismic uplift (180 cm) in all the area affected later by 2010 Maule earthquake. Then during the 2010 Mw=8.8 Maule earthquake an anomalous vertical coseismic ground uplift occurred in the study area, opposite to that of 1960 since Mocha Island experienced lower (25 cm) ground uplift than Tirua (90 cm). Subsequently, during the Araucania 2011 earthquake a ground uplift in Mocha Island (50 cm) and subsidence at Tirua (20 cm) occurred. These unexpected vertical ground movements can be explained by the existence of an upper plate splay fault located below the sea bottom between Tirua and Mocha Island: the Tirua-Mocha splay fault. Considering the last seismic cycle, the activity of this fault would have started after the 1960 Valdivia earthquake. During 2010 Maule earthquake, the main slip occurred at Tirua Mocha splay fault. Finally during 2011 Araucania earthquake, the slip occurred mainly at the updip of Wadati-Benioff plane with probable normal activity of Tirua-Mocha splay fault. Simple elastic dislocation models considering the Wadati-Benioff plane and the Tirua-Mocha splay fault activity, can account for all the vertical ground movements observed during 1960 earthquake, the 1960-2010 interseismic period, the 2010 Maule earthquake and the 2011 Araucania earthquake.