The Mw8.8 Maule, Chile earthquake of February 2010 presents an excellent opportunity to characterize the deformation of the upper plate forearc that occurs during coseismic rebound. The largest aftershocks were recorded at Pichilemu nearly two weeks after the main shock and had magnitudes of Mw6.9 and 6.7. Because they were normal faults, they contribute to coseismic extension of the upper plate and are very consistent with coseismic GPS data. However, most seismologists consider coseismic rebound to be elastic (i.e., non-permanent) and use GPS data to determine the rupture area of great earthquakes using elastic models. The Pichilemu normal fault aftershocks, however, represent permanent deformation of the Chilean forearc. This research project will address two questions: First, how and why does the upper plate deform in response to great earthquakes and, second, do upper plate discontinuities control the location of the rupture zones along convergent plate boundaries. The project will involve fieldwork to document the distribution and longevity of normal faults in the Chilean forearc overlying the Maule segment and numerical inversions of GPS data and Coulomb stress calculations of the stress changes on pre-existing geological faults in the forearc. Particular attention will be paid to a suite of NW striking fault zones that coincide with the northern limit of the Maule rupture. The role of these structures in long term seismic segmentation of the Chilean subduction zone will be determined through mechanical analysis of the structures.
The size of great earthquakes, such as the 2010 Maule, Chile and the 2011 Tohoku, Japan events, around the Pacific Ring of Fire is determined by the length of the plate margin that ruptures. Increasing evidence from Chile suggests that the same or similar segments break repeatedly. If the average length of these rupture segments were known, better predictions of the typical earthquake to be expected in any one segment could be made; these predictions could then be used as the basis for building codes and disaster preparation. This project will use the February 2010 Maule earthquake, the sixth largest on historic record, to investigate how physical weaknesses in the crust of western South America may control the length of rupture segments as well as how those weaknesses respond during major earthquakes. Because of their relatively shallow depth, secondary earthquakes on those weaknesses can be particularly destructive. The March 2010 Pichilemu aftershocks to the Maule earthquake were the largest of any associated with the main shock. At 6.7 and 6.9 magnitudes, they were as large as the Haiti earthquake and they disrupted the Chilean presidential inauguration.