Andean Backarc Deformation and the Plate Boundary Earthquake Cycle

BROOKS, B., BEVIS, M., SMALLEY, R., FOSTER, J., POLLITZ, GIAMBIAGI, L., FOLGUERA, A., BLANCO, M., CIMBARO, S., PARRA, H., BAEZ, J., SIMONS, M., LIN, Y. AND WANG, K.

Congreso; AGU Meeting; 2011

Of the 3 great earthquakes to have occurred in the GPS observational era, the proximity to Andes of the 2010 Maule event offers the most promise in relating subduction-zone forcing to active orogenic process. In particular, the backarc in this part of the Andes is unique because Quaternary fold and thrust belt shortening rates are almost negligible (less than ~ 1mm/yr, in comparison to rates of 5-10mm/yr to the north) and because there are widespread examples of Quaternary extensional faulting. In response to the Maule event, we installed more than 30 continuous GPS (CGPS) stations distributed along the rupture zone and from the Chilean forearc to the backarc thrust front in Argentina. In a South America-fixed reference frame, displacement time series are in the reverse sense from the interseismic period and they record oceanward post-seismic transient displacements of as much as 30 cm with a characteristic logarithmic decay. Forearc contractional strain rates are as much as ~ .5 micro-strains/yr whereas backarc extensional rates can be as high as 1 micro-strain/year. Whether we model the post-seismic transient as afterslip on the Maule rupture plane or visco-elastic mantle relaxation, the signal will last for at least one and more likely multiple decades, albeit at a decreasing rate. When considering that the backarc associated with the 1960 Valdivia rupture zone immediately to the South still records oceanward post-seismic displacements it becomes clear that at any given time during the ~ 200 year recurrence interval for the Maule and Valdivia events, the backarc has a high probability of being in extension. This type of strain cycling could explain the preservation of the backarc extensional structures in a predominantly contractional wedge. Furthermore it should be a general aspect of subduction-related orogeny although the importance of the extensional periods is directly proportional to the size of the subduction zone great earthquake and its related post-seismic transient deformation. We suggest that the effect is most pronounced in regions, such as the Maule&Valdivia rupture zones, where trench sediments are most abundant for two reasons: 1) in interseismic periods, stress transmission across the interface is minimized and so the stresses available for driving effect is most pronounced in regions, such as the Maule&Valdivia rupture zones, where trench sediments are most abundant for two reasons: 1) in interseismic periods, stress transmission across the interface is minimized and so the stresses available for driving backarc deformation are also minimal and, 2) sediment-filled trench ruptures appear to promote the largest earthquakes and the length of the post-seismic extensional transient.