SEISMIC FOLDING IN COASTAL SOUTH CENTRAL CHILE

The great (M(w)=9.3) Valdivia earthquake of May 21-22, 1960, was assoc iated with ground deformation over a distance of over 800 km parallel to the Peru-Chile trench. Downwarping by as much as 2.3 m affected a b elt averaging a width of 100 km which was bordered by a major upwarped zone in the west, where uplift locally attained 5.7 m, and a minor up warped zone in the east. The deformation has been explained by thin vi scous sheet models or by dislocation models involving predominantly di p-slip movement on thrust faults. We show that it can be modeled by el astic buckling of the overriding South American plate without the need to invoke specific fault geometries. The proposed buckling mechanism accounts for both zones of upwarping and for the exceptionally rapid u plift experienced by Mocha Island to the west. Moreover, elastic relax ation provides a simple explanation for the postseismic recovery repor ted by observers at several sites. It is also consistent with the resu lts obtained by C-14 dating of fossil shorelines within and outside th e area affected by the 1960 events, which show that central and northe rn Chile has undergone little uplift during the last 30,000 years desp ite convergence between the South American and Nazca plates at over 90 mm/yr, although the presence of numerous older shorelines shows that coastal uplift was very active earlier in the Cenozoic.