PERMANENT FOREARC STRAIN IN NORTHERN CHILE

Though forearc crustal earthquakes release only a fraction of the energy of that released by subduction mega-thrust earthquakes, the local intensity of shaking can be greater thanks to the shallower depth of the crustal events. This phenomenon was clearly shown by the Mw7.0 Pichilemu aftershock sequence following the Mw8.8 Maule, Chile earthquake in 2010. Thus, it is important to identify active forearc crustal structures and attempt to quantify their potential rupture areas and recurrent intervals. Permanent forearc deformation may be associated with subduction megathrust events or it may occur during the interseismic interval. Moment release of co/post-seismic crustal earthquakes is generally less than 1% of the main shock (~0.3% in the case of Pichilemu). Our work has focused primarily on permanent structures in northern Chile, including margin-parallel normal and reverse faults, margin-perpendicular reverse faults, and coseismic surface cracking.

Abundance active fault scarps in northern Chile attests to the ongoing permanent deformation of the forearc. Because of extremely low erosion rates in the hyperarid Atacama Desert, the offset surfaces involved in the deformation may be extremely old, exceeding 1 million years. Thus, despite the youthful appearance of the scarps, deformation rates are probably slow. Trenching of fault scarps on the Mejillones Peninsula and along the Atacama fault zone suggest recurrence intervals of ~5000 years. Assessment of the magnitude of the deformation is hampered by the fact that many margin parallel structures have been reactivated as both normal and reverse faults. Because most of the faults dip away from the mountains that they border, it appears that margin-perpendicular forearc extension dominates over shortening. Coseismic cracks also demonstrate permanent, margin orthogonal surface extension. Long term strain rates due to cracking are ~1e-15, indicating a small but finite proportion of geodetically measured coseismic extension is not elastic. Permanent margin parallel shortening is on-going between 22° and 19°S, including significant crustal activity in the region of Salar Grande. This activity was implicated in the triggering sequence of the Mw8.1 Pisagua earthquake and bears striking similarity to margin-parallel shortening in Cascadia.