ASSESSING THE ROLE OF SEISMIC PUMPING IN THE FORMATION OF CO-SEISMIC SOIL CRACKS, ATACAMA DESERT, CHILE

The Atacama Desert is one of the driest and most stable landscape surfaces on earth. Consequently, soils in the Atacama are generally thick (~2-5m) and cemented with soluble salts including sulfates, chlorides, and nitrates. As the Atacama Desert is adjacent to the Nazca-South America plate boundary, this region is also tectonically active. Evidence of past earthquakes are preserved in brittle soils as co-seismic surface cracks. These cracks are typically 5 to 50 cm wide and are filled with vertically bedded eolian silt, surface clasts, and soluble salts. Recent copper exploration studies in the Atacama have identified high concentrations of copper in soils adjacent to soil cracks overlying copper ore bodies, suggesting that salts and trace elements in and around co-seismic cracks may originate from groundwater pumped to the surface during seismic events. These results are intriguing in that they suggest surficial soil chemistry may be used to identify subsurface copper deposits, and that seimic pumping may play a role in the formation of co-seismic cracks in the Atacama.

To test the potential role of seismic pumping in both the formation of co-seismic surface cracks and the precipitation of salts within crack fill material, we analyzed the δ³⁴S isotopic values of soil, surface crack fill material, and evaporites deposited by groundwater at three sites in the Coastal Cordillera around Salar Grande in the northern Atacama Desert. Soils in this region are strongly influenced by marine aerosols and soil salts generally have δ³⁴S isotopic values >10‰. Goundwater in this region, however, is sourced from precipitation along the western flank of the Andes. Evaporites derived from this water have δ³⁴S isotopic values of ~5‰. Sulfate δ³⁴S vales of the co-seismic crack fill material at one locality had isotopic values of between 11.2 and 12.2‰, similar to the δ³⁴S values of the surrounding soil. At the two other locations, however, δ³⁴S values of co-seismic crack fill material were mostly between 5.8 and 9.4‰, whereas soil δ³⁴S values were between 11.5 and 13.7‰, suggesting that salts precipitated in co-seismic cracks were in part derived by groundwater pumped to the surface during seismic events. Therefore, seismic pumping of groundwater does appear to be an active process in some locations of the Atacama Desert.