Recovery estimates for the Río Cruces after the May 1960 Chilean earthquake

Abstract

The May 22nd 1960 Chilean earthquake was the largest earthquake on record with a magnitude 9.5. The event caused extensive subsidence and uplift over wide areas (at least 130,000 km 2) with uplift on the offshore South American Plate margin and downwarping in the coastal and landward areas which ranged in magnitude from 1–2 m. The subsidence deepened coastal rivers creating extensive shallow banks (now wetlands) in many rivers including the Río Cruces. The amount of subsidence was well-documented with field measurements recorded soon after the event. This study using sediment accumulation histories predicts the recovery time of the Río Cruces to pre-1960 conditions, an important factor for assessing the fidelity of seismic recorders in the geological record. Sediment cores ( n = 139) from the subtidal areas of the Río Cruces were lithologically logged and selectively analyzed using high-resolution particle-size distribution (PSD) plots, microfossil analysis (thecamoebians) and 210Pb, 137Cs dating, documenting the post-1960 thickness of sediment. The analysis differentiated four main facies which include: 1) Floodplain Soil Horizon, 2) Tsunami or Flood Unit, 3) Post-Earthquake Adjustment, and 4) River Sedimentation. The post-1960 river sediment is clearly demarcated in the cores by highly-peaked PSDs (4–5 phi), abundant thecamoebian tests and high diversities compared to the underlying facies. Species compositions are typical of lacustrine or slow-moving river systems ( Cucurbitella tricuspis, Difflugia oblonga, Centropyxis constricta, Centropyxis aculeata, Lagenodifflugia vas, and Difflugia protaeiformis). Based on this core data, the shallow banks along the Río Cruces will likely shoal in less than 100 yrs (< 50 yrs from present). Between 1960 and 2008 the water depths of the shallow banks were reduced by more than half, to an average water depth of less than 1 m due to 60 to 80 cm (1.25–1.68 cm/yr) of sedimentation over that period. Time estimates for recovery to a floodplain are estimated to be in the 100–200 yr range due to uncertainty in compaction rates of the organic rich sediment. These estimated recovery times in a microtidal setting (< 1 m) are longer than those measured in the macrotidal (∼ 9 m) setting of Portage, Alaska (10–20 yrs) and more similar to the estimated rates of 100–150 yrs for the mesotidal (∼ 3 m) Willapa Bay, Washington at the Cascadia subduction zone. Based on these estimates, the submerged mid-lower reaches of the Río Cruces are at the limit of being a good seismic recorder and it is possible that this setting would under represent large events if they had recurrence intervals less than several centuries.