Streamflow Response to the 2010 M8.8 Maule Earthquake

Stream discharge in 10 forest-covered catchments on the eastern slopes of the Coastal range in the Bío-Bío Region of Chile changed after the M8.8 Maule earthquake of 27 February 2010. Since all catchments are restricted to the very uplands, the changes in flow can only be attributed either to changes in the hydraulic head or in the permeability within the catchments. Most streams showed a decrease in discharge immediately after the earthquake followed by a gradual increase for variable amount of time. Maximum post-seismic base flow in discharge exceeded the pre-seismic flow by factor four and discharge did not return to the pre-seismic flow conditions 3 months after the earthquake, despite the lack of rainfall throughout the post-quake period. Unchanged water temperature after the earthquake suggests a near-surface source of the excess flow. Recession analysis revealed unchanged lateral permeability conditions between pre- and post-seismic conditions. Models of enhanced vertical permeability and increased hydraulic head can both describe the post-seismic increase in streamflow but have difficulty in explaining the initial decline. The initial decline in streamflow suggests the occurrence of coseismic dilantancy of the near-surface material, which led to an initial decline in water table and thus in the baseflow to the streams. Coseismic dilatancy of the near-surface material may be consistent with the model of enhanced vertical permeability but inconsistent with the model of increased hydraulic head. The basins of this study are part of an investigation performed under the Fondecyt 1070218 and with additional support from Forestry SA Mininco and the Potsdam Graduate School. Keywords: earthquake, hydrologic response, vertical permeability, dilatancy.