Aquifer Sensitivity to Earthquakes: the 1755 Lisbon Earthquake

Abstract

The development of a theoretical analytical model leads us to consider the recession coefficient as a useful hydrological parameter for studying the hydraulic impacts of an earthquake on spring flow in terms of the increase of persistent discharges and the overall alteration of springs. Both the inertia of the aquifer emptying and the increased pore pressure in the lower part of the aquifer caused by the earthquake contribute to maintaining the discharge of persistent springs. Abundant information on the hydrological phenomena induced or modified by the 1755 Lisbon earthquake (M ∈ [8,9]) and its relationship with present‐day knowledge of the geology and hydrogeology of Portugal and Spain, as well as Spanish spring statistics, have allowed us to identify the factors that most influence the hydraulic sensitivity of aquifers to that earthquake: regional faults, geological boundaries between large geological units, the granite lithology, and aquifers with high recession coefficients. We observe an overall single response, which was an increase in postseismic spring discharges over the entire of the SW quadrant of the Iberian Peninsula (1–24 months). The persisting discharges were concentrated close to the major NE‐SW directional faults located in SW Portugal that are linked to the Azores‐Gibraltar strike‐slip fault. In terms of the geographical distribution of the hydrological phenomena, we observe that in the near and intermediate field of the earthquake source, there were persistent increases in spring flow in hard rocks occurring alongside persistent diffuse discharges, liquefaction, and rising water levels in wells due to consolidation of superficial geological formations.