Earthquakes and extreme rainfall induce long term permeability enhancement of volcanic island hydrogeological systems

Abstract

Earthquakes affect near-surface permeability, however temporal permeability evolution quantification is challenging due to the scarcity of observations data. Using thirteen years of groundwater level observations, we highlight clear permeability variations induced by earthquakes in an aquifer and overlaying aquitard. Dynamic stresses, above a threshold value PGV > 0.5 cm s−1, were mostly responsible for these variations. We develop a new model using earth tides responses of water levels between earthquakes. We demonstrate a clear permeability increase of the hydrogeological system, with the permeability of the aquifer increasing 20-fold and that of the aquitard 300-fold over 12 years, induced by fracture creation or fracture unclogging. In addition, we demonstrate unprecedented observations of increase in permeability due to the effect of extreme tropical deluges of rainfall and hurricanes. The water pressure increase induced by the exceptional rainfall events thus act as piston strokes strong enough to unclog congested fractures by colloids, particles or precipitates. Lastly, an analysis of regional permeabilities also highlights a permeability increase over geological timeframes (× 40 per million years), corroborating the trend observed over the last decade. This demonstrates that permeability of aquifers of andesitic volcanic islands, such as the Lesser Antilles, significantly evolve with time due to seismic activity and extreme rainfall.

Determining Stress State of Source Media with Identified Difference between Groundwater Level during Loading and Unloading Induced by Earth Tides

“…To remove data extrema from the water level sequences. For the rock media, the magnitude of increased volume result from the cracks, in terms of the studies of Brace et al [25], is no more than 2.0 times the elastic volume variations. Thus, the data points whose values exceed twice the average amplitude of the water level sequence are removed.…”

“…Traditional LURR practice is based on the calculation of the ratio between Benioff strain releases during the time periods of loading and unloading, involving the evaluation of the Coulomb failure stress change [21] induced by earth tides and a regional earthquake catalog. However, recent studies have found that the tidal responses can also be observed in the well water level [22][23][24][25]. Meanwhile, the circulation of groundwater in the crust, especially in fault zones, can reach 15~20 km, corresponding to the depth of most earthquakes [26,27].…”

Hydrogeologic and Thermal Effects of Glaciations on the Intracontinental Basins in Central and Northern Europe

“…This is especially the case in intraplate stable areas where seismicity does not seem to correlate with tectonic loading rates, but shows a rather diffusive spatial distribution (Grünthal et al, 2018 and references therein). There is growing evidence of correlations between spatio-temporal variations in rainfall and changes in seismicity rates (Hainzl et al, 2006;Vittecoq et al, 2020). Crustal thermal weakening associated to active fluid convection has also been considered as a possible causative dynamic to increased seismic activities (Acosta et al, 2018).…”

Aquifer Permeability Decreases Before Local Earthquakes Inferred From Water Level Response to Period Loading

“…Both processes can change aquifer permeability. This is the generally accepted viewpoint, especially for co‐seismic hydrologic response beyond the near field (Shi et al., 2015; Vittecoq et al., 2020).…”