Changes In Permeability And Groundwater Level In Groundwater System With Unconsolidated Sediments In Near-field Induced By Large Earthquakes

The change of hydraulic conductivity of subsurface media is the key to understand the evolution of groundwater dynamic field,chemical field and temperature field caused by the earthquake.However,most of the current studies were focused on aquifers,and there is almost no systematic research on the hydraulic conductivity change of aquifer system caused by the earthquake.Based on the field data,this paper systematically analyzed the temporal and spatial evolution characteristics of the permeability changes in the groundwater system of the loose sediments in the Canterbury Plain caused by the Darfield earthquake sequence(including the main earthquake and three aftershocks with large magnitude),and revealed the formation mechanism for the permeability changes of the groundwater system caused by these earthquakes.The results show that large earthquakes can cause the change of permeability in the groundwater system of the near field.Under the influence of the Darfield earthquake and its aftershocks,the horizontal and vertical permeability in the groundwater system where liquefaction occurred showed changes(partly enhanced,partly weakened),while the other parts of the plain without liquefaction did not show obvious changes.This heterogeneous permeability change is attributed to heterogeneous consolidation of unconsolidated sediments and is mainly dependent on the heterogeneity of the aquifer system.This paper also discusses the influence of permeability change in the groundwater system on the temporal and spatial distribution of groundwater level change in Canterbury Plain.The results show that the change of permeability further causes the permanent change of groundwater level at a plain scale,which makes the well water level in the upstream area of groundwater flow system increase permanently.The water level of most wells in the downstream area has been permanently reduced.The formation of this permanent additional head difference between the upstream and downstream is mainly caused by the reduced permeability of the groundwater system at the plain scale.In addition,there was a permanent rise in well water level in some downstream areas,which was mainly due to the destruction of the aquifer system in the downstream areas.The research results in this paper can provide the theoretical basis for the study of the hydrological phenomena and water resources changes caused by earthquakes,as well as the changes of the crustal permeability caused by earthquakes and the earthquake-induced underground fluid phenomena,and the study of the cause of triggering earthquakes.Seismic disruption to aquifer systems and temporal and spatial changes in permeability may have a significant impact on the safety of groundwater resources and underground reservoirs of pollutants.These unintended effects deserve further study.