Remote Triggering of Damage Followed by Healing Recorded in Groundwater Pressure

Water levels in three adjacent water wells in the Yarmouk Gorge area have all responded to the 2020 Elazığ Mw 6.8 teleseismic earthquake. Water levels in two aquifers exhibited reciprocal behavior: during the first eight days after the earthquake, water level decreased by 40 cm in the deeper highly confined aquifer, and increased by 90 cm in the shallower less confined aquifer. The recovery of the water levels in both aquifers continued for at least three months. We interpret these observations as reflecting the increase in damage along the fault at the Yarmouk Gorge. Ground shaking increased the damage and permeability of this fault, temporarily connecting the two aquifers, allowing flow from the deep aquifer to the shallow one. Model results showing decreased permeability suggest that the fault healed by one order of magnitude within three days. This is the first documentation of decrease in permeability in a fault zone within such short time scales

Water-level oscillations caused by volumetric and deviatoric dynamic strains

International audienceTravelling seismic waves and Earth tides are known to cause oscillations in well water levels due to the volumetric strain characteristics of the ground motion. Although the response of well water levels to S and Love waves has been reported, it has not yet been quantified. In this paper we describe and explain the behaviour of a closed artesian water well (Gomè 1) in response to teleseismic earthquakes. This well is located within a major fault zone and screened at a highly damaged (cracked) sandstone layer. We adopt the original Skempton approach where both volumetric and deviatoric stresses (and strains) affect pore pressure. Skempton's coefficients B and A couple the volumetric and deviatoric stresses respectively with pore pressure and BKu and N are the equivalent coupling terms to volumetric and deviatoric strains. The water level in this well responds dramatically to volumetric strain (P and Rayleigh waves) as well as to deviatoric strain (S and Love waves). This response is explained by the nonlinear elastic behaviour of the highly damaged rocks. The water level response to deviatoric strain depends on the damage in the rock; deviatoric strain loading on damaged rock results in high water level amplitudes, and no response in undamaged rock. We find high values of N= 8.5 GPa that corresponds to -0.5 < A < -0.25 expected at highly damaged rocks. We propose that the Gomè 1 well is located within fractured rocks, and therefore, dilatency is high, and the response of water pressure to deviatoric deformation is high. This analysis is supported by the agreement between the estimated compressibility of the aquifer, independently calculated from Earth tides, seismic response of the water pressure and other published data

Effective seismic wave velocities and attenuation in partially molten rocks

International audienceSignificant reduction in mechanical properties, i.e., elastic moduli and seismic wave velocities, as well as enhanced inelastic attenuation is often associated with areas of partially molten rocks. In this paper we suggest a new mechanism responsible for significant reduction of wave velocity and enhanced attenuation. The suggested mechanism considers solid-melt phase transition at thermodynamic equilibrium. Any pressure change, that takes the system out of thermodynamic equilibrium, causes solidification or melting which modifies the heat balance according to the Clausius-Clapeyron equation. The latent heat (sink or source) is transferred away or towards the interface by conductive-advective mechanism heating or cooling the entire rock mass leading to energy loss and dissipation of the mechanical energy and to seismic wave attenuation. We use simplified geometry and derive analytical solutions for wave velocity reduction and attenuation associated with a moving solid-melt interface (Stefan problem). We demonstrate that the latent heat generation due to wave-induced pressure oscillations around thermodynamic equilibrium is an efficient mechanism for energy dissipation and leads to significant reduction in mechanical properties (seismic velocities and attenuation). The highest attenuation occurs when the period of oscillation is close to the heat transfer time scale associated with the size of melt inclusions. The predicted values are approximately in agreement with large scale seismological observations, showing that seismic waves are mostly attenuated within the shallow parts of Earth's crust and mantle, and are associated with possible presence of melt

Earthquake Swarms Triggered by Groundwater Extraction Near the Dead Sea Fault

In 2013 and 2018, earthquake swarms with a maximum moment magnitude of 4.5 occurred ~5 km from the northern section of the Dead Sea Transform Fault. Here we show that aquifer pressure data, interferometric synthetic aperture radar surface deformation time series, and seismic monitoring suggest that groundwater withdrawal triggered these earthquakes. Continuous groundwater extraction from several wells located ~10 km west of the swarms has accelerated since 2010 and resulted in a total decrease of ~50 m of the groundwater level at the time of the 2018 earthquake swarm. The withdrawal also corresponds to surface subsidence of ~10 mm/year based on repeat interferometric synthetic aperture radar measurements. The temporal correlation, extensive subsidence, anomalous swarm characteristics, and normal faulting orientation suggest a connection between the groundwater withdrawal and recent earthquakes. Poroelastic modeling demonstrates that pumping-induced pore pressure decrease west of the earthquake could have caused significant dilatational stresses that led to normal faulting events outside the aquifer