Stress transfer and seismic instabilities in the upper crust: example of the western Pyrenees

Abstract

When the regional state of stress is close to the condition for earthquakes to occur, small stress and strain changes could trigger seismic instabilities. Stress diffusion ( Δσ = 1–2 MPa) has been suggested to explain, at lithospheric scale, space and time correlated earthquakes and surface deformations in subduction zones (100–200 km, 10–40 years). We test the possible regional mechanisms, at the upper crustal scale, in the western Pyrenees, where major regional events (M max = 5.7), locally induced events and subsurface fluid pressure fluctuations are temporally and spatially correlated. In a first phase, fluid withdrawal (decrease of pore pressure of the aquifer at the gas-water interface) affects the natural regional seismicity rate. The locked fault inhibits small events for a time, during which tectonic stresses accumulate. Major regional events then occur on the Pyrenean Fault, when the energy of deformation is large enough to allow seismic instabilities to occur. These major events (M 1 > 5) trigger seismic instabilities (M 1 = 4), in the vicinity of the gas extraction area, 30 km away. As a result of gas extraction, the Lacq field zone has local weakness characteristics similar to the neighbouring main active fault. The Pyrenean earthquakes lead the gas field events by 2 years. This correlation is consistent with a viscoelastic model, where salt rock acts as a viscoelastic channel, below an elastic bed. The possible connections proposed here may give a new insight worldwide in understanding the role of fluids in seismic instabilities and seismic risk assessment in the neighbouring of man-made fluid pressure fluctuations, when natural seismic activity is high.