Non-linear elasticity, earthquake triggerring and seasonal hydrological forcing along the Irpinia fault, Southern Italy

Abstract Pump-probe experiments investigate the strain sensitivity of crustal elastic properties, showing nonlinear variations during the strain cycle. In the laboratory, pre-seismic reductions in seismic velocity indicate that asperity contacts within the fault zone begin to fail before macroscopic frictional sliding. The recognition of such effects in natural seismic-cycles has been challenging. By exploiting seasonal hydrological strains, we used a quasi-static natural pump-probe experiment to investigate the nonlinear response of crustal rocks and its role in seismic failure along the tectonically-active Irpinia Fault System (Southern Italy). By comparing 14-years-long series of spring discharge, strain, seismic velocity variations and earthquakes rate, we find that seismicity peaks during maximum hydrological forcing and minimum seismic velocity. Seasonal strains of ~10-6 are required for both earthquakes triggering and significant nonlinearity effects arising from modulus reduction. We suggest that, for faults in a critical state, cyclical softening may lead to failure and seasonal seismicity.