On tidal triggering of earthquakes at Campi Flegrei, Italy

Abstract

SUMMARY The caldera at Campi Flegrei underwent an inflationary episode during 1982–84 that produced a maximum uplift of 1.6 m at Pozzuoli, Italy. The seismicity at Pozzuoli increased enormously during the time of the uplift, but was delayed by several months. Ground deformation during inflation has been previously well modelled with a finite element model of a pressurized magma chamber in an elastic medium that takes into account the effects of increasing pressure and temperature with depth on elasticity. We used the output from this model to estimate the temporal change in the stress field that presumably controls the seismicity during inflation. The result is that the solid-earth tidal stress should modulate heavily the seismogenic inflationary stress, which in turn should result in some tidally triggered earthquakes. This expectation is based on the assumptions that: (a) the inflationary model is valid; (b) tidal and inflationary stresses can be superimposed; (c) the inflation is smooth on the time-scale of periodic tidal stress variations; and, most importantly (d) earthquakes occur when a critical level of stress has been reached. We checked the Pozzuoli catalogue for evidence of tidal triggering with the Schuster test and found none. The Schuster test is sensitive enough to easily detect a diurnal variation of reported seismicity caused by day-to-night changes in noise levels. The lack of tidal triggering suggests that one (or more) of the above assumptions is wrong. After evaluating each assumption, we conclude that the most likely explanation is that the failure threshold for seismicity is time dependent at Pozzuoli. In other words, earthquakes do not necessarily occur when the stress exceeds the yield strength of a fault for a short time only.