Focal mechanisms of recent seismicity at Campi Flegrei, Italy

Abstract

The present study analysed volcano tectonic earthquakes that occurred in the caldera of Campi Flegrei, Italy from2000 to 2019 to compute source focalmechanisms.Weuseddata froma largenumber of seismic stations to estimate reliable fault plane solutions for 71 events of a maximum duration magnitude (MD) ranging between 0.5 ≤ MD ≤ 2.5. We found that most solutions were of a normal type and were rake- angle negative focal mechanisms in 86% of the cases. Only a few earthquakes occurred on normal dip-slip faults; the majority occurred on varying degrees of oblique slip. Only one event had a pure reverse- mechanism, and it was located far from the caldera centre. The spatial distribution of the computed mechanisms exhibited a remarkable mix of fault orientations without any relationship to the location area or source depth. The predominance of normal kinematics on high-angle and nearly vertical faults and the very shallow source depth exhibited good agreement with the fact that seismicity at Campi Flegrei occurred during periods of ground uplift. In fact, ground uplift elongated the shallowest crust, thus reducing the normal stress on existing locked faults; this facilitated earthquakes, particularly those characterised by normal, dip-slip, and strike-slip mechanisms. In contrast, ground subsidence produced a horizontal shortening of the upper crust, which increased the normal stress on high-angle faults and reduced earthquake occurrence. Our results indicate that the driving force of the recent volcano-tectonic seismicity in the study region is the ongoing ground uplift.