Earthquake dynamics during unrest episodes at Campi Flegrei Caldera (Italy): A comparison with Rabaul (New Guinea)

Abstract

We have analysed the earthquake sequence occurred at Campi Flegrei during an unrest episode of strong ground uplift and seismicity, occurred in the period 1982–1984. The maximum magnitude of these events was about 4. Both earthquake occurrence and ground deformation have been interpreted in terms of the role played by a ring fault system, inward dipping, related to phenomena of caldera collapse and resurgence. Earthquakes are of mixed strike-slip and normal fault type. They show a dip movement opposite to the static ground deformation. The rising of the internal block with respect to the zone external to the ring fault, as observed by ground deformation, should cause thrust fault slip on the fracture system, whereas a normal fault dip component is observed. The simulation of the stress field generated by overpressure in a magma chamber in presence of lateral discontinuities, as performed by a boundary element method, allows to hypothesise that reverse fault slip on the ring fault is mainly aseismic, and such aseismic movement is able to focus normal fault shear stress along the lateral discontinuities. Aseismic slip on the ring fault in response to static deformation is also supported by the low amount of seismic moment released (M0 ≅ 1015 Nm), about two orders of magnitude lower than expected from the shear slip on the discontinuities needed to accomplish the total static surface deformation (1.8 m). Such results have been compared with observations at Rabaul caldera, during a similar unrest episode. In such area, the seismic moment release is in good agreement with shear slip produced on a system of outward dipping ring faults, and seismicity is much more focused on the fault structures. The comparison between the two areas shed new light about the dynamics of earthquakes in calderas, as due to the role of bordering ring fault systems.