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

We have analysed the earthquake sequence occurred at Campi Flegrei during a n unrest episode of strong ground uplift and seismicity, occurred in the pe riod 1982-1984. The maximum magnitude of these events was about 4. Both ear thquake occurrence and ground deformation have been interpreted in terms of the role played by a ring fault system, inward dipping, related to phenome na of caldera collapse and resurgence. Earthquakes are of mixed strike-slip and normal fault type. They show a dip movement opposite to the static gro und 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 compo nent is observed. The simulation of the stress field generated by overpress ure in a magma chamber in presence of lateral discontinuities, as performed by a boundary element method, allows to hypothesise that reverse fault sli p on the ring fault is mainly aseismic, and such aseismic movement is able to focus normal fault shear stress along the lateral discontinuities. Aseis mic slip on the ring fault in response to static deformation is also suppor ted by the low amount of seismic moment released (M-o congruent to 10(15) N m), about two orders of magnitude lower than expected from the shear slip o n the discontinuities needed to accomplish the total static surface deforma tion (1.8 m). Such results have been compared with observations at Rabaul c aldera, during a similar unrest episode. In such area, the seismic moment r elease 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 stru ctures. The comparison between the two areas shed new light about the dynam ics of earthquakes in calderas, as due to the role of bordering ring fault systems. (C) 1999 Elsevier Science Ltd. All rights reserved.