The role of hydrothermal fluids in triggering the July–August 2000 seismic swarm at Campi Flegrei, Italy: evidence from seismological and mesostructural data

Abstract

This study presents a detailed analysis and interpretation of the seismicity that occurred on July 2–7 and August 22, 2000, during a ground uplift episode which started on March 2000 at Solfatara crater, Campi Flegrei. Earthquakes are located using a probabilistic grid-search procedure acting on a 3-D heterogeneous earth structure. The mainshock of the July swarm depicts a spectrum characterized by a few narrow peaks spanning the 1–5-Hz frequency band. For this event, we hypothesize a direct involvement of magmatic fluids in the source process. Conversely, the spectra of the August events are typical of shear failure. For these latter events, we evaluate the source properties from P- and S-wave displacement spectra. Results for the most energetic shocks (M d around 2) yield a source radius in the order of 100 m and stress drop around 10 bars, in agreement with most of the earthquakes that occurred during the 1982–1984 bradyseismic crises. For the August swarm we identify two clusters of similar earthquakes. Application of high-resolution relative location techniques to these events allows for the recognition of two parallel alignments trending NE–SW. The relationship among source dimension and relative location evidences overlapping of sources. This may be interpreted in terms of either a heterogeneous stress field or a lubrication process acting over the fault surface. For a selected subset of the August events, we also analyze the splitting of the shear waves: results are indicative of wave propagation through a densely fractured medium characterized by a distribution of cracks oriented NE–SW. The pattern of faulting suggested by relative locations and shear-wave splitting is not consistent with the surface trace of NW–SE striking faults. However, a detailed mesostructural analysis carried out over the Solfatara area indicated the occurrence of two main crack systems striking NW–SE and NE–SW. This latter system shows a strike consistent with that derived from seismic evidence. Results from a stress analysis of the crack systems indicate that a fluid overpressure within the NW–SE-striking faults is able to form NE–SW cracks. We found that the pressure of fluids P f required to activate the NW–SE faults is less than σ Hmin, while the P f value required to open the NE–SW cracks is higher than σ Hmax. Our main conclusions are: (a) the Solfatara area is affected by two orthogonal fracture systems, and the fluid pathway during the 2000 crisis mainly occurred along the NNE–SSW/NE–SW-striking crack system; (b) the July seismicity is associated to the upward migration of a pressure front triggered by an excess of fluid pressure from a small-size magmatic intrusion; conversely, the August events are associated to the brittle readjustment of the inflated system occurring along some lubricated structures.