Repeated fluid-transfer episodes as a mechanism for the recent dynamics of Campi Flegrei caldera (1989-2010)

摘要

We have analyzed a multiparametric data set of seismological, geodetic and geochemical data recorded at Campi Flegrei caldera since 1982. We focus here on the period 1989-2010 that followed the last bradyseismic crisis of 1982-1984. Since then, there have been at least five repeated minor episodes of ground uplift accompanied by seismicity. We have reanalyzed old paper and digital seismic data sets dating back to 1982. The paper recordings show evidence of long-period events in January 1982 and March 1989, and we have digitized some of these significant waveforms. Furthermore, the revision of digital seismograms dating back to 1994 shows a significant swarm of long-period events in August 1994. Volcano-tectonic and long-period events hypocenters have been relocated in a three-dimensional velocity model. Statistical analysis of volcano-tectonic seismicity shows many similarities and few differences between 1982-1984 and the following period 1989-2010. Long-period waveforms have been analyzed using spectral analysis, which shows a grouping into three macrofamilies. Similarities in the seismic signature of episodes of minor uplift suggest that they originate from the injection of fluids into the deep part of a geothermal reservoir (about 2.5 km depth) and in its transfer toward a shallower part (about 0.75 km depth). Most of the observed geophysical signals are related to this second phase. The evidence consists of spatial and temporal connections between the ground deformation, long-period and volcano-tectonic seismicity and changes in the geochemical parameters of fumaroles. In this study we focused our analysis on two uplift episodes observed in 2000 and 2006. The joint inversion of Differential Synthetic Aperture Radar (DInSAR) and tiltmeter data show that during these periods the ground deformation was generated by at least two distinct sources located at different depths, with the shallower activated in the later stages of the uplift episodes. Our interpretation of the recent dynamics of Campi Flegrei is that the deep part of the geothermal reservoir inflates in response to mass and heat input from a magmatic source. When the pressure exceeds a threshold, fluids starts to migrate into the shallower part. During this transfer, long-period sources are activated in response to the fluid motion. The gradual diffusion of fluids in the surrounding rocks lowers the resistance of a pervasive fracture system generating shallow microseismicity. Finally, fluids reach the surface, which gives a distinct geochemical signature to the overlying fumaroles.