Deep CO2 emissions characterize many nonvolcanic, seismically active regions worldwide, and the involvement of deep CO2 in the earthquake cycle is now generally recognized. However, no long-time records of such emissions have been published, and the temporal relations between earthquake occurrence and tectonic CO2 release remain enigmatic. Here, we report a 10-year record (2009–2018) of tectonic CO2 flux in the Apennines (Italy) during intense seismicity. The gas emission correlates with the evolution of the seismic sequences: Peaks in the deep CO2 flux are observed in periods of high seismicity and decays as the energy and number of earthquakes decrease. We propose that the evolution of seismicity is modulated by the ascent of CO2 accumulated in crustal reservoirs and originating from the melting of subducted carbonates. This large-scale, continuous process of CO2 production favors the formation of overpressurized CO2-rich reservoirs potentially able to trigger earthquakes at crustal depth.

On 6 April 2009, a Mw = 6.3 earthquake occurred in the central Apennines (Italy) damaging the city of L'Aquila and the surrounding country. We relocate the October 2008 to 6 April 2009 foreshocks and about 2000 aftershocks occurred between 6 and 30 April 2009 by applying a double‐difference technique and determine the stress field from focal mechanisms. The events concentrate in the upper 15 km of the crust. Three main NW‐SE to NNW‐SSE striking, 30°–45° and 80°–90° dipping faults were activated during the seismic sequence. Among these, a normal fault and a thrust were reactivated with dip‐slip movements in response to NE‐SW extension. The structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern Apennines because of the lower strain rate of the central sector of the chain with respect to the southern one. VP/VS increases progressively from October 2008 to the 6 April 2009 main shock occurrence along a NW‐SE strike because of an increment in pore fluid pressure along the fault planes. Pore pressure diffusion controls the space‐time evolution of aftershocks. A hydraulic diffusivity of 80 m2 s−1 and a seismogenic permeability of about 10−12 m2 suggest the involvement of gas‐rich (CO2) fluids within a highly fractured medium. Suprahydrostatic, high fluid pressure (about 200 MPa at 10 km of depth) within overpressurized traps, bounded by preexisting structural and/or lithological discontinuities at the lower upper crust boundary, are required to activate the April 2009 sequence. Traps are the storage zone of CO2‐rich fluids uprising from the underlying, about 20 km deep, metasomatized mantle wedge. These traps easily occur in extensional regimes like in the axial sector of Apennines but are difficult to form in strike‐slip regimes, where subvertical faults may cross the entire crust. In the Apennines, fluids may activate faults responsible for earthquakes up to Mw = 5–6. Deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges.

Summary Scattered waves observed at the seismographs of the National Italy's seismic network have been used to investigate the intrinsic dissipation and scattering properties of the lithosphere under the Southern Apennines, Italy. First, we investigate the coda‐Q properties, then we apply the MLTW analysis in the hypothesis of velocity and scattering coefficient constant with depth, and finally we interpret these results with the aid of numerical simulations in a medium with depth dependent velocity and scattering coefficient. Results obtained in the hypothesis of a uniform model show that a low scattering‐Q−1 and a relatively higher intrinsic‐Q−1 characterize the lithosphere of the Southern Apennines. Numerical simulations of the seismogram energy envelopes were performed hypothesizing a strongly scattering crust and trasparent upper mantle, both with reasonable intrinsic dissipation coefficients. In these symplifying assumptions the theoretical curves calculated for the homogeneous model fit to the synthetic envelopes with scattering attenuation coefficients always greater than the synthetic values. This results lead to the consideration that scattering‐Q−1 obtained using MLTW analysis under the assumption of uniform medium are overestimated. The values of the scattering‐Q−1 estimated for Apennines at low frequency (1–2 Hz) in the hypothesis of uniform medium are of the same order of those obtained in several areas around the world. The estimates obtained for frequencies ranging from 2 to 12 Hz are very low if compared with those obtained in the same hypothesis for other areas around the world. Coda Q−1 closely resembles intrinsic Q−1.

S U M M A R YWe investigated the intrinsic dissipation and scattering properties of the lithosphere under the Friuli region (northeastern Italy) using two hypotheses: (i) a uniform earth model and (ii) two 'reasonable' non-uniform, layered crustal models. For case (i) we measured the coda Q, and used the multiple-lapse time window analysis (MLTWA) technique to obtain separate estimates of intrinsic absorption and scattering attenuation.Results for the uniform earth model show that the lithosphere in northeastern Italy is characterized by a low-scattering attenuation (small scattering Q-inverse, Q −1 s ), and by a relatively high intrinsic attenuation (high intrinsic Q-inverse, Q −1 i ). A comparison between the investigated region and other areas around the world shows that both Q −1 i and Q −1 s for the Friuli region are among the lowest values ever measured, with the exception of the southern Apennines, which has the lowest measured Q −1 s . For case (ii), numerical simulation of the energy envelopes was performed using two-layered earth models, where the values of the intrinsic and scattering attenuation coefficients are both within 'reasonable ranges' when compared with the geological information. The theoretical envelopes calculated for the homogeneous model give a good fit to the synthetic envelopes calculated for the layered models; the best fit is obtained for scattering attenuation coefficients of the uniform model always greater than those of the layered model. The main result is consequently that scattering Q −1 s obtained using the MLTWA under the assumption of a uniform medium is overestimated, on average, by a factor 2.Finally, coda Q −1 appears to be closer to the total Q −1 than to the intrinsic Q −1 i , as predicted by the theory.The first attempt to obtain separate estimates of the intrinsic and scattering attenuation parameters was based on the application of the radiative transfer theory to the seismic energy density (Wu 1985). This theoretical tool allows one to model the multiple-scattering processes in seismology using the equation of transfer (the equivalent of the Boltzmann equation for the kinetic theory of gases and neutron scattering). In the hypothesis of a homogeneous Earth, with constant attenuation and velocity, Fehler et al. (1992) modified Wu's technique and developed a method that integrates the energy versus distance in three time windows. This technique, called multiplelapse time window analysis (MLTWA), separately estimates the coefficient of intrinsic and scattering attenuation. The method has been widely applied to several tectonic areas (see e.g. Mayeda et ala relative comparison of the attenuation properties of areas with different tectonic histories. Hoshiba (1995, 1997) and Hoshiba et al. (2001) applied the MLTWA technique to a medium characterized by a non-uniform velocity and scattering coefficient, finding that the quantitative estimate of scattering attenuation depends strongly on the assumed velocity structure. Bianco et al. (2002) estimated the coefficients of in...

We find that the double-difference relocated seismicity, which occurred over the last 30 years at Campi Flegrei, was triggered by the uprising of fluids preferentially concentrated along onshore and offshore NW striking preexisting caldera faults. Focal volumes of the 2005-2014 seismicity do not overlap that of the 1982-1984 period, when a major uplift of 1.8 m occurred in the central sector of the caldera. This indicates a transition from an elastic to a plastic behavior due to fluid saturation and heating of the rocks in the hydrothermal reservoir. The 2012-2014 deeper earthquakes are located in a low V P ∕V S zone at the western boundary of the hydrothermal reservoir, where a volume increase from a magmatic body at 3.5 km depth has been recognized. The progressive rheological change from elastic to plastic in the upper 4 km of the crust implies that a slow upward migration of magma may not necessarily be preceded by earthquakes or swarms.