Search

Earthquake-triggered volcanic activity promoted by dynamic and static stresses are considered rare and difficult-to-capture geological processes. Calderas are ideal natural laboratories to investigate earthquake–volcano interactions due to their sensitivity to incoming seismic energy. The Campi Flegrei caldera, Italy, is one of the most monitored volcanic systems worldwide. We compare ground elevation time series at Campi Flegrei with earthquake catalogues showing that uplift events at Campi Flegrei are associated with large regional earthquakes. Such association is supported by (yet non-definitive) binomial tests. Over a 70-year time window we identify 14 uplift events, 12 of them were preceded by an earthquake, and for 8 of them the earthquake-to-uplift timespan ranges from immediate responses to 1.2 yr. Such variability in the response delay may be due to the preparedness of the system with faster responses probably occurring in periods during which the Campi Flegrei system was already in a critical state. To investigate the process that may be responsible for the proposed association we simulate the propagation of elastic waves and show that passing body waves impose high dynamic strains at the roof of the magmatic reservoir of the Campi Flegrei at about 7 km depth. This may promote a short-lived embrittlement of the magma reservoir's carapace otherwise marked by a ductile behaviour. Such failure allows magma and exsolved volatiles to be released from the magmatic reservoir. The fluids, namely exsolved volatiles and/or melts, ascend through a nominally plastic zone above the magmatic reservoir. This mechanism and the associated inherent uncertainties require further investigations but the new concept already implies that geological processes triggered by passing seismic waves may become apparent several months after passage of the seismic waves. [ABSTRACT FROM AUTHOR]

We report new measurements of the seismic properties of an upper amphibolite facies metapelite from the Serie dei Laghi basement (southern Alps, northern Italy), presenting evidence for the α-ß transition using elastic wave velocity methods and under in-situ conditions. Measurements were made using an internally heated gas apparatus under hydrostatic conditions to 500 MPa and temperatures to 1075 K. P-wave and S-wave velocities were measured parallel and normal to both lineation and foliation at room temperature. An average Vp (at 400 MPa) of 6.4 km/s and Vp anisotropy of 15% were found, and an average Vs1 of 3.6 km/s (8.2% anisotropy) and Vs2 of 3.5 km/s (4.1 % anisotropy) were also measured. To observe the effects of the α-ß quartz transition under hydrostatic conditions, elastic wave velocity was monitored continuously as the sample was heated to 1075 K. At 400 MPa, V was seen to decrease monotonically with increasing temperature to 950 K; as temperature continued to increase, V rapidly increased, until 1075 K. The effect was found to be reversible and is interpreted in terms of the α-ß transition in quartz, in broad agreement with previous studies. A simple interpretation of the data, taking into account a field case from southern Tuscany as well as natural and elevated geothermal gradient, suggests that thermal and hydraulically driven fracture is a likely cause of "bright spots" seen in these areas. With knowledge of the depth of such features, the transition additionally allows the temperature at depth to be estimated. [ABSTRACT FROM AUTHOR]

We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method. [ABSTRACT FROM AUTHOR]

We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to- PP reflection amplitude ratios. The velocity structure above the reflector is constrained by traveltime modelling, and the amplitude ratios are determined using the same source–receiver pair for the measured PP and PS amplitudes (common-offset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray theoretical forward modelling of theoretical PS-to- PP ratios. A minimization of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest ( P velocity, P-to- S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 to 3.5 km s−1 and a decrease of the P-to- S velocity ratio from 3.6 to 1.75. The proposed PS-to- PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known. [ABSTRACT FROM AUTHOR]

We estimated the attenuation laws of high-frequency seismic waves in the shallow crust (depth ≤5 km) and earthquake source parameters by using a selected data set of 320 shallow events (2.6 ≤ MD ≤ 4.2), recorded at Mt. Etna volcano during the last two flank eruptions occurring in 2001 and 2002-2003. The quality factor (Q) was estimated from spectra of P and S waves for 24 stations of the local permanent network by applying a spectral ratio technique. The results show variations in both QP and QS as a function of frequency, according to the power law Q = Q0ƒn, with n ranging between 0.3 and 1.3 for P waves and between 0.2 and 0.9 for S waves. As typical of volcanic environments, strong azimuthal variations of QP were also found, suggesting the presence of local strong lateral heterogeneities and/or of fluid-filled cracked volumes. After correction for attenuation, we estimated the source parameters (seismic moment, source radius, and stress drop) of a subset of 66 shallow events, under the assumption of a circular dislocation. The estimated seismic moments M0 range from 1013 to 1015 N m. The source radii (r) are confined between 100 and 1000 m and stress drop (Δσ) ranges between 0.2 MPa and about 4 MPa. Combining the source parameters obtained in this study with those calculated by Patanè et al. (1997) for an old data set of smaller microearthquakes (109 ≤ M0 ≤ 1014 N m) recorded in the same area, we re-evaluated the scaling relationship between seismic moment (M0) and corner frequency (ƒc) for the earthquakes with M0 ranging between 1013 and 1015 N m. We confirm that microearthquakes at Mt. Etna seem not to obey a M0 α ƒc-3 scaling relationship, as generally observed for moderate to large earthquakes, as the slope of the scale dependence is about 4.3 or higher. Assuming that this dependence is real, within the uncertainty in the results, a departure from the self-similarity exists for the volcanotectonic earthquakes at Mt. Etna. [ABSTRACT FROM AUTHOR]

Recently it was shown, studying data acquired with in-situ measurements on the Gran Sasso mountain (Italy), for about ten years, by means of a high sensitivity transducer coupled to the free-end section of a stainless steel rod fixed by cement in a rock-drill hole 10 m high, about 2500 m above sea level, that Acoustic Emission (AE) can be affected by more than 90% False Acoustic Emission (FAE) of an electromagnetic origin. A very simple method to solve the problem of the discrimination between AE events due to elastic waves, from FAE signals, due to electromagnetic noise, both coming from the same "reception-point," is presented. The reliability of the obtained separation is confirmed also by the reported amplitude and time distribution of AE events, typical of fracture dynamics and those of FAE events, similar to those of noise. [ABSTRACT FROM AUTHOR]

Static and dynamic elastic moduli of Calcare Massiccio mudstone-limestone, the typical seismogenic rock in the Italian Apennines, are measured using a standard uniaxial static compression test, a dual cantilever forced oscillation test and ultrasonic measurement of elastic wave velocities. These measurements cover nine decades in frequency including the seismic domain. Neither a significant frequency dependence nor a pronounced strain amplitude dependence was observed, providing a Young's modulus of and a Poisson's ratio of . These values are characteristic of Calcare Massiccio in undamaged condition. [ABSTRACT FROM AUTHOR]