Paolo Capuano

Seismic Evidence for a Low-Velocity Zone in the Upper Crust Beneath Mount Vesuvius

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower (P-wave velocity VP ∼ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a P to S phase converted at a depth of about 10 kilometers at the top of a low-velocity zone (VP < 3 kilometers per second), which might represent a melting zone.

Three‐dimensional seismic tomography from P wave and S wave microearthquake travel times and rock physics characterization of the Campi Flegrei Caldera

[1] The Campi Flegrei (CF) Caldera experiences dramatic ground deformations unsurpassed anywhere in the world. The source responsible for this phenomenon is still debated. With the aim of exploring the structure of the caldera as well as the role of hydrothermal fluids on velocity changes, a multidisciplinary approach dealing with three-dimensional delay time tomography and rock physics characterization has been followed. Selected seismic data were modeled by using a tomographic method based on an accurate finite difference travel time computation which simultaneously inverts P wave and S wave first-arrival times for both velocity model parameters and hypocenter locations. The retrieved P wave and S wave velocity images as well as the deduced V p /V s images were interpreted by using experimental measurements of rock physical properties on CF samples to take into account steam/water phase transition mechanisms affecting P wave and S wave velocities. Also, modeling of petrophysical properties for site-relevant rocks constrains the role of overpressured fluids on velocity. A flat and low V p /V s anomaly lies at 4 km depth under the city of Pozzuoli. Earthquakes are located at the top of this anomaly. This anomaly implies the presence of fractured overpressured gas-bearing formations and excludes the presence of melted rocks. At shallow depth, a high V p /V s anomaly located at 1 km suggests the presence of rocks containing fluids in the liquid phase. Finally, maps of the V p *V s product show a high V p * V s horseshoe-shaped anomaly located at 2 km depth. It is consistent with gravity data and well data and might constitute the on-land remainder of the caldera rim, detected below sea level by tomography using active source seismic data.

An image of Mt. Vesuvius obtained by 2D seismic tomography

Abstract A high-resolution seismic tomography of Mt.Vesuvius was started in May 1994, with the aim of reconstructing the detailed shallow crustal structure underneath the volcano and define its feeding system. The first phase of the experiment was to perform a 2D profile, using three underground explosions as active sources. Data from controlled sources and microearthquakes were jointly used to determine the shallow structure of the volcano. A high-velocity body (Vp=3.5–4 km/s) was identified at about 2 km beneath the Somma-Caldera. It is likely to represent a sub-volcanic structure, formed by a dense network of solidified dikes. A prominent converted P-to-S phase at about 10 km of depth indicates the occurrence of a sharp transition to a very low-velocity zone. This may represent the top of an extended magmatic reservoir.

Space and Time Behavior of Seismic Activity at Mt. Vesuvius Volcano, Southern Italy

We analyze the space and time behavior of seismicity at Mt. Vesuvius during the last 20 yr to characterize the seismic regime of the volcano during the present quiescent period. The new results on the volcano structure inferred from active seismic tomography experiments, the newly implemented 10-yr arrival time catalog, and a high-quality digital waveform data set have been analyzed. The background seismicity is concentrated near and beneath the Mt. Vesuvius crater, with depths lying above and below the discontinuity, which marks the transition from the shallow alluvium/volcanic sediments and the Mesozoic carbonate basement. The focal mechanisms of microearthquakes show variable stress-axis orientations as a function of depth, although there is evidence for a clustering around roughly the north–south to vertical directions for the tension axes and east-southeast–west-northwest to vertical directions for the pressure axes. The statistical analysis of the seismic catalog confirmed the tendency of background seismicity to cluster in time, according to a trigger model (as denoted by Vere-Jones and Davies, 1966), that is, the generalized Poisson process. A significant increase of the average seismic energy release with time is observed, which is related to the occurrence of several M D > 3 events in the past 10 yr, accompanied by intense swarm activity. This is consistent with the decrease of the b -value from about 2 to 1 during the same period. The ( M D > 3) events are located in the same area and depth range of the whole seismicity, and their fault-plane solutions also show variable stress-axis and nodal-plane orientations. In particular, the moment tensor inversion of P and S waveforms from the largest earthquake in the catalog ( M D 3.6, on 9 October 1999) shows no significant departure from a pure shear, double-couple mechanism, thus suggesting a dominant tectonic-like fracture mechanism. The decrease of parameter b with time is interpreted as a dominant effect of fluid pressure variations on the present seismic regime at Mt. Vesuvius, which could be driven by the progressive cooling of the volcanic system.

An integrated geophysical investigation of the upper crust in the epicentral area of the 1980, Ms=6.9, Irpinia earthquake (Southern Italy)

Abstract In this paper, we investigate the upper crustal structure of the Irpinia region, Southern Apennines thrust belt, Italy, through analysis and joint interpretation of gravity data, seismic reflection lines and subsurface information from many deep wells. The investigated region includes the epicentral area of the 1980 (Ms=6.9) Irpinia earthquake and is one of the Italian regions with the highest seismic hazard. The upper crustal structure is imaged by modeling a series of 15 SW-trending gravity profiles, spaced about 5 km apart, plentifully constrained by seismic reflection lines and wells, thus reducing the inherent ambiguity of the gravity modeling. Despite of the complexity of the modeled Bouguer anomalies, the application of a calibrating procedure to constrain the range of variability of the density values, as well as the use of geometric constraints, results in a good level of stability in the final density cross-sections, which in fact appear coherent both in the density values and in the geometrical features. The inferred model shows important lateral density variations that can be mostly related to NW-trending geologic structures. High-density bodies delineate carbonate platform thrust sheets and broad antiforms involving Mesozoic basinal rocks, while low-density shallow bodies are associated with Pliocene basins. In addition, important density (i.e. lithological) variations are evident along the strike of the range, the most relevant being an abrupt deepening of the Apulia Carbonate Platform in the southeastern part of the investigated region. In the epicentral region of the 1980 event, we find that the geometry of the high-density, high-velocity carbonates of the Apulia Platform appears correlated with the distribution of the aftershocks and with the P-wave velocity anomaly pattern as inferred from a previous local earthquake tomography. The structural highs of the Apulia Platform correspond to high-velocity regions, where aftershocks and coseismic slip of the mainshock are concentrated. This correlation suggests that the Apulia Carbonate Platform geometry played an important role in the rupture propagation and in the aftershock distribution.

Seismicity at Somma-Vesuvius and its implications for the 3D tomography of the volcano

Abstract A research project to define the feeding system of the Vesuvius volcano and the upper crustal structure of the area was started in 1993. The core of the project is the high resolution seismic tomography study by using explosive sources. Results of a preliminary 2D seismic profile have been discussed by Zollo et al. (1998) [J. Volcanol. Geotherm. Res., this volume]. The study of local seismicity at Vesuvius has many implications for the determination of the substructure of the volcanic area. The shape and size of the seismic volume and the study of the focal mechanisms put important constraints on the stress field of the area. The local seismicity can greatly improve the knowledge of the tridimensional velocity distribution of both P and S seismic waves inside the volcano, using passive tomographic techniques. In this paper we obtain a first three-dimensional tomographic image of the volcanic structure, obtained by inversion of first P and S arrival times of local earthquakes. The results show the existence of a sharp velocity contrast, along a lineation oriented NW–SE, cutting the crater along the line separating the relict of the Somma caldera from the southwest part of the volcano. This study confirms, also for S wave velocities, the absence of indications for a magma chamber in the first 4–5 Km below the sea level, as already evidenced, just for P waves, by 2D tomography. Furthermore, we discuss the improvement of resolution obtainable by deploying a small set of three component seismic stations to get a better coverage of the area.

Rupture mechanism and source parameters of Umbria-Marche mainshocks from strong motion data

A long sequence of earthquakes causing few casualties and considerable damage in a wide zone struck Central Italy starting on September 26, 1997. Theearthquakes are characterized by normal faulting mechanism, with a NE-SW(anti-Apenninic direction) tension axis. In this paper we analyze the accelerometric recordings collected by the accelerograph stations belonging to the National Accelerograph Network. About 10 stations were triggered by the mainshocks of the sequence. In particular, a small size foreshock and the two mainshocks that occurred on September,26 (00:33(GMT) MW = 5.7 and 09:40 MW = 6.0) have been recorded by two digital 3-C accelerometers located at near source distances (within 30 km from the faults). These records are relevant to investigate the detail of therupture kinematics, due to the close epicentral distance and azimuthallocation relative to the fault orientation and geometry. Using a trial and error approach we modeled the source mechanism through the fit of the arrival times, the apparent source time duration, the main polarization features and the entire waveforms of the recorded signals, in order to get some insight on the rupture evolution, the location of the fracture origin point and the fault geometry. Based on this fault kinematic model, inferences on fault slip distribution are obtained by modeling the S acceleration waveform, comparing the ray theory synthetics with 1–5 Hz band filtered ground velocity records.The final model shows that the seismic ruptures occurred along two adjacent,sub-parallel, low angle dipping normal faults. Ruptures bothnucleated from the fault bottom and propagated up-dip, showing differentrupture velocity and length. The presence of a transfer zone (barrier)can be suggested by the mainshocks rupture evolution. This transfer zonehas probably controlled the amplitude increase of local stressreleased by the first rupture at its NW edge which triggered about 9 hourslater the second rupture. The inferred model was used to compute the predictedground acceleration in the near source range, using a hybridstatistical-deterministic approach.A similar trial and error method has been also applied to the October 14, 199715:23 earthquake (MW = 5.6). The inferred kinematic model indicates a rupture nucleating from the faultbottom and propagating up-dip, toward the SE direction. Thus the three mainshocks ruptured distinct fault segments, adjacent and slightly offsetfrom one to another.

The 1997 Umbria‐Marche (central Italy) Earthquake Sequence: Insights on the mainshock ruptures from near source strong motion records

A small size foreshock and the two mainshocks of the Umbria Marche earthquake sequence which occurred on September 26, 1997 have been recorded by two digital 3C accelerometers located at near source distances. The close epicentral distance and azimuthal location relative to the fault orientation and geometry make these records relevant to look at the detail of the rupture kinematics. S-wave polarizations, apparent source time duration and waveforms from strong motion records are used to constrain the location of the fracture origin point, the fault geometry, the final slip distribution, size and mechanism of the events. The final model shows that the seismic ruptures occurred along two adjacent, sub-parallel, low angle dipping normal faults. The relative timing, location and geometry of the mainshock faults suggest the presence of a transfer zone (barrier) which has probably controlled the amplitude increase of local stress released by the first rupture at its NW edge which triggered about 9 hours later the second rupture.

Internal stress field at Mount Vesuvius: A model for background seismicity at a central volcano

We propose a model to explain the background seismicity occurring at Somma-Vesuvins in its present, mostly quiescent period. A finite element procedure has been used to simulate the stress field due to gravitational body forces in an axisymmetric volcano characterized by a central high-rigidity anomaly. Results emphasize the important effect of axial high-rigidity, which concentrates at its borders stresses resulting from the gravitational load of the volcanic edifice, as well as external (regional) stresses. The joint effect of the gravitational loading and of the presence of the anomaly produces stresses very close to or above the critical rupture threshold. The observed spatial concentrations of seismicity and moment. release correlate well with peaks of computed maximum shear stress. Seismicity is then interpreted as due to small stress perturbations concentrated around the high-rigidity core and added to a system already close to the failure threshold. This model can explain the widely observed occurrence of background seismicity at central volcanoes worldwide.

Source parameters of earthquakes in the Strait of Messina, Italy, during this century

Abstract Basic source parameters of earthquakes that have occurred in the seismogenic source region of the Strait of Messina, southern Italy, are analyzed from seismological data and vertical ground displacements. In particular faulting parameters associated with the 1908 Ms = 7.1 Messina earthquake have been analyzed through inversion of coseismic displacements and available seismograms. This normal faulting earthquake was located in the upper 15 km of the crust, and had a source dislocation of 1.5 m on a fault plane striking N-S, and dipping 40° E. This seismogenic region was quiescent before 1908 and later on this century, but its surroundings, i.e. the southern segment of Calabrian arc, is one of the most seismically active regions of Italy, with sequences of large earthquakes (M = 7–7.5) in 1638, 1693, 1783 and 1905, probably also associated with normal faulting, as suggested by the regional tectonics. During this century in the Strait of Messina small earthquakes occurred in 1952 (ML = 4.5), 1961 (ML = 4.7), 1975 (ML = 4.7) and in 1985 (ML = 4.1), this last being the first documented swarm sequence. Hypocenter distribution, fault-plane solutions, seismic moments and stress drops have been computed from the data of a local seismic network consisting of up to eight stations. The scaling law of the swarm events seems to show a departure from self-similarity, with earthquakes having a constant source radius of about 300 m.