R. de Franco

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.

Crustal shortening and duplication of the Moho in the Northern Apennines: a view from seismic refraction data

Abstract A reappraisal of the DSS refraction seismic campaigns of 1978 and 1974 in the Northern Apennines and the northern Tyrrhenian Sea, after digitization of the original analog data, and a new interpretation of crustal structures are presented in this paper. The layering of the Adria crust consists of a low-velocity lower crust topped by a 6.7 km/s horizon and an upper crust again formed by a low-velocity layer capped by a faster one. In the Tuscany sector the crust-mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the Adriatic counterpart. These differences are due to the extensional tectonics, restricted to Tuscany and occurred in a high heat flow regime related to an uplifting astenosphere. It has been confirmed that a deep thrust is responsible for Moho doubling in correspondence with the zero-Bouguer anomaly line: the Tuscan Moho overlaps the Umbro-Marchean one for a length of about 30 km. It is hypothesized that another shallower thrust involves upper crust in the Mid-Tuscany Range, Mt. Cetona and Perugia Massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks. No conclusive proof of the existence of a European Moho below a Tuscan one in the area underneath Elba has yet been found. Revision of DSS data excludes the presence of a refractor-reflector 60 km deep; the same data, however, are compatible with the existence of a crust-mantle boundary at 30–35 km depth.

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.

Seismic structure of the Central Tyrrhenian basin: Geophysical constraints on the nature of the main crustal domains

In this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back-arc basin combining refraction and wide-angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterraneo OCcidental)-2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a ~450 km long 2-D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time-migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity-derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin, extension has thinned the crust from ~20 km under the coastline to ~13 km ~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back-arc oceanic settings. The velocities-depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10–20 km wide, relatively low-velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

Seismotectonic study of the Friuli (Italy) area based on tomographic inversion and geophysical data

Abstract The upper crustal structure of the Friuli area involved in the seismic sequence which started on May 6, 1976 (main shock M L = 6.4) is investigated, together with the distribution of seismicity, in order to define the main Seismotectonic features. Three-dimensional P-velocity modelling is obtained by tomographic techniques using local earthquakes recorded by the seismic network of the Osservatorio Geofisico Sperimentale. Lateral velocity variations and velocity inversions are related to structural heterogeneities. The shortened and thrusted sedimentary cover is detached from the basement; the 3-D velocity structure shows a southward verging, thrusted wedge basement below 5 km depth. Analysis of the distribution of earthquakes, together with three-dimensional velocity modelling, shows that active faulting is represented by north-dipping thrusts uplifting the basement wedge. This high-velocity block forms the main seismogenic zone.

Velocity structure of the Vulsinian Volcanic Complex (Latium, Italy) from seismic refraction data and three‐dimensional inversion of travel times

We have interpreted three reversed seismic refraction profiles and three related fans in the Monti Vulsini Volcanic Complex, modelling travel times and amplitudes calculated using the asymptotic ray theory. The interpretation of the refraction lines revealed complex structure in the shallow crust (0–7 km), characterized by strong lateral heterogeneities. The three-layer seismic model of the volcanic area is characterized by relatively high P wave velocities, generally ranging from 4.0–4.5 km/s in the uppermost layer to 6.7–7.1 km/s at a depth of about 7 km. The upper layer (0.5–2.7 km thick) corresponds to the volcanic cover and the upper part of the flysch sequence. The depth to the top of the middle layer, corresponding to the lower part of the flysch unit and to the Meso-Cenozoic carbonate sequence, is very irregular and is strongly controlled by the tectonic evolution of the area. The thickness of this layer ranges between 2.2 and 5.0 km. The third layer extends beneath the whole Vulsinian region and is characterized by high P velocity (6.7–7.1 km/s) at relatively shallow depth (5–7 km). We interpret the high velocity observed in the third layer as being caused by mafic intrusive rocks, such as gabbros, or by high-grade metamorphic rocks, such as schists, granulates, or metatuffs. The results of the refraction modelling have been compared with a three-dimensional (3-D) P velocity model calculated by inverting travel time residuals from explosions and local earthquakes. Generally, close agreement is observed between refraction and inversion models. The fan profiles, as well as three unreversed refraction profiles and 3-D inversion, showed a velocity decrease, as low as 5%, within the deepest layer in the central part of the volcanic complex. The low-velocity zone in the central region could be related either to high temperatures and/or partial melt in the intrusive (or metamorphic) body, or to the deepening of the top of the third layer.

Application of pattern recognition techniques to long-term earthquake prediction in central Costa Rica

Abstract The expert system GEO 2.5 has been applied to the analysis of geophysical and geological data from Costa Rica. Geophysical data consist of seismic tomography, Bouguer anomalies, seismic catalogue (RSN-ICE, 1984–1993) and seismic historical catalogue. Among the geological data, faults, lineaments, topography and a structural zonation of the region concerned have been used. Many other features have been obtained through the calculation of statistical functions of the original data such as nearness, density, length and b -value, other functions were secondary derivatives and modulus of horizontal gradient. A first analysis, mainly based on seismic tomography data and seismic catalogue, has led to the identification of a few seismogenic structures in the upper crust characterized by a high statistical correlation coefficient between seismic activity and positive velocity anomalies. Pattern recognition techniques have been applied to the long-term earthquake prediction by the definition, in the feature space, of non-linear regression functions between geophysical/geological data and a preliminary estimate of expected maximum magnitude. This preliminary estimate has been formulated on the basis of both the historical seismic catalogue and the structural zonation. This approach gives the empirical relationships among different geophysical and geological features, which are potentially related to the phenomenon of stress release. Due to the high non-uniqueness of such an approach, the results have to be physically understandable and each function has to be interpreted. The obtained multi-dimensional function has been applied to the calculation of a forecast maximum magnitude field for central Costa Rica. The results are to be considered preliminary. An improvement of such a forecast could be achieved by new data, such as heat flow, depth of Mohorovicic surface, active faults, vertical movement velocities, etc.

Reinterpretation of a wide angle reflection “fan” across the Central Alps

Abstract Within the framework of a new phase of crustal exploration in the Alpine range (by vertical reflection seismics) all existing seismic data (refraction-wide-angle reflection profiles and fans) have been reprocessed and reinterpreted. This study discusses the results of the reinterpretation of the fan ZE (recorded in 1986), which crosses the Central Alps east of the European Geotraverse (EGT). The geophones (3 components) were positioned 130 km east of the shot-point C (on the EGT); therefore, the “mirror” of the supercritical reflections from the Moho is located approximately to the north of the Iseo lake. Reprocessing included a spectral analysis (on noise, P m P and S m S signals), a polarization analysis (on 3-component reoriented recordings), and a reflectivity analysis (on the vertical component). A line-drawing was prepared and finally the depth of the correlated reflection segments was calculated using average velocities constrained mainly by the Sudalp 77 profile, which intersects the fan ZE. The analysis shows that the Penninic crust is more reflective than the South-alpine crust and that the transparency of the lower crust increases where the total thickness is greater, here, the frequency of the signal reflected from the Moho also seems to increase. The geometry shown in the depth cross-section is discussed also considering the resolution power of the fan. Furthermore, like in the Western Alps as well as along the EGT, the asymmetry of the lower crust clearly indicates that a trace of lithospheric subduction still exists in the present configuration.

Imaging and Tomographic Interpretations of Seismic Refraction Data for Near Surface Basement Assessment in Somaliland

We present a comparison between the results of the processing of seismic profiles acquired in Somaliland, near the town of Boroma, in the frame of an UNESCO project for groundwater research. The main objective of the work was to support the siting of drilling water wells. The aim of geophysical survey was the identification of the geophysical parameters of the basement and overlying limestone unit. The data processing was performed with a classical approach based on the tomographic inversion of P first arrivals. Whereas the picking of first arrival times is sometimes difficult, we present the application of the multi refractor imaging technique (MRI) based on the stack of refraction convolution sections of the refracted signals. The results show that MRI is a suitable technique to processing and interpret the refracted data and it allows to obtain an accurate image of refracting interfaces. Further it complements and integrates the interpretation of the results obtained with tomographic approach also in presence of complex structures.