Giovanni Iannaccone

Seismotectonics of the Calabrian arc

Abstract Relocation of intermediate and deep earthquakes of Tyrrhenian Sea area through joint hypocenter determination for the period 1962–1979, has allowed a more detailed definition of the geometry of this peculiar Benioff zone. Earthquakes dip along a quasi-vertical plane to 250 km depth; there is a 50° dip in the 250–340 km depth range, and a low dip angle to 480 km depth. The structure sketched from the hypocenters is almost continuous, but most energy has been released in the 230–340 km depth interval. An evaluation of fault plane solutions of intermediate earthquakes in this area indicates predominance of down-dip compressions in the central part of the slab. At the border, strike-slip motion occurs independent of depth. Some earthquakes that occurred at intermediate depth (less than 100 km) along the Ionian margin of Calabria show predominance of reverse faulting, with the P-axis oriented SE-NW. However, shallow earthquakes in the Calabria-Sicily region indicate a more complex motion, with predominance of normal faulting. A possible interpretation of these features according to the available geological history, which involves subduction of continental lithosphere, is discussed.

Fault-plane solutions and seismicity of the Italian peninsula

Abstract A new fault-plane solution map of the Italian peninsula is presented in this paper. The earthquakes analyzed are included in the period 1905–1980, with magnitudes ranging 4–7, 75 earthquakes are located in the crust, while 31 are related to the deep and intermediate zone of the Calabrian arc. The large seismic events of the Italian peninsula are generally associated with normal faulting, while strike-slip motion is mostly related to small earthquakes, located along lateral segments of the mountain chain. The deep and intermediate earthquakes of the Tyrrhenian Sea indicate predominant down-dip compression, and strike-slip motion at the boundaries of this Benioff zone. This last is interpreted as a remnant of a subduction zone, active since Oligocene, extending to 500 km depth, with a very small lateral size (about 300 km). The present tectonics of this Benioff zone is strongly conditioned by the lateral bending, more so than the gravitational sinking process. The coexistence of thrust and normal faulting motion associated to the earthquakes, within a few tens of kilometers of each other, seems to be explained by the strong lateral inhomogeneities of the crustal rocks present in this region, more so than to the depth of the seismogenetic zone and the nature of the faulting process.

Seismic Sources and Attenuation Properties at the Campi Flegrei Volcanic Area

AbstractMicroearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:1.Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.2.Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.3.A constant Brune stress drop pattern (∼4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.4.Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.5.The scaling of peak ground motion parameters (Amax andVmax vs seismic moment) can be explained by an ω2 source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation. These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.

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.

Seismic Investigation of the Campi Flegrei: A Summary and Synthesis of Results

Campi Flegrei, Italy is a 10-km-diameter Quaternary explosive caldera near Naples and Mount Vesuvius. Numerous eruptive vents lie within it, the last of which formed in 1538 a.d. An elevation change of ≈10m has occurred within the caldera during the past 2000 years. Recently (mostly 1982–1985), following more than 400 years of subsidence, there has been approximately 2 m of uplift in a region 2.5 km in diameter in the central portion of the caldera. Local earthquakes in the magnitude range 0.6 ⩽ MD ⩽4.2 and depth range 1.5 < z < 5 km associated with this uplift were recorded by a portable 3-component digital network and by a telemetered network of single-component stations. The results of seismological studies arising from this data set are considered in consort with geodetic measurements, geothermal drilling, and gravity data to constrain the structure and dynamics of the shallow caldera. The largely aseismic central region, where the greatest uplift occurred is spatially correlated with a 10mgal gravity low and with a zone of low seismic velocities with a high vP/vs ratio. This area is interpreted as an incompetent, highly fractured, water saturated zone of low-density material which extends to at least 3 km. The shear-wave Q of the central caldera measured from direct arrivals is 110 ± 50, somewhat lower than the range of coda-Q estimates for the region (120–250). Hypocenter locations indicate a minimum magma chamber depth of 3.5 km and suggest the presence of an inward-dipping, elliptical ring fault. Fault plane mechanisms are highly heterogeneous, even for highly restricted regions, and thus do not indicate coherent reverse faulting at the perimeter of the ring. Moment tensor inversion indicates that seismic sources are well-modeled by low stress drop brittle fracture due to uplift; no clear indications of non-double couple focal mechanisms are seen which might indicate a significant seismogenic role for intrusion.

Development and Testing of an Advanced Monitoring Infrastructure (ISNet) for Seismic Early-warning Applications in the Campania Region of Southern Italy

In the framework of an ongoing project financed by the Campania Region, a prototype system for seismic early and post-event warning is being developed and tested, based on a dense, wide dynamic seismic network (ISNet) and under installation in the Apennine belt region.

A comparison of sea floor and on land seismic ambient noise in the Campi Flegrei caldera (Southern Italy)

The Campi Flegrei (southern Italy) is one of the most active calderas in the world. This caldera is characterized by episodes of slow vertical ground move- ment, called bradyseism. With several hundred thousand people living within its bor- ders, this area is in a high-risk category should there be an eruption. The seismological monitoring system in the Campi Flegrei is based on nine seismic stations, eight of which are equipped with short-period seismometers (1 Hz), and one with a broadband seismometer (60 sec-50 Hz). While all of the seismic stations are located on land, part of the seismic activity occurs in the undersea area of the Pozzuoli Gulf (Campi Fle- grei), where there are no seismic stations. This gap in the data coverage produces a biased and incomplete image of the volcanic area. We carried out an experiment in the Pozzuoli Gulf with the installation of two broadband seismic stations on the seafloor with remote and continuous data acquisition for a duration of 31 days between January and March 2005. Using the data acquired, we have computed the power spectral den- sity (PSD) to characterize the background seismic noise, and to evaluate the true noise variation, we have generated the seismic noise probability density functions from the computed PSD curves. The results of our analysis show that the broadband seismic noise is high when compared with the Peterson noise model (land model), but for periods less than 0.3 sec, the seismic noise on the seafloor is lower than the recordings on land over the same period range. The last bradyseismic crisis (1982-1984) high- lights the importance of this frequency range, where most of the spectral content of the recorded earthquakes was observed. Finally, we evaluate the detection threshold of a new seismic station located on the seafloor of the Campi Flegrei caldera consider- ing the characteristics of the local seismicity. This analysis shows that the detection threshold for the sea-floor stations (Mw ∼ 0:2) is less than that for land stations (Mw ∼ 0:8).

Low Energy Sequences in Areas with High Seismic Potential: Benevento (Southern Apennines), April 1990

In the southern sector of the Apennines chain (Italy), destructive earthquakes with medium-high magnitude occurred within a narrow belt along the axis of the chain. Low-energy seismic swarms have frequently been observed.

A Strong Motion Attenuation Relation for Early-warning Application in the Campania Region (Southern Apennines)

For early-warning applications in particular, the reliability and efficiency of rapid scenario generation strongly depend on the availability of reliable strong ground-motion prediction tools. If shake maps are used to represent patterns of potential damage as a consequence of large earthquakes, attenuation relations are used as a tool for predicting peak ground-motion parameters and intensities. One of the limitations in the use of attenuation relations is that these have only rarely been retrieved from data collected in the same tectonic environment in which the prediction has to be performed. As a consequence, strong ground motion can result in underestimations or overestimations with respect to the recorded data. This also holds for Italy, and in particular for the Southern Apennines, due to limitations in the available databases, both in terms of distances and magnitude. Moreover, for “real-time” early-warning applications, it is important to have attenuation models for which the parameters can be easily upgraded when new data are collected, whether this has to be done during the earthquake rupture occurrence or in the post-event, when all the strong motion waveforms are available.