Site effects in the Aterno River Valley (L’Aquila,Italy): comparison between empirical and 2D numerical modelling starting from April 6th 2009 Mw 6.3 earthquake

In this paper, we focused our attention on a cross-section of the Aterno River Valley where a good quality geological and geophysical dataset allowed to reconstruct accurately the geometry and the Vs profiles along all the plane of the section. Its trace is deliberately aligned close to the strong motion stations that recorded the Mw 6.3 (April 6th 2009) L’Aquila earthquake. We analysed strong and weak motion data available at these latter stations as well as at one of the temporary stations installed during the Microzonation activities and located on outcropping bedrock, in proximity of the cross-section. We used the H/V technique to select a reliable reference site and once we found it, we applied the SSR technique to compute amplification functions in correspondence of two strong motion stations. In turn, for both sites we performed a site response numerical modelling with two different 2D codes and we compared simulated versus experimental transfer functions. We found that the cross-section is well constrained based on the very reasonable agreement between results of numerical modelling and earthquake data analysis. We pointed out also a strong amplification of the deposit at the centre of the valley due to the constructive interference of S and surface waves, not predictable by means of 1D numerical modelling. We also compared the H/V as well as the SSR obtained from strong motion data with the ones computed from weak motion finding evidences of non-linearity in soil behaviour.     

Seismic waves in 3-D: from mantle asymmetries to reliable seismic hazard assessment

A global cross-section of the Earth parallel to the tectonic equator (TE) path, the great circle representing the equator of net lithosphere rotation, shows a difference in shear wave velocities between the western and eastern flanks of the three major oceanic rift basins. The low-velocity layer in the upper asthenosphere, at a depth range of 120 to 200 km, is assumed to represent the decoupling between the lithosphere and the underlying mantle. Along the TE-perturbed (TE-pert) path, a ubiquitous LVZ, about 1,000-km-wide and 100-km-thick, occurs in the asthenosphere. The existence of the TE-pert is a necessary prerequisite for the existence of a continuous global flow within the Earth. Ground-shaking scenarios were constructed using a scenario-based method for seismic hazard analysis (NDSHA), using realistic and duly validated synthetic time series, and generating a data bank of several thousands of seismograms that account for source, propagation, and site effects. Accordingly, with basic self-organized criticality concepts, NDSHA permits the integration of available information provided by the most updated seismological, geological, geophysical, and geotechnical databases for the site of interest, as well as advanced physical modeling techniques, to provide a reliable and robust background for the development of a design basis for cultural heritage and civil infrastructures. Estimates of seismic hazard obtained using the NDSHA and standard probabilistic approaches are compared for the Italian territory, and a case-study is discussed. In order to enable a reliable estimation of the ground motion response to an earthquake, three-dimensional velocity models have to be considered, resulting in a new, very efficient, analytical procedure for computing the broadband seismic wave-field in a 3-D anelastic Earth model.     

The ITalian ACcelerometric Archive (ITACA): processing of strong-motion data

The Italian strong-motion database was created during a joint project between Istituto Nazionale di Geofisica e Vulcanologia (INGV, Italian Institute for Geophysics and Vulcanology) and Dipartimento della Protezione Civile (DPC, Italian Civil Protection). The aim of the project was the collection, homogenization and distribution of strong motion data acquired in Italy in the period 1972–2004 by different institutions, namely Ente Nazionale per l’Energia Elettrica (ENEL, Italian electricity company), Ente per le Nuove tecnologie, l’Energia e l’Ambiente (ENEA, Italian energy and environment organization) and DPC. Recently the strong-motion data relative to the 23th December 2009, Parma (Mw = 5.4 and Mw = 4.9) and to the April 2009 L’Aquila sequences (13 earthquakes with 4.1 ≤ Mw ≤ 6.3) were included in the Italian Accelerometric Archive (ITACA) database (beta release). The database contains 7,038 waveforms from analog and digital instruments, generated by 1.019 earthquakes with magnitude up to 6.9 and can be accessed on-line at the web site . The strong motion data are provided in the unprocessed and processed versions. This article describes the steps followed to process the acceleration time series recorded by analogue and digital instruments. The procedures implemented involve: baseline removal, instrumental correction, band pass filtering with acausal filters, integration of the corrected acceleration in order to obtain velocity and displacement waveforms, computation of acceleration response spectra and strong motion parameters. This procedure is applied to each accelerogram and it is realised to preserve the low frequency content of the records.     

Is there a correlation between lithosphere structure and the statistical properties of seismicity?

The elastic parameters and the thickness of the lithospheric part of the mantle, the lid, are correlated with the seismicity of Italy and surrounding areas through the estimation of several statistical parameters. In particular, the intensity of the flow of seismic events, deprived of aftershocks, reveals concentrations associated with large gradients in the lithospheric thickness. The average depth of crustal shocks tends to be very shallow in areas of thin and soft (low shear-wave velocities) lid. The positive influence, probably reflecting the thermal state in the lithosphere, is very large in connection with the thin and soft lid and where strong gradients in lid thickness are observed, and is small in connection with a thick and hard lid (high shear-wave velocities).     

Source Parameters of Weak Crustal Earthquakes of the Vrancea Region from Short-period Waveform Inversion

High-frequency records of nine low magnitude shallow earthquakes of the Vrancea (Romania) seismic region are inverted for the seismic moment tensor (MT). An approach is suggested regarding how to obtain at least a rough estimate of the MT when the information on the structure of the crust is poor. Here simple 1-D layered models are used in the Greens function synthesis despite the fact that the structure of the region is undoubtedly very complex. Different 1-D models were used for different source-station paths to approximate lateral variations. Record of a station located on a ray path which crosses a structure differing substantially from a 1-D model may however bias the retrieved MT essentially. Therefore, we did not collectively process all records, but subsets of stations separately. We check the consistency of the MTs resulting from these individual bootstrap solutions, and reject those which differ substantially, assuming that this is due to the oversimplification of the forward modeling. Thereafter, the averaged moment tensor yielded by the consistent subsets is accepted. Moreover, the distribution of the T, N and P axes from the moment tensors used for averaging provides a rough estimate of their reliability.Following this simplistic procedure, we found seven acceptably constrained solutions among nine events processed. Their P axes are compared with the general trend of the stress in the area: three comply with it, the others should be attributed to the complex stress field active in the region.Acknowledgment This research has been made possible by MURST (40% and 60%), by UNESCO-IGCP project 414 Realistic Modelling of Seismic Input for Megacities and Large Urban Areas and NATO SfP 972266. Partial support issued from the Grant Agency of Czech Rep. (Grant 205/02/0383) and from the National Agency for Science, Technology and Innovation of Romania (Grant 6185 GR/2000).     

Three Decades of Seismic Activity at Mt. Vesuvius: 1972–2000

We analyse the seismic catalogue of the local earthquakes which occurred at Somma-Vesuvius volcano in the past three decades (1972–2000). The seismicity in this period can be described as composed of a background level, characterised by a low and rather uniform rate of energy release and by sporadic periods of increased seismic activity. Such relatively intense seismicity periods are characterised by energy rates and magnitudes progressively increasing in the critical periods. The analysis of the b value in the whole period evidences a well-defined pattern, with values of b progressively decreasing, from about 1.8 at the beginning of the considered period, to about 1.0 at present. This steady variation indicates an increasing dynamics in the volcanic system. Within this general trend it is possible to identify a substructure in the time sequence of the seismic events, formed by the alternating episodes of quiescence and activity. The analysis of the source moment tensor of the largest earthquakes shows that the processes at the seismic source are generally not consistent with simple double-couples, but that they are compatible with isotropic components, mostly indicating volumetric expansion. These components are shown to be statistically significant for most of the analysed events. Such focal mechanisms can be interpreted as the effect of explosion phenomena, possibly related to volatile exsolution from the crystallising magma. The availability of a reduced amount of high quality data necessary for the inversion of the source moment tensor, the still limited period of systematic observation of Vesuvius micro-earthquakes and, above all, the absence of eruptive events during such interval of time, cannot obviously permit the outlining of any formal premonitory signal. Nevertheless, the analysis reported in this paper indicates a progressively evolving dynamics, characterised by a generally increasing trend in the seismic activity in the volcanic system and by a significant volumetric component of recent major events, thus posing serious concern for a future evolution towards eruptive activity.