Maurizio Vassallo

Earthquake early warning system in southern Italy: Methodologies and performance evaluation

We investigate the effect of extended faulting processes and heterogeneous wave propagation on the early warning system capability to predict the peak ground velocity (PGV) from moderate to large earthquakes occurring in the southern Apennines (Italy). Simulated time histories at the early warning network have been used to retrieve early estimates of source parameters and to predict the PGV, following an evolutionary, probabilistic approach. The system performance is measured through the Effective Lead-Time (ELT), i.e., the time interval between the arrival of the first S-wave and the time at which the probability to observe the true PGV value within one standard deviation becomes stationary, and the Probability of Prediction Error (PPE), which provides a measure of PGV prediction error. The regional maps of ELT and PPE show a significant variability around the fault up to large distances, thus indicating that the systems capability to accurately predict the observed peak ground motion strongly depends on distance and azimuth from the fault.

A Local Magnitude Scale for Southern Italy

local magnitude scale has been defined for southern Italy, in the area monitored by the recently installed Irpinia Seismic Network. Waveforms recorded from more than 100 events of small magnitude are processed to extract synthetic Wood–Anderson traces. Assuming a general description of peak-displacement scaling with the distance, by means of linear and logarithmic contributions, a global exploration of the parameter space is performed by a grid-search method with the aim of investigating the correlation between the two decay contributions and seeking for a physical solution of the problem. Assuming an L2 norm, we found M=logA+1.79 log R - 0.58 yielding an error on the single estimation smaller than 0.2, at least when the hypocenter location is accurate. Station corrections are investigated through the station residuals, referring to the average value of the magnitude. Using a z test, we found that some stations exhibit a correction term significantly different from 0. The use of the peak acceleration and peak velocity as indicators of the magnitude is also investigated.

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).

The Seismic Microzonation of San Gregorio Through a Multidisciplinary Approach. Seismic Amplification in a Stiff Site

The village of San Gregorio (L’Aquila, Italy) was severely damaged by the April 6, 2009 earthquake. San Gregorio is situated at the base of a carbonate relief bounded by the Aterno river alluvial plain. The geological features of the area are very complex: jointed carbonate bedrock, cemented gravels and alluvial fan crop out in the village. Co-seismic ground fractures were seen along SW dipping active fault segments crossing San Gregorio. We integrated the microzonation studies with new geological, geotechnical and geophysical data for supporting the reconstruction planning of the village. Noise measurements show strong and polarized peaks in the horizontal-to-vertical spectral ratios (H/V) in the 3–7 Hz frequency band. Interestingly, the alluvial fan and the outcropping rock masses show both H/V peaks. To understand the influence of rock mass joint condition on site effect, we performed structural surveys on carbonate bedrock to look for a possible correlation between rock fracturing and ground-motion amplification.

Active Normal Faulting and Large-Scale Mass Wasting in Urban Areas: The San Gregorio Village Case Study (L’Aquila, Central Italy). Methodological Insight for Seismic Microzonation Studies

The 2009 L’Aquila earthquake determined ground cracks in the area of San Gregorio, along a normal fault branch NW–SE trending and SW dipping. We dug two paleoseismological trenches across the fault to investigate its Late Quaternary activity and to verify whether the co-seismic ground ruptures could be effectively related to the fault activation. The trenches showed that, in the past few millennia, the fault branch may have been responsible for surface displacement much larger than that determined by the 2009 earthquake. Moreover, geological/geomorphological field survey defined that the central portion of the structure is utilised in its shallowest portion as sliding plane of a large-scale gravitational mass movement. In particular, one of the paleoseismological trenches revealed that the large scale mass wasting is probably characterised by both continuous displacement and abrupt events of movement. Seismological investigations defined small amplification on rock site along the investigated fault, to be likely related to the joint and fracture condition determined by the fault activity.

Low shear velocity in a normal fault system imaged by ambient noise cross correlation: The case of the Irpinia fault zone, Southern Italy

We extracted the Greens functions from cross correlation of ambient noise recorded at broadband stations located across the Apennine belt, Southern Italy. Continuous records at 26 seismic stations acquired for 3 years were analyzed. We found the emergence of surface waves in the whole range of the investigated distances (10–140 km) with energy confined in the frequency band 0.04–0.09 Hz. This phase reproduces Rayleigh waves generated by earthquakes in the same frequency range. Arrival time of Rayleigh waves was picked at all the couples of stations to obtain the average group velocity along the path connecting the two stations. The picks were inverted in separated frequency bands to get group velocity maps then used to obtain an S wave velocity model. Penetration depth of the model ranges between 12 and 25 km, depending on the velocity values and on the depth of the interfaces, here associated to strong velocity gradients. We found a low-velocity anomaly in the region bounded by the two main faults that generated the 1980, M 6.9 Irpinia earthquake. A second anomaly was retrieved in the southeast part of the region and can be ascribed to a reminiscence of the Adria slab under the Apennine Chain.

Local Seismic Response in a Large Intra-mountain Basin as Observed from Earthquakes and Microtremor Recordings: The Avezzano Area (Central Italy)

Buildings close to each other can perform different behaviour despite its similar seismic vulnerability. This effect is mainly due to the local seismic response connected to the characteristics of the shallow soil layers, especially when we move away from the epicentral area and the near field motion reduces its importance among the total amount of shaking. In this paper we show some results of the microzonation project of the Avezzano municipality, a town located in the southwestern portion of the Abruzzi region, which experienced the severe effects of the January 13th, 1915 M 7.0 earthquake. Starting from a particularly detailed knowledge of the geological characteristics of outcropping lithologies and inferring the trend of subsoil geometries, we explored the role played by the near-surface geology in causing variability of the ground motion by analysing a large database of earthquakes and microtremor recordings acquired by temporary seismological networks using classical site-reference and non-reference spectral techniques. Based on the obtained results we can seismically characterize all the municipal territory not only in terms of fundamental resonance frequency, useful in drawing maps of seismic microzonation and design geological sections, but also of amplification factors helpful in verifying numerical modelling of seismic response as required by national microzonation guidelines. We have also found many criticisms that need a more detailed analysis in order to establish the cause of these anomalies.

Cross-Correlation Analysis of Seismic Noise Data in the Fucino Plain (Central Appennines, Italy)

Several months of ambient seismic noise recordings are used for investigating the distribution of elastic properties in the Fucino Plain, one of the largest intermontane tectonic depressions of the Italian Apennine chain (Central Italy). The Plain is characterized by a low level of seismicity but the presence of several active faults makes it an Italian area of high seismic hazard. The most recent and strongest seismic event in Fucino Plain occurred in the 1915 (Avezzano earthquake) and it represents one of the most energetic events (Ms = 7.0) happened in central Apennines. Inter-stations Green’s functions are reconstructed by the cross-correlation of continuous ambient noise data recorded from twelve seismic velocimeters deployed around the Avezzano city, and organized in two different temporally sub-networks. The aim of cross-correlation analysis is to extract surface waves from Green’s functions for investigating the dispersive response of the structure. We analyzed the temporal stability of the cross-correlated signals that is used as an indicator of reliability of measurements and as criteria to select the Green’s functions to analyze.

The Seismic Site Characterization of Palazzo Centi in L’Aquila City Centre: The Case Study of a Historical Building Damaged by the April 6th 2009 Earthquake

An extensive geological, geotechnical and geophysical investigation was performed in L’Aquila city centre to restore Palazzo Centi, a historical building, damaged by the April 6, 2009 L’Aquila earthquake. This site investigation consisted of punctual and linear tests that allowed to define a detailed 3D model of the subsoil, irregularly affected by some peculiar conditions and characterized by low and variable values of the shear wave velocity V S in the near surface volume. In particular, the variable thickness of the upper fine-grained residual soils probably determined different ground motion amplifications during the main shock.