Paolo Augliera

Pan-European ground-motion prediction equations for the average horizontal component of PGA, PGV, and 5 %-damped PSA at spectral periods up to 3.0 s using the RESORCE dataset

This article presents a set of Ground-Motion Prediction Equations (GMPEs) for Europe and the Middle East, derived from the RESORCE strong motion data bank, following a standard regression approach. The parametric GMPEs are derived for the peak ground acceleration, peak ground velocity, and 5 %-damped pseudo-absolute acceleration response spectra computed over 23 periods between 0.02 and 3 s, considering the average horizontal-component ground-motions. The GMPEs are valid for distances less than 300 km, hypocentral depth up to 35 km and over the magnitude range 4–7.6. Two metrics for the source-to-station distance (i.e. Joyner-Boore and hypocentral) are considered. The selected dataset is composed by 2,126 recordings (at a period of 0.1 s) related to 365 earthquakes, that includes strong-motion data from 697 stations.The EC8 soil classification (four classes from A to D) discriminates recording sites and four classes (normal, reverse, strike-slip, and unspecified) describe the style of faulting. A subset which contains only stations with measured Vs30 and earthquakes with specified focal mechanism (1,224 records from 345 stations and 255 earthquakes) is used to test of the accuracy of the median prediction and the variability associated to the broader data set. A random effect regression scheme is applied and bootstrap analyses are performed to estimate the 95 % confidence levels for the parameters. The total standard deviation sigma is decomposed into between-events and within-event components, and the site-to-site component is evaluated as well. The results show that the largest contribution to the total sigma is coming from the within-event component. When analyzing the residual distributions, no significant trends are observed that can be ascribed to the earthquake type (mainshock-aftershock classification) or to the non-linear site effects. The proposed GMPEs have lower median values than global models at short periods and large distances, while are consistent with global models at long periods \((\hbox {T} > 1)\) s. Consistency is found with two regional models developed for Turkey and Italy, as the considered dataset is dominated by waveforms recorded in these regions.

Application of the Generalized Inversion Technique (GIT) to a Microzonation Study: Numerical Simulations and Comparison with Different Site-Estimation Techniques

The reliability of the results obtained by applying the Generalized Inversion Technique (GIT) to site response estimates is assessed by means of numerical simulations. This study was developed in the framework of the UMBRIA-MARCHE microzonation project. The analysis with synthetic data demonstrates that the generalized inversion technique is capable of revealing the site effects. Furthermore, this approach shows that a priori information relevant to the Q quality factor is needed if a source estimate is required. We also analyze data recorded by five seismological stations installed in Fabriano (central Italy) with a minimum interstation distance of 100 m. The results computed with the generalized inversion technique are compared with those obtained by the reference site method and the receiver function technique. Whereas the first two methods provide highly consistent site-response estimates, the receiver function technique provides lower levels amplification. Furthermore, the time duration used in the analysis seems to be a critical parameter for the receiver function technique alone. Manuscript received 15 April 1999.

Source Parameters Estimated from the Aftershocks of the 1997 Umbria–Marche (Italy) Seismic Sequence

We derived the source parameters of 563 aftershocks (1.4 ≤ M L ≤ 4.5) of the 1997 Umbria–Marche seismic sequence by analyzing three-component digital recordings of a temporary network and developed a local magnitude scale using the whole available data set (more than 25,000 waveforms) from synthetic Wood–Anderson seismograms. Then, considering a subnetwork composed of nine portable stations, more than 12,000 earthquake spectra were corrected for the near-surface attenuation parameter k and the path attenuation. The corrected spectra were used to compute the seismic moment, the source radius, and stress drop assuming an ω–2 Brune source model. We calculate the relationship log M = 17.46 + 1.12 M L between seismic moment and local magnitude for the studied area. The source dimension versus seismic moment relationship did not show any breakdown in the similarity of the rupture process, even when events of magnitude less than 2.2 (the threshold value for the completeness of the considered catalog.) were considered. The average stress drop obtained analyzing the S -wave spectra was 38 ± 10 bar, and the source dimension ranged between about 40 and 500 m.

SEISMIC MULTIPLETS ANALYSIS AND ITS IMPLICATION IN SEISMOTECTONICS

Abstract Doublets analysis techniques suitable to obtain very accurate determination of arrival times of seismic phases on different stations are presented. These methodologies represent a very powerful tool to recognize the seismotectonic structures and to study the propagation characteristics of seismic waves. The signals similarity analysis, performed in the frequency and time domain, has been applied to increase the precision in arrival times definition of seismic phases generated by the same sources and having the same path. Tests on synthetic events have shown how it is possible to recognize the time delay between digital traces with a sensitivity less than the sample rate. In particular, in the time-domain analysis the results obtained using different interpolation functions (parabolic and sinc for the cross-correlation operator) are compared. The analysis slightly suffers from a random noise component added to the signal, allowing the phase picking of highly noised traces. On the contrary, the frequency domain analysis offers the best results in case of high signal-to-noise ratio, while it appears less ‘robust’ with respect to high noise influence. The application of relative location techniques on the obtained arrival times allows to define with very high detail the geometrical shape of seismogenetic structures. As an example, a case of spatial doublets, recorded by the seismic network of Northwestern Italy, that occurred during two days in an area of the Maritime Alps (Italy), has been considered. In spite of the low magnitude of the events, an accurate reconstruction of the spatial distribution of the sources was obtained, allowing to discriminate the fault plane from the auxiliary one in the focal mechanism solution.

An ML Scale in Northwestern Italy

A local-magnitude scale is derived for northwestern Italy from wave-form data recorded at 17 stations from 2600 local earthquakes, ranging in distance from 10 to 300 km. By averaging the horizontal components in a single measure, we used 8127 zero-to-peak amplitudes from synthetic Wood–Anderson seismograms to determine, in a least-squares sense, the appropriate –log A attenuation function, the event local magnitude, and the station corrections. Both a parametric and a non-parametric description of –log A is considered while performing the inversion. In both cases, the constraint of 1-mm motion recorded at 100 km for a magnitude 3.0 earthquake was used. The resulting parametric distance correction is given by –log A = 1.144log( r /100) + 0.00476( r – 100) + 3. The remarkable agreement between the parametric and nonparametric results confirms that the assumptions on the attenuation function that we made for deriving the parametric distance correction are reasonable. Moreover, inversion of bootstrap replications of our data set furnished stable solutions. Station magnitude corrections range between –0.28 and 0.48, suggesting a variable and significant effect of station site properties on recorded amplitudes. Finally, the local-magnitude scale has been used to evaluate the magnitude values for the whole catalog of earthquakes recorded by the IGG–University of Genoa network from 1996 until March 2001 (about 3200 events) and for a subset relevant to the southwestern Alps (about 1900 events). Statistics performed using the new local-magnitude scale and the usual duration magnitude demonstrate the influence of this choice on both completeness and b -value estimation. For example, in the southwestern Alps, the completeness threshold decreases from 2.0 ( M d) to 1.7 ( M L) and the b -value from 1.38 ( M d) to 1.08 ( M L).

Anomalously deep earthquakes in northwestern Italy

It is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of ‘normal’ seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991–1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area.

Site Classification Assessment for Estimating Empirical Attenuation Relationships for Central-Northern Italy Earthquakes

The aim of this article is to investigate the ground motion attenuation of the most industrialized and populated regions of Italy, evaluating the capability of different approaches to estimate site dependent models. The 5.2 local magnitude earthquake on November 24, 2004 shocked the areas of Northern Italy producing damage of about 215 million euros. The data set, including 243 earthquakes of local magnitude up to 5.2, has been collected in the period December 2002–October 2005 by 30 three-component seismic stations managed by Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano (INGV-MI). Empirical attenuation relationships have been estimated for horizontal peak ground velocity (PGHV), acceleration (PGHA), displacement (PGHD), and for response spectral acceleration (SA) for periods between 0.1 and 1.5 s. To estimate suitable attenuation models, in particular for sites characterized by thick sedimentary geological formations, a soil discrimination based on EU8 code can lead to wrong evaluations. On the contrary, a classification based on H/V spectral ratios of seismic ambient noise (NHV) allows the models to fit better real and predicted data and to reduce the uncertainties of the process. For each receiver, NHV have been strengthened by additional H/V spectral ratio of earthquake data (EHV), calculated considering different portions of the analysed signals. In order to validate the PGHA attenuation relationship for greater magnitudes, accelerometric records, relative to Central-Northern Italy strong motions occurring in the last 30 years, have been collected and superimposed to our attenuation curves.

Regional coda Q variations in the western Alps (northern Italy)

Abstract To study the microseismicity of the central segment of the western Alps, a local temporal seismic network of six stations was installed in addition to the regional centralized network operating in the area. In 3 months about 220 shocks, with magnitude ranging between 1.0 and 3.8, have been analyzed and located. The epicenter distribution shows a possible subdivision into seismogenetic bands. This subdivision is also recognizable from the spectral characteristics of the analyzed seismic signals. A study of the wave attenuation, in a restricted area well covered by the local network, was carried out through a ‘coda’ analysis. The results show strong spatial Qc variations, in agreement with the strong lateral heterogeneities that characterize this area. In particular, the Brianconnais domain, the crystalline massifs and the inner part of the Alpine arc appear to behave as decreasing attenuation bands.

Reliability of earthquake location procedures in heterogeneous areas: synthetic tests in the South Western Alps, Italy

Abstract The accuracy of procedures for locating earthquakes and tomographically imaging 3-D velocity structures, is influenced by factors such as network geometry, errors in picking seismic phases, modeling errors and velocity model uncertainties. The main purpose of this study is to investigate the importance of different error sources through synthetic experiments. The synthetic data sets were generated for a realistic 3-D velocity model of the South Western Alps adopting a precise raytracing technique. The actual station configuration and recorded seismicity during the passive seismological experiment Geofrance3D-Alps, conducted between August 1996 and February 1997, were used. The station configuration corresponded to an enhancement of the regional networks operating in the area, with the addition of temporary stations. In addition, synthetic data sets were corrupted by applying different error schemes. Furthermore, to assess the locating capabilities of the regional networks, synthetic data sets were constructed for synthetic events placed in correspondence to the nodes of a regular three dimensional (3-D) grid. We mimicked the actual data management scheme for synthetic data processing: we re-located the earthquakes, performed an inversion for a minimum 1-D velocity model with station corrections and, then, ran a complete 3-D velocity model inversion. One of the main results, confirming earlier studies was that earthquake locations in strongly heterogeneous areas, obtained using a regional 1-D model, can have significant errors. Even a minimum 1-D model with station corrections does not significantly improve the accuracy of parameter locations in the South Western Alps. For high precision locations, a 3-D model is needed. However, a 3-D model cannot be reconstructed from noise contaminated data recorded by a sparse regional network.

A Microtremor Survey in the Area Shocked by the ML 5.2 Salò Earthquake (North Italy): An Empirical Approach to Determine the Effects of Ground Motions

In this work, the results of a quick microtremor survey performed in the municipalities situated in the epicentre area of the Ml 5.2 2004 Salò earthquake (North Italy) are presented. The aim of this study is to understand if the large amount of damage caused by the event (about 215 million euros only in the areas near to the epicenter) is correlated more to the local surface geology conditions or to the vulnerability of ancient Italian historical centers. A preliminary seismic zonation was carried out in 5 villages including about 30 measurements of microtremors analyzed by the Nakamura technique (hereinafter HVNR). The points of measurement were carefully selected considering sites located both near damaged buildings and over different local geology conditions (alluvium deposits, fluvial-glacial deposits, debris fans and rock). In order to strengthen the HVNR results and to evaluate the reliability of the Nakamura analysis, a comparison with spectral ratios calculated on earthquakes (hereinafter HVSR) recorded at the strong motion station of Vobarno was made. In general, the outcomes of the survey highlight a possible correlation between local geology conditions and ground motion amplification for different frequency bands. In order to check if this evidence is linked with the damage, a series of macroseismic intensity values were collected for different zones of the investigated area, and a nonparametric correlation approach was used to establish a possible correlation between damage and ground motion amplification for selected frequency bands. The results show, from a statistical point of view, that in the area surrounding the epicenter of the November 24, 2004 mainshock, the damage pattern is not strongly dependent upon the local surface geology but more correlated to the low quality of the civil structures present in the area, including old buildings of the last century.