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Coda-Q is used to estimate the attenuation and scattering properties of the Earth. So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5-5 s, 5-10 s and 10-20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5-5 s and 5-10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5-5 s and 5-10 s. In the period band 10-20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected data sets and longer time-series result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data.

A review of the recent achievements of the innovative neo-deterministic approach for seismic hazard assessment through realistic earthquake scenarios has been performed. The procedure provides strong ground motion parameters for the purpose of earthquake engineering, based on the deterministic seismic wave propagation modelling at different scales— regional, national and metropolitan. The main advantage of this neo-deterministic procedure is the simultaneous treatment of the contribution of the earthquake source and seismic wave propagation media to the strong motion at the target site/region, as required by basic physical principles. The neo-deterministic seismic microzonation procedure has been successfully applied to numerous metropolitan areas all over the world in the framework of several international projects. In this study some examples focused on CEI region concerning both regional seismic hazard assessment and seismic microzonation of the selected metropolitan areas are shown.

Il progetto è orientato al problema della prevenzione dei disastri: valutazione, prevenzione e ubicazione del rischio, in connessione con l’attività sismica e le vibrazioni artificiali. La definizione realistica della sollecitazione sismica è stata ottenuta col calcolo di un ampio insieme di serie temporali (sismogrammi sintetici) e relative informazioni spettrali, corrispondenti ai possibili scenari sismotettonici. La tecnica innovativa di modellazione, che costituisce il filo conduttore dell’intero progetto, prende in considerazione l’origine delle onde sismiche, la loro propagazione e gli effetti locali nel luogo di interesse. Ciò è stato realizzato utilizzando i principi fisici della generazione e propagazione delle onde nei mezzi anelastici, evitando di rifarsi ad un approccio convolutivo che si è dimostrato del tutto irrealistico, soprattutto in caso di strutture geologiche complesse, che sono poi le più interessanti dal punto di vista pratico. Molte tecniche, che sono state proposte per stimare empiricamente gli effetti locali tramite la convoluzione di osservazioni con segnali, calcolati in corrispodenza di modelli semplificati, forniscono informazioni attendibili sulla risposta sismica del sito a fasi sismiche non interferenti. Essi sono inadeguati nella maggior parte dei casi reali, dove la sequenza sismica è formata invece da diverse onde interferenti. La disponibilità di simulazioni numeriche realistiche ha reso possibile una stima attendibile dell’effetto di amplificazione locale, persino nel caso di strutture geologiche complesse, quando sono disponibili i parametri geotecnici, litologici e geofisici, i dati topografici, tettonici, storici e paleosismologici, nonche’ i modelli sismotettonici. La modellazione realistica dei movimenti tellurici è un punto fondamentale nella preparazione di scenari di scuotimento del suolo (groundshaking) che rappresentano un valido ed economico strumento per la microzonazione sismica. Tale conoscenza è facilmente utilizzabile negli ambiti dell’ingegneria civile per i progetti di nuove costruzioni antisismiche e per migliorare le strutture già esistenti, ed inoltre rappresenta un ottimo strumento nell’ambito dei servizi di prevenzione della protezione civile. Vene Qui presentata una selezione dei principali risultati ottenuti nelle città di: Algeri, Pechino, Bucarest, Cairo, Debrecen, Delhi, Napoli, Roma, Russe, Santiago de Cuba, Sofia, Salonicco e Zagabria.

A hybrid technique consisting of modal summation and subsequent ®nite differences modelling is applied for the computation of synthetic accelerograms along a pro®le crossing the city of Zagreb, the capital of Croatia. Assuming the source geometry is known, the ampli®cation properties of the underlying soil may be determined by comparison of synthetics and their response spectra computed for a bedrock model with the ones obtained under the assumption of a realistic laterally varying local model. The peak ground acceleration is larger by a factor of up to 3.5 than the value obtained for the bedrock model. The ampli®cation of the response spectra is most prominent for frequencies below 2 Hz, and increases sharply to the SW from the mapped fault running through the centre of the city. q 2002 Published by Elsevier Science Ltd.

Report submitted to the International Association of Seismology and Physics of the Earth’s Interior of the International Union of Geodesy and Geophysics

Seismic activity in Croatia and surrounding areas in the 1993-1996 period was mostly confined to the previously identified seismically active areas. Nine events (excluding aftershocks) with the magnitude equal to or exceeding 4.5 occurred during that time. The most important earthquake sequence is the one that started on September 5, 1996 (Ml=6.0, Imax=VIII MSK) in the Ston-Slano area (greater Dubrovnik region). Microseismic locations of hypocentres of thousands of aftershocks, as well as the best double-couple CMT solution for the main-shock indicate that the earthquakes occurred on the NW-SE striking reverse fault system dipping towards NE.

Based on the catalogue of all located earthquakes in Croatia and the surrounding areas for the period 1990 – 1992 and on the macroseismic data analyses for individual earthquakes, certain seismicity features have been analysed. All together, 1188 earthquakes were located and the catalogue was found to be complete for the magnitudes ML ≥ 3.0. Seismically the most active was the coastal part of Croatia, where the strongest earthquake in the analysed period occurred on November 27, 1990 with the epicentre beneath the Dinara Mt. The composite fault-plane solution for the main shocks indicate the presence of a tectonic stress-filed directly approximately SSW-NNE which is compatible with the assumed anticlockwise rotation of the Adriatic plate and its subduction under the Dinarides.

Some of the elements of regional seismic hazard in Croatia are assessed by computing synthetic accelerograms at a predetermined set of sites. The input dataset consists of structural models, parameters of seismic sources, and an updated earthquake catalog. Synthetic strong-motion time series for frequencies below 1 Hz are computed on a grid of sites using the modal summation technique. The long-period hazard is described by the distribution of estimated peak values of ground displacement, velocity and acceleration, while the short-period hazard is represented by the map of design ground acceleration values (DGA). The highest values of DGA exceeding 0.35 g on the base-rock level are found in the southeastern coastal part of the country, in the greater Dubrovnik area.

The linear amplification of the larger horizontal component of strong ground motion along a selected profile in the city of Zagreb is estimated by examining the synthetic waveforms corresponding to a suite of 16 realistically chosen scaled point sources. The accelerograms, computed for the average bedrock model by modal summation, are propagated through local laterally heterogeneous anelastic models by the finite-difference algorithm. The ratio of peak ground acceleration (PGA) and of the response spectra (RS), obtained by using local and bedrock models, define the PGA and RS amplification AMP(PGA) and AMP(RS), respectively. Even variations of the order of commonly observed uncertainties of only dip and rake angles of the causative fault show that both AMP(PGA) and AMP(RS) vary at some sites by more than a factor of two. It follows that, especially for strongly laterally heterogeneous structures, local effects must be determined for each of the relevant sources considering all associated uncertainties as completely as possible. Such a conclusion certainly holds for the case of the microzonation of Zagreb, where the local geology is quite complex, and the seismicity is not confined to a single seismic source zone.

The project addressed the problem of pre-disaster orien- tation: hazard prediction, risk assessment, and hazard mapping, in connection with seismic activity and man- induced vibrations. The definition of realistic seismic input has been obtained from the computation of a wide set of time histories and spectral information, corre- sponding to possible seismotectonic scenarios for dif- ferent source and structural models. The innovative modeling technique, that constitutes the common tool to the entire project, takes into account source, propaga- tion and local site effects. This is done using first princi- ples of physics about wave generation and propagation in complex media, and does not require to resort to con- volutive approaches, that have been proven to be quite unreliable, mainly when dealing with complex geologi- cal structures, the most interesting from the practical point of view. In fact, several techniques that have been proposed to empirically estimate the site effects using observations convolved with theoretically computed signals corresponding to simplified models, supply reli- able information about the site response to non-interfer- ing seismic phases. They are not adequate in most of the real cases, when the seismic sequel is formed by several interfering waves. The availability of realistic numeri- cal simulations enables us to reliably estimate the amplification effects even in complex geological struc- tures, exploiting the available geotechnical, lithologi- cal, geophysical parameters, topography of the medium, tectonic, historical, palaeoseismological data, and seismotectonic models. The realistic modeling of the ground motion is a very important base of knowl- edge for the preparation of groundshaking scenarios that represent a valid and economic tool for the seismic microzonation. This knowledge can be very fruitfully used by civil engineers in the design of new seismo- resistant constructions and in the reinforcement of the existing built environment, and, therefore, supply a par- ticularly powerful tool for the prevention aspects of Civil Defense. We present a selection of the main results obtained for the cities of Algiers, Beijing, Bucharest, Cairo, Debrecen, Delhi, Naples, Rome, Russe, Santiago de Cuba, Sofia, Thessaloniki and Zagreb.

Modal summation technique is used to generate 5000, three-component theoretical seismograms of Love and Rayleigh waves, assuming modified PREM (PREM-C) and AK135F global earth models. The focal depth h and the geometrical fault parameters are randomly chosen so as to uniformly cover possible source mechanisms and obtain uniform distribution of log h in the interval 1 < h < 600 km. The amplitudes of 20 ± 2-s waves measured on each of the synthetic seismograms yield curves of amplitude vs. depth, and consequently the theoretical surface waves magnitude depth correction for all three components of ground motion. Predicted surface wave amplitudes are practically constant for h < 20 km, then decrease with the focal depth. This decrease is not uniform, and depends on the excitation level of higher modes. For PREM-C model and for shallow sources, computed Love wave amplitudes are nearly an order of magnitude larger than those of Rayleigh waves, which is why the AK135F model is given preference over PREM-C. The theoretical depth correction ranges between zero, for shallow sources, to about +1 magnitude unit, for the deepest ones. The theoretical results are compared with 74,480 individual station measurements of 20-s surface wave amplitudes reduced to the same distance, period and seismic moment. It is found that empirical data closely match the predictions made by using the AK135F model. Considering both theoretical modeling and observational data, we propose the introduction of a piecewise log-linear depth correction for M s of the form: ΔM s (h) = 0 for h < 20km, ΔM s (h) = 0.314 1og(h) - 0.409 for 20 ≤ h < 60km, ΔM s (h) = 1.351 log(h) - 2.253 for 60 ≤ h < 100km, ΔM s (h) = 0.400 log(h) - 0.350 for 100 ≤ h < 600 km.