Source parameters and stress release of seismic sequences occurred in the Friuli Venezia Giulia region (Northeastern Italy) and in Western Slovenia

The source parameters of the major events of a swarm and of two seismic sequences, occurred in the Friuli area (Northeastern Italy) and in Western Slovenia, were estimated. The Claut swarm (C96) occurred since the end of January to June 1996, with a MD 4.3 major shock and it appears composed of three sub-sequences. The two sequences are the Kobarid sequence (K98) started on April 12, 1998 with a MD 5.6 mainshock and the M.te Sernio (S02) sequence caused by the February 14, 2002 earthquake (MD = 4.9). Acceleration and velocity data recorded by the local seismic network of the Istituto Nazionale di Oceanografia e di Geofisica Sperimenale (OGS) and corrected for attenuation, were employed to estimate seismic moments and radiated energies. Source dimensions were inferred from the computed corner frequencies and the stress release was estimated from the Brune stress drop, the apparent stress and the RMS stress drop. On the whole, seismic moments range from 1.7×1012 to 1.1×1017 Nm, and radiated energies are in the range 106–1013 J. Brune stress drops are scattered and do not show any evidence of a self-similarity breakdown for sources down to 130m radius. The radiated seismic energy scales as a function of seismic moment, with a slope of the scaling relation that decreases for increasing seismic moments. The mechanism of stress release was analyzed by computing the ε parameter of Zuniga [Zuniga, R., 1993. Frictional overshoot and partial stress drop. Which one? Bull. Seismol. Soc. Am. 83, 939–944]. The K98 and S02 sequences are characterized by a wide range of the ε parameter with stress drop mechanism varying from partial locking to overshoot cases. The ε values of the C96 swarm are more homogeneous and close to the Orowan’s condition. The radiated seismic energy and the ratio of stress drop between mainshock and aftershocks appear different among the analyzed cases.We therefore investigated the relationship between the stress parameters of the main shock and the energy radiated by the aftershock sequences. For this purpose, we also estimated the source parameters of two other sequences occurred in the area, with mainshocks of MD 4.1 and 5.1, respectively. We found a positive correlation between the Brune stress drop of the mainshock and the ratio between the radiated energy of the mainshock and the summation of the energies radiated by the aftershocks

Source parameters and stress release of seismic sequences occurred in the Friuli - Venezia Giulia region (Northeastern Italy) and in Western Slovenia

The source parameters of the major events of a swarm and of two seismic sequences, occurred in the Friuli area (North-eastern Italy) and in Western Slovenia, were estimated. The Claut swarm (C96) occurred since the end of January to June 1996, with a MD 4.3 major shock and it appears composed of 3 sub-sequences. The two sequences are the Kobarid sequence (K98) started on April, 12, 1998 with a MD 5.6 mainshock and the M.te Sernio (S02) sequence caused by the February, 14, 2002 earthquake (MD =4.9). Acceleration and velocity data recorded by the local seismic network of the Istituto Nazionale di Oceanografia e di Geofisica Sperimenale (OGS) and corrected for attenuation, were employed to estimate seismic moments and radiated energies. Source dimensions were inferred from the computed corner frequencies and the stress release was estimated from the Brune stress drop, the apparent stress and the RMS stress drop. On the whole, seismic moments range from 1.7 x 1012 to 1.1 x 1017 Nm, and radiated energies are in the range 106 – 1013 J. Brune stress drops are scattered and do not show any evidence of a self-similarity breakdown for sources down to 130 m radius. The radiated seismic energy scales as a function of seismic moment, with a slope of the scaling relation that decreases for increasing seismic moments. The mechanism of stress release was analyzed by computing the ε parameter of Zuniga (1993). The K98 and S02 sequences are characterized by a wide range of the ε parameter with stress drop mechanism varying from partial locking to overshoot cases. The ε values of the C96 swarm are more homogeneous and close to the Orowan’s condition. The radiated seismic energy and the ratio of stress drop between mainshock and aftershocks appear different among the analyzed cases. We therefore investigated the relationship between the stress parameters of the main shock and the energy radiated by the aftershock sequences. For this purpose, we also estimated the source parameters of two other sequences occurred in the area, with mainshocks of MD 4.1 and MD 5.1, respectively. We found a positive correlation between the Brune stress drop of the mainshock and the ratio between the radiated energy of the mainshock and the summation of the energies radiated by the aftershocks

Deterministic ground-motion scenarios for engineering applications: the case of thessaloniki, Greece.

In this paper we present a deterministic study to estimate seismic ground motions expected in urban areas located near active faults. The purpose was to generate bedrock synthetic time series to be used as seismic input into site effects evaluation analysis and loss estimates for the urban area and infrastructures of Thessaloniki (Northern Greece). Two simulation techniques (a full wave method to generate low frequency,~< 1Hz, and a hybrid deterministic-stochastic technique to simulate high-frequency seismograms, ~> 1 Hz) were used to compute time series associated with four different reference earthquakes having magnitude from 5.9 to 6.5 and located within 30 km of Thessaloniki. The propagation medium and different source parameters were tested through the modeling of the 1978 Thessaloniki earthquake (M 6.5). Moreover two different nucleation points were considered for each fault in order to introduce additional variability in the ground motion estimates. Between the two cases, the quasi-unilateral rupture propagation toward the city produces both higher median PGA and PGV values and higher variability than bilateral ones. Conversely, the low-frequency ground motion (PGD) is slightly influenced by the position of the nucleation point and its variability is related to the final slip distribution on the faults of the reference earthquakes and to the location of the sites with respect to the nodal planes of the radiation pattern. To validate our deterministic shaking scenarios we verified that the synthetic peak ground motions (PGA, PGV) and spectral ordinates are within one standard deviation of several ground-motion prediction equations valid for the region. At specific sites we combined the low- and high-frequency synthetics to obtain broadband time series that cover all the frequency band of engineering interest (0-25 Hz). The use of synthetic seismograms instead of empirical equations in the hazard estimates provides a complete evaluation of the expected ground motions both in frequency and time domains, including predictions at short distances from the fault (0 – 10 km) and at periods larger than 2 – 3 seconds

Seismic risk assessment of italian seaports: the case of Ancona (Italy)

A National research project was recently carried out to develop a robust methodology for the design and retrofit of wharves structures located in areas of high to moderate seismicity. A detailed census of the Italian major seaports was performed using purposely devised questionnaires and Ancona harbor was chosen for a detailed investigation with the aim of providing risk assessment guidelines. This port has been selected as representative of a moderate seismicity area (expected peak ground acceleration of 0.25 g with a return period of 475 yrs). Ancona is the first harbor in the Adriatic sea, with more than a 1.500.000 passenger service and 150.000 trucks transit. It is equipped with 30 wharfs, 25 of which built in 1965-1975. To compute deterministic ground shaking scenarios, predictive empirical models have been used. At the same time advanced numerical simulation have been carried out both at high (0.7-30 Hz) and low (0-1.3) frequency ranges. The contributions of site effects and liquefaction have been also taken into account in damage estimation. Standard risk assessment has been performed using the empirical curves implemented in HAZUS program (NIBS, 2004), supported by recent studies (Lessloss, 2006, Del. 89) on damage observed after the 2003 event in Lefkas (Greece)

Approcci deterministici per la stima del moto del suolo: vantaggi e limiti

Nel Progetto DPC-INGV S3 “Scenari di scuotimento in aree di interesse prioritario e/o strategico”, le stime del moto del suolo sono state ottenute attraverso l’applicazione di diverse tecniche di simulazione di sismogrammi sintetici. Le esperienze maturate nel corso del progetto hanno condotto alla stesura di linee guida per il calcolo degli scenari di scuotimento al bedrock attraverso approcci deterministici. In questo ambito è stata introdotta una classificazione degli scenari deterministici secondo tre diversi livelli di complessità

Modelling directivity effects of the October 21, 2002 (Mw = 5.7), Molise, Southern Italy, earthquake

Acceleration time series recorded by the Italian Strong Motion Network (RAN) during the October 31, 2002 (Mw=5.8), Molise earthquake, are employed in order to investigate source effects on the ground motion in the epicentral area. We consider two different seismogenic sources: a fault model inferred from inversion of teleseismic, regional and local seismic signals [Vallée and Di Luccio, 2005], and a fault model based on seismotectonic data [Basili and Vannoli, 2005]. Both source studies suggest a deep location of the earthquake fault plane (ranging from 6.0 to 20.1 km and from 12.0 to 19.9 km, respectively), however, with considerably different fault lengths (5.2 and 10.5 km, respectively), and widths (14.2 and 8 km, respectively). Due to these differences, only the second model allows for effective horizontal unilateral rupture propagation. Finite fault effects are modelled by the Deterministic-Stochastic-Method (DSM) [Pacor et al., 2005], and the Hybrid Integral-Composite source model (HIC) [Gallovic and Brokesova, 2006]. In both methods k-square slip distributions on the faults are considered. We simulate the October 31, 2002 earthquake considering: 1) Vallée and Di Luccio [2005] faultwith a bilateral rupture propagation, and 2) Basili and Vannoli [2005] fault with unilateral directions of the rupture propagation. The spectral attenuation is modelled using a regional estimate of the quality factor [Castro et al., 2004] and k values estimated from acceleration records. Comparison between synthetic and recorded data at nearby stations (hypocentral distances < 60 km) performed in terms of frequency content and peak ground motion, favours the model with unilateral propagation of the rupture. Assuming the source model with unilateral rupture propagation, we utilize both asymptotic and full wave field methods in order to simulate ground shaking scenarios for an area extending up to 150 km epicentral distance. These results are then subjected to comparison with peak ground accelerations recorded in the far field

Damage distribution and seismological model of the November 24, 2004, Salo' (Northern Italy) earthquake

The West side of lake of Garda, in Northern Italy, was struck by a ML=5.2 earthquake on November 24, 2004. The felt area is rather large (from Venice to Milan) and the damaged area consists of 66 municipalities, with a number of homeless of about 2200 and estimated direct damages of 215 millions of euros. Most of the damaged structures are old masonry buildings and churches, while there were almost no damage to reinforced concrete structures. The observed distribution of macroseismic intensity shows a strong azimuthal dependence, with high intensity level in a 10x10 km2 area located SW to the epicentre and rather large dispersion of values (ranging from V to VII-VIII) in the first 10 km epicentral distance. Taking into account the vulnerability level of the damaged structures and the features of the geological formations, we tried to explain the observed damage distribution in terms of finite fault properties of the source, despite the moderate magnitude of the event. Thus we hypothesised a fault geometry from seismotectonic considerations and we simulated the event by a high frequency simulation technique (Deterministic Stochastic Method, DSM). The synthetic ground motion parameters were converted into intensity values by empirical relationships and local geological conditions were considered to explain some discrepancies between simulated and observed intensities. It was possible to adequately reproduce both the observed distribution of macroseismic intensity and the ground motion recorded by an accelerometric station located at about 13 km epicentral distance

Ground motion shaking scenarios for the 1997 Colfiorito earthquake

In the recent years, two Italian research projects have been devoted to the simulation of ground shaking scenarios in different areas. A large part of the activities has been performed in the Umbria region and was in particular related to the 1997 Colfiorito earthquake. In general the statistical-deterministic approach was adopted for evaluating the scenarios for strong motion parameters (peak values, spectral ordinates, signal integral quantities, and so on) associated with the occurrence of a characteristic earthquake on a given fault. This approach is based on the realistic occurrence of a single earthquake related to the fracture of an a priori well identified active fault. According to the characteristic earthquake model, an earthquake rupture can repeatedly occurs along the same fault (or fault system) with an almost constant geometry, mechanism and seismic moment, these parameters being mainly related to the direction and intensity of the large scale tectonic stress regime. These ideas are supported by numerous paleoseismic studies of active faults in different tectonic environments [e.g., Pantosti and Valensise, 1990]. On the other hand, each faulting process may not repeat the same style of nucleation, propagation and arrest during successive rupture episodes occurring along a given fault zone, depending these characteristics on the pre-fracturing conditions of rock strength and/or yielding stress along the fault zone. It is therefore assumed that the large scale source characteristics (i.e., fault size and position, focal mechanism and seismic moment) are a priori known as the result of previous geological, geophysical and historical seismicity investigations. The variability of the rupture process is expected to produce variable strong ground motions at the earth surface, depending on the distribution of the kinematic parameters (final slip distribution, rupture velocity, slip duration …) along the faulting surface. In order to account for the possible variation of the source process from one rupture event to another, a large number of synthetic seismograms should be computed for different (and possible) rupture histories occurring along the characteristic fault selected, so to provide a representative set of strong motion records to be used for hazard estimation. By this strategy, the massive computation of synthetics for different possible rupture models does not provide a single earthquake scenario (as for the standard deterministic approach) but a set of possible scenarios whose variability substantially reflects the heterogeneity of the source process. The advantage of this approach is that the variability of the selected strong ground motion parameter at a given site can be described by the statistical quantities inferred from the large number of simulations available. The earthquake scenario can then be represented, for example, by a couple of maps, one describing the spatial distribution of the mean value of the considered ground motion parameter and the other representing the associated variability for example in terms of standard deviation

THESSALONIKI SEISMIC HAZARD ASSESSMENT: PROBABILISTIC AND DETERMINISTIC APPROACH FOR ROCK SITE CONDITIONS

Within the framework of four research projects (RISK-EU, EUROSEISRISK, SRM_LIFE and LESSLOSS) extensive calculations were carried out assessing the seismic hazard in the Thessaloniki and surrounding area. The main results were derived from probabilistic and deterministic approaches taking into account rock site conditions for each examined site in the Metropolitan area of Thessaloniki. The expected strong-ground motions were calculated applying different methodologies. Two different groups worked for the assessment of the seismic hazard, the first one constituted of the INGV (Istituto Nazionale di Geofisica e Vulcanologia, Italy) and LSMF (Laboratory of Soil Mechanics and Foundation Engineering, Thessaloniki, Greece) and the second one of LSMF and ITSAK (Institute of Engineering Seismology and Earthquake Engineering, Thessaloniki, Greece)

Ground motion scenarios for the 1997 Colfiorito, central Italy, earthquake

In this paper we report the results of several investigations aimed at evaluating ground motion scenarios for the September 26th, 1997 Colfiorito earthquake (Mw 6.0, 09:40 UTC). We model the observed variability of ground motions through synthetic scenarios which simulate an earthquake rupture propagating at constant rupture velocity (2.7 km/s) and the inferred directivity. We discuss the variability of kinematic source parameters, such as the nucleation position and the rupture velocity, and how it influences the predicted ground motions and it does not account for the total standard deviation of the empirical predictive model valid for the region. Finally, we used the results from the scenario studies for the Colfiorito earthquake to integrate the probabilistic and deterministic approaches for seismic hazard assessment