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

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

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

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 models for Molise region (Southern Italy)

On October 31st and November 1st, 2002 two moderate earthquakes of moment magnitude Mw=5.7 (INGV-Harvard European-Mediterranean Regional Centroid-Moment tensor project) occurred in southern Italy. After the mainshocks, felt in many municipalities of the Molise and Puglia region, a strong motion and a seismic temporary network were installed in the epicentral area and surrounding regions. The strong motion network was composed by 9 stations, integrating the accelerometers of the permanent Rete Accelerometrica Nazionale (RAN network), and operated until December 2003. The strong motion data set is composed by 195 recordings from 51 earthquakes (2.5<Ml<5.4) recorded by 29 accelerometers (Dipartimento della Protezione Civile et al., 2004). In addition to the strong motion network, several Italian research institutions (Istituto Nazionale di Geofisica e Vulcanologia, INGV; Istituto Nazionale di Oceanografia e Geofisica, INOGS; Dipartimento per lo studio del Territorio e delle sue Risorse, University of Genoa, Dip.Te.Ris) installed a temporary regional network, composed by 35 seismic stations. This network aimed at monitoring and studying the evolution in time and space of the seismic sequence. More than 1900 aftershocks were recorded in the period November 1st - December 5th, 2002 (Chiarabba et al., 2005). The unified velocity-acceleration data set has been considered to derive ground motion models for peak ground acceleration and peak ground velocity for both maximum horizontal and vertical components. The results obtained for the Molise area have been compared with the attenuation pattern of the Umbria-Marche region (central Italy), that was recently investigated by Bindi et al. (2006). The remarkable differences observed indicate the need of a regional attenuation relation for the area and the need of further investigations, to better identify the role of source characteristics, anelastic and geometric attenuation and site effects in the evaluation of peak ground motion values

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

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

What can we learn from the January 2012 Northern Italy earthquakes?

This note focuses on the ground motion recorded during the recent moderate earthquakes occurred in the central part of Northern Italy (panel 1), a region characterized by low seismicity. For this area the Italian seismic hazard map (Stucchi et al., 2011) assigns a maximum horizontal acceleration (rock site) up to 0.2 g (10% probability of exceedance in 50 yrs). In the last 4 years, the region was struck by 9 earthquakes in the magnitude range 4≤Mw≤5.0, with the three largest located in the Northern Apennines (Mw 4.9 and 5.0 Parma events, December 2008 and January 2012) and in the Po plain (Mw 4.9 Reggio Emila event of January 2012). We analyze the strongmotion data (distance < 300 km) from these events recorded by stations belonging to the INGV (RAIS, http://rais.mi.ingv.it; RSNC http://iside.rm.ingv.it) and DPC (RAN, www.protezionecivile.it; http://itaca.mi.ingv.it). The 2008 and 2012 Parma events, both characterized by reverse focal mechanisms (http://cnt.rm.ingv.it/), have depths of 27 and 60 km respectively. The deep event produced a maximum peak ground acceleration (PGA) of 97 cm/s2 at Novellara (NVL, EC8 C class) station (70 km from the epicenter). The 25th January 2012 event (depth of 34 km) produced a maximum PGA of 114 cm/s2 at Sorbolo (SRP) station (7 km from the epicenter). Preliminary analyses show: 1) a peculiar ground-motion attenuation of the deep Parma event with respect to the shallow one. In panel 2, the PGAs for the two Parma events are plotted as a function of hypocentral distance and compared to the global ground motion prediction equation (GMPE) calibrated by Cauzzi and Faccioli (2008) using events with depth < 30 km. The different distance-decay of PGA for the deep event (blue for A class of EC8 and red for B and C classes, CEN 2003) is evident, in particular for distance up to 100 km. On the other hand, the PGAs of the 2008 Parma crustal event (grey) are well explained by this GMPE. In panel 3, the PGAs for the deep 2012 event, grouped for EC8 classes, are compared to the national GMPE calibrated by Bindi et al. (2011) using crustal events and epicentral distance. Also in this case, the GMPE underestimates the PGAs up to 200 km. Although most of the class C sites (red) show the largest PGAs, the underestimation cannot be completely ascribed to site effects. The large PGAs from the Parma deep event, with respect to the shallow one, could be explained in terms of source effects (e.g. large stress drop values enhancing the high-frequency radiation). In addition, as explained by Castro et al. (2008), the different attenuation in the lower and upper crust could explain the large PGAs recorded for the 2012 deep event. 2) seismic amplification at Po Plain sites: In panel 4, the PGAs of the January 25th, Mw 4.9, Reggio Emilia event are plotted as a function of the epicentral distance, together with the Bindi et al. (2011) GMPE. In general, the largest amplitudes occur at the Po plain sites (red), suggesting possible peculiar site response. An overall increase of the PGAs is observed around 100km, in agreement with the results of Bragato et al. (2011) that studied the regional influence of Moho S-wave reflections in the area. An example of site response is shown in panel 5, considering TREG (class C) and ZEN8 (class A) stations (panel 5a), located at 88 km from the Reggio Emila epicentre. The rotational standard spectral ratio (panel 5b) for 10 s of S wave shows polarized amplifications around 2 Hz, detected also at others Po plain sites (not reported), as well as amplification on the vertical component. The points discussed above should to be interpreted as a warning for future applications dealing with ground motion estimation in the aftermath of an earthquake in this area (e.g. ShakeMap calculation): currently used GMPEs, based on different events and sites characteristics could lead to significant bias in the final results

Task 3 - Molise - Deliverable D7: Validation shaking scenarios.

The main goal of this report is the computation of the bedrock seismic motion at 5 municipalities located in the Molise area (Bonefro, S.Giuliano, Colletorto, S.Croce di Magliano, Ripabottoni, hereafter referred to as sites BNF, SGI, CLT, SCM and RPB, respectively). This area represents one of the validation case studies, planned in the framework of Project S3 which aim is the production of ground shaking scenarios for moderate magnitude earthquakes. Indeed, the recently occurred Molise earthquake represents a proper opportunity to compare synthetic simulations with real data. Acceleration time series were recorded during the October 31, 2002 and November 1, 2002 main shocks by analog and digital instruments managed by the Italian Civil Protection Department [DPC-SSN, 2004] while acceleration and velocity records were collected during the first month of seismic activity by DPC, INGV, INOGS, Dip.Te.Ris.(Genoa) (see §2.1 and Deliverable D6). Both strong and weak motion data were employed to infer regional ground motion prediction equations and spectral attenuation models (§2.3 and §2.4) while acceleration time series recorded during the first main shock by nearby stations were used to constrain the seismogenic sources of the October 31 and November 1, 2002 twin earthquakes (§4.1). Bedrock shaking scenarios at different level of complexity were produced by ground motion prediction equations (scenarios of level 0, §4.2), high frequency (f>1Hz) simulations (scenarios of level I, §4.3) and broad band (0-12 Hz) simulations (scenarios of level II, §4.4). Comparison of results obtained with different simulations methods confirms the complexity of the Molise area as regard to both seismogenic and attenuation properties of the crust. Especially for this area the ground motion prediction is constrained by the demand of simulations reproducing different features of the seismic wavefield. In particular, the input motion for site effect modelling, performed at sites located in the epicentral area, was computed with a broad band technique able to reproduce the complete wave field in the frequency band 0-10 Hz in terms of acceleration time series (scenarios of level II scenarios)

Qui INGV

L’INGV, a partire dal 2006, ha iniziato una fase di potenziamento del monitoraggio accelerometrico, installando nelle aree centrali della pianura padana 22 sensori strong-motion (Rete Accelerometrica Italia Settentrionale, RAIS, http://rais.mi.ingv.it/). Dal 2008, sensori accelerometrici sono stati via via installati in 105 siti a Rete Sismica Nazionale (RSN), gestita dal Centro Nazionale Terremoti (CNT). Nel complesso le 127 stazioni accelerometriche presenti sul territorio nazionale costituiscono a tutti gli effetti la rete accelerometrica nazionale INGV. I dati acquisiti da tutte le stazioni accelerometriche sono attualmente distribuiti in tempo reale tramite il portale EIDA (European Integrated Data Archive; http://eida.rm.ingv.it/) e sono principalmente utilizzati per il calcolo delle Shakemaps a scala nazionale. Attualmente, l’INGV sta realizzando un portale web per la distribuzione dei dati accelerometrici registrati dalle stazioni INGV, composto da 2 moduli distinti: il primo, denominato ISMD, ha lo scopo di archiviaziare e distribuire in tempo quasi reale (poche ore dopo l’evento) le forme d’onda accelerometriche in formato non corretto ed i relativi metadati ottenuti a seguito di una procedura di processamento automatico; il secondo, denominato DYNA, è una banca dati relazionale, contenente le forme d’onda di accelerazione, velocità e spostamento e gli spettri di risposta di accelerazione, ottenuti attraverso il processamento manuale dei segnali non corretti, oltre ai relativi metadati associati agli eventi sismici ed alle stazioni di registrazione Il prototipo del portale dei dati accelerometrici INGV (Figura 1) è stato pubblicato lo scorso maggio, a seguito della sequenza sismica Emiliana