ERROR DISTRIBUTIONS Ground Motion Prediction Equations derived from the Italian Accelerometric Archive (ITACA) Bindi + D., Pacor* F., Luzi* L., Puglia* R., Massa* M. and Paolucci^ R. + Helmholtz Centre Potsdam GFZ *Istituto Nazionale di Geofisica e Vulcanologia – Milano-Pavia ^Politecnico di Milano-Dipartimento di Ingegneria Strutturale ABSTRACT In the framework of the S4 project Italian Strong Motion Database funded by the Italian Civil Protection (DPC), a new version of the ITalian ACcelerometric Archive (ITACA, has been released, including revised strong-motion data recorded in Italy from 1972 to 2007 and some records from recent seismic events, in primis, the April 6, 2009 L'Aquila earthquake (Mw 6.3) and its main aftershocks. In this work, we use records from earthquakes in the magnitude range , recorded at distance smaller than 200 km, to derive ground motion prediction equations (ITACA-GMPEs) for Peak Ground Acceleration and Velocity (horizontal components) and ordinates of acceleration response spectra at 5% damping. To investigate the presence of regional features in the derived models, the predictions obtained with the ITACA-GMPEs are compared to the models recently derived for Turkey (Akkar and Çağnan, 2011) and based on global data-set (Atkinson and Boore,(2008). Finally, the inter-event, inter-station and record-to-record distributions of error are evaluated using a random effect approach (Abrahamson and Youngs, 1992), and the different components of variance are analyzed as function of the period. INGV RESULTS REGRESSION SCHEME DATA SET Earthquakes ( ): 218 Stations (analog and digital): 353 Records: 1213 (Analog 500 Digital 713) Key points i.compatibility of all corrected records ii.re-establish, after filtering, the original time scale (whenever feasible) iii.late triggered records are tagged and an ad-hoc procedure applied iv.comparisons with records from other sources (ESMDB, CESMD, PEER) Magnitude (Mw or Ml) versus distance scatter plot (points with Joyner-Boore distance less than 0.1km plotted at 0.1 km). Left: the circle colors indicate the EC8 site classification. Right: the circle colors indicate the style of faulting. ITACA data processing 1.baseline correction (constant de-trending); 2.application of a cosine taper, based on the visual inspection of the record; records identified as late- triggered are not tapered; 3.visual inspection of the Fourier spectrum to select the band-pass frequency range; application of a 2nd order acausal frequency-domain Butterworth filter to the acceleration time-series; 4.double-integration to obtain displacement time series; 5.linear de-trending of displacement; 6.double-differentiation to get the corrected acceleration. The ground motion prediction equations have been derived over the magnitude range , considering epicentral distances up to 200 km and hypocentral depths smaller than 30 km. The epicentral distance (Repi), for M < 5.5 events, and the Joyner-Boore distance (Rjb) for stronger earthquakes are considered, based on the fault geometry data available in the DISS database (DISS Working Group, 2009). Nine 3-component records with epicentral distance Repi 10 km are available in the range 5.9 Mw 6.3 (5 from the 2009, M 6.3, L'Aquila earthquake, 3 from the Friuli aftershocks of September 1976, and 1 from the Umbria-Marche September 1997 mainshock). The strongest events in ITACA, i.e., the Mw 6.4 May Friuli and the Mw 6.9 November Irpinia earthquakes, were recorded at Repi > 10 km. For the strongest earthquakes, the focal mechanisms were assigned following the classification of Zoback (1992). Most events with magnitude less than 5 have unknown focal mechanisms. SITE CLASSIFICATION All station in ITACA were classified following the EC8 (CEN 2004). Measurements of Vs30 are available for 131 stations, 50 of which were characterized in the S4 Project, using surface waves methods. Among stations with Vs30, according to the EC8, 15% were classified as A, 49% B, 25 % C, 6% D and 5% E. All the other stations are classified only based on the existing geological/geophysical information and denoted by a star (*) in the ITACA database. CATANIA STATION METHOD: MASW Vs30 = 160m/s CLASS D GRUMENTO NOVA STATION METHOD: ESC-FK Vs30 = 283 m/s CLASS C Explanatory variables: Mw, R JB, style of faulting and site classifications (only linear site terms) Response variables Y: PGA, PGV, SA (5%, 0.04T 4sec) Components: GeoMean of the horizontal components; vertical component MMhMMh M>M h The regressions are performed by applying a random effect approach (Abrahamson and Youngs, 1992) to evaluate the inter-station (a) and inter-event error distributions (b) Earthquake i recorded at station k Inter-event and inter-station distributions of error and : they assume a value for each earthquake and for each station and describe the correlation among the errors for different recordings of the same earthquake and the same station. They are normal distributions with standard deviation equal to eve and sta Intra-event distribution of error : it assumes a value for each recording. It is a normal distribution with standard deviation equal to rec-rec. Magnitude term Distance term (e.g. Boore and Atkinson, 2008) Model Median predictionError distributionsObservationError distributions 0.1s 0.5s 2.0s TOTAL STANDARD DEVIATION ab COMPARISON WITH DATA Irpinia 1980 LAquila 2009 TURKEY (red) Akkar and Cagnan 2011 (AK11) GLOBAL (red) Boore and Atkinson 2008 (BAT08) COMPARISON WITH OTHE MODEL Comparisons between predictions and observations for the strongest normal earthquakes occurred in Italy), for SA ordinates at 0.1 and 1s. The comparisons are shown for three EC8 soil classes A, B, and C Comparisons between predictions (PGA, SA at periods 0.1s and 1s) from regional and global GMPEs and from Italian model for LAquila earthquake (M 6.3, normal fault). The observations at rock site are also reported. Observations versus Ak11 (left) and BAT08 predictions (class A – 760 m/s) for PGA and SA at T= 1s as a function of Magnitude (Top) and distance (Bottom). For BAT08, the overall biases are negative, denoting a general overestimation of the prediction, while the biases are negligible for AK11. A strong dependence is observed with distance, particularly at high frequency and for Bat08, which means that Italian data attenuate faster than the Global and Turkey predictions Residual = log10(Y obs ) – log10(Y pred ) Inter-event (left) and inter-station error (right) distributions, at different periods, distinguished for focal mechanisms and soil classes. Most earthquakes and stations show errors within the ±0.3 range. The largest inter-station errors are at short period, while the largest inter- event errors are for small magnitudes ( < 4.5) and un- known focal mechanisms The total standard deviation of Italian GMPE is about 0.4. Unlike other GMPEs, the dominant components of variance are related to the terms inter-event and inter-station. At long periods and up to 0.3s, the two components are approximately the same sizes, at shorter periods the inter- station variance assumes higher values (about 0.3) 50 bootstrap replications COEFFICIENTS Applied Constraints e A =0 (class A used as reference) f U =0 and sum(f i )=0 b3=0 (i.e. F(M) const for M>Mh) INTER- STATION ERROR The station error normalized to the inter-station standard deviation can be used to identify stations with anomalous response site (different by the average behavior of their site class) Example: Catania Piana station belong class D (vs30 = 160 m/s). This station strongly amplify the long period (horizontal spectral ordinates exceed the standard dispersion band) and de- amplify the short periods (< 0.2 s)