Strong motion data and empirical ground motion models for low seismicity areas: the case of the Po Plain (Northern Italy) Massa M. (1), Pacor F. (1), Bindi D. (2), Ameri G. (1), Luzi L. (1), Puglia R. (1) and Augliera P. (1) (1)Istituto Nazionale di Geofisica e Vulcanologia, Milano-Pavia, Italy (2)Helmholtz Zentrum Potsdam, Germany INGV Northern Italy strong-motion dataset The development of reliable Ground Motion Prediction Equations (GMPEs) with related uncertainties (sigma) is fundamental for probabilistic seismic hazard assessments. This aim is particularly difficult to achieve in regions of low seismic activity, due to the lack of instrumental data for moderate to large events. On the other hand, the use of global GMPEs in such context is not straightforward since they are generally developed for moderate to large magnitude events (M > 5). Thus, a quantitative comparisons of locally recorded ground motions with these models often require extrapolating the global models to a lower magnitude range. The selected case-study is the Po plain (Northern Italy), a large alluvial basin ( ~ km 2 ) with sediments depth varying from few hundred meters up to few kilometres. In the last 30 years, the Po Plain was characterized by the occurrence of thousands of small-energy events ( ~ 200/year) and few moderate earthquakes (e.g. 23 december 2008, Mw 5.4, Parma event, 17 July 2011, Mw 4.7, Ferrara event and 25 January 2012, Mw 4.9, Reggio Emilia event). However, in the past, the area was struck by relevant historical earthquakes (I up to X MCS). We construc a regional strong motion dataset including records from the 21 RAIS strong-motion stations (Augliera et al., 2011), installed by the INGV since 2006, and the 71 RAN (National Accelerometric Network) stations, operated by DPC (Italian Civil Protection). The Northern Italy dataset includes 236 events occurred since 1972 ( ~ components recors), in the magnitude range The preliminary results are presented in terms of residuals analysis, obtained considering the Italian GMPEs calibrated by Bindi et al. (2011), using data over the M range and R up to 200 km. The residuals, decomposed into the inter and intra-event components, allow to investigate the dependence of errors on magnitude and distance. Finally, merging the Italian strong motion data (M down to 3) and the NGA data (M up to 8), the effect of the minimum M on the calibration of GMPEs (for PGA) was preliminary evaluated. Corresponding : Merging ITACA and NGA data, different GMPEs were calibrated varyng the minimum M (step of 0.25). Two minimum values of M (3 and 5.25) were chose, while the maximum M was fixed to 8. The functional form used in Bindi et al. (2011) and in Boore and Atkinson (2008) was considered. Red: 2038 ITACA records Events: 390; M [ ]. R km. The inter-event component of variability decreases as the M increases (black NGA, grey ITACA) Comparison between Italian GMPEs (Bindi et al., 2011) and the Po Plain strong-motion data Blue: 2712 NGA records Events: 57 ; Mw [ ] R km Teleseisms as estimator of long period site amplifications? In the figure on the right, some examples of spectral analyses performed at selected Po Plain (Italy) stations are reported. As input the 2011, Mw 9.0, Tohoku-oki (Japan) recordings were used. Rotational Standard Spectral Ratio (<1 Hz) estimated at 2 Po Plain stations (CTL8 and MANT). LR=surface waves. DOI (Western Alps) was used as a reference station. (Massa and Augliera, 2012, submitted). Left top : historical earthquakes occurred in the area under study (from: Left middle : available earthquakes recorded by RAIS ( , and RAN ( ) strong motion networks. RAN data are availble at (Pacor et al., 2011) Left bottom : source station paths desumed from the available strong motion dataset Right panels : example of 25 January 2012, Mw 4.9, Reggio-Emilia recordings for a station installed on rock-site (ZEN8, red) and one instelled in the Po Plain (TREG, black). Both stations are located 88 km from the epicenter. Bottom : main features of the available North Italy strong motion dataset ( ). Analyses on a Po Plain subset (Mw > 4.0 and R < 200 km). As a reference the GMPes calibrated by Bindi et al. (2011) were used. Top left : total bias estimated as explained in Spudich et al. (1999). Positive values indicate an underestimation of the predictive models. Top right : dependence of inter-event error on magnitude. The trend indicates a slight underestimation of the model for lower magnitude. Bottom right : dependence of intra event error on distance. The trend indicates a relevant underestimation of the model with increasing distance. Residuals analysis on Po Plain data for SA at 1.0 s period The same analyses, as explained for PGA, were performed considering the elastic acceleration response spectra (5% damping) at period of 1.0 s. Top left : total bias Top right : dependence of inter-event error on magnitude. Bottom right : dependence of intra event error on distance. In general the agreement between observed and predicted data increases with increasing periods. Example 1 : 25 January, Mw 4.9 (depth 33 km), Po Plain earthquake (08:06:36 UTM) Example 2 : 24 January, Mw 4.2 (depth 10.3 km), Po Plain earthquake (23:54:46 UTM) Horizontal componentsVertical componentAll stations Long period site amplifications estimated by teleseisms Influence of minimum M on GMPEs calibration References : Bindi D., Pacor F., Luzi L., Puglia R., Massa M., G. Ameri and R. Paolucci, Ground Motion Prediction Equations Derived from the Italian Strong Motion Data Base, Bull. Earth. Eng, vol. 9, n. 6, Boore DM, Atkinson GM (2008) Ground-Motion prediction equations for the average horizontal component of PGA, PGV, and 5%-damped PSA at spectral periods between 0.01 s and 100 s. Earthq Spectra 24:99–138. P.L. Bragato, M. Sugan, P. Augliera, M. Massa, A. Vuan and A. Saraò, Moho reflection effects in the Po Plain (northern Italy) observed from instrumental and intensity data, Bull. Seism. Soc. Am., vol 101, 5, Massa M. and Augliera P. (2012), Teleseisms as estimator of long period site amplifications: example in the Po Plain (Italy) from the 2011, Mw 9.0, Tohoku-oki (Japan) earthquake, submitted to Seism. Res. Letters. Pacor F., Paolucci R., Ameri G., Massa M. and Puglia R., Italian strong motion records in ITACA: overview and record processing, Bulletin of Earthquake Engineering, DOI: /s x, vol. 9, n. 6, Paolo Augliera, Marco Massa, Ezio D'Alema, Simone Marzorati, RAIS: a real time strong-motion network in northern Italy, Annals of Geophysics, 54, 1, 23-34, doi: /ag Residuals analysis on Po Plain data for PGA Horizontal (geom. mean) Horizontal (1.0 s) 1 minute of recording Stations in the distance range km highlight, both on the horizontal and vertical components, high frequency amplifications, independently from the site category. The underestimation of GMPEs decreases with increasing periods. Bragato et al. (2011) indicate as in the Po Plain area the ground motion is affected by the S-wave Moho reflections in the distance range km. From Bragato et al. (2011) RAIS recording stations B site C site B site C site