Agreement between DPC – INGV Project S4 – Italian Strong Motion Data Base Coordinators: F. Pacor (INGV Milan) R. Paolucci (Politecnico di Milano) Seismic response of the Gubbio basin during the 26 September 1997 (Mw 6), Umbria-Marche (Central Italy) Earthquake M. Stupazzini, M. Pilz, L. Luzi, F. Pacor, R. Paolucci, S. Parolai, S. Pucci, and C. Smerzini

Progetto S4 – Wien, EGU, 20th April 2009 The Italian Project S4 (Agreement DPC-INGV ) Italian Strong Motion Database, coordinated by R. Paolucci e F. Pacor The main objective of this Project is to make available through the Internet an updated and improved release of the Italian accelometric database (ITACA), originally developed within project S6, in the framework of the DPC-INGV agreement. TaskTopic 1ITACA update 2Geological-geotechnical catalogue of ITACA sites 3Site characterization by surface waves methods 4Identification of anomalous sites and records 5Site classification One of the objectives of Task 4 concerns the numerical modelling of some of representative case studies, using up-to-date tools for 2D-3D seismic wave propagation and soil-structure interaction analyses (Stupazzini et al., 2009). Such numerical modelling will contribute to understand the physical reasons of the anomalies and the period range in which they mostly affect earthquake ground motion.

Progetto S4 – Wien, EGU, 20th April 2009 The Italian Project S4 (Agreement DPC-INGV ) Italian Strong Motion Database, coordinated by R. Paolucci e F. Pacor

Progetto S4 – Wien, EGU, 20th April 2009 Umbria Fault System and historical seismicity. Focal mechanisms and magnitudes are for the Colfiorito sequence, for the Norcia earthquake and for the Gubbio earthquake (from Mirabella et al., 2004). The Colfiorito event of 26th September 1997 ( , h9:40, M W 6.0) is shown with a red star. Seismic response of the Gubbio plain to the main shock of the 1997 Umbria-Marche sequence (Mw6)

Progetto S4 – Wien, EGU, 20th April 2009 Anomalous sites and records: Gubbio, Umbria-Marche Complete record of the horizontal component On the vertical component, identification of the onset of surface waves 2 Surface waves Onset of surface waves Response spectrum Complete record Surface waves removed

Progetto S4 – Wien, EGU, 20th April D Numerical Model by Spectral Elements Layer # H (m)V P [m/s] V S [m/s] ρ [kg/m 3 ]QSQS B B B B B Deep crustal model 3D Numerical model: Model size (approximately): 62 km X 86 km X 11km Spectral Degree = 4 (Order 5) fmax = 2 [Hz] # of elements = # of nodes = Tmax = 120 [s] Element size: min = 100 [m] max = 900 [m] t = 5.0e-4 [s]

Progetto S4 – Wien, EGU, 20th April 2009 Investigations after Gubbio, Umbria-Marche 1997

Progetto S4 – Wien, EGU, 20th April 2009 Investigations after Gubbio, Umbria-Marche 1997

Progetto S4 – Wien, EGU, 20th April D Numerical Model by Spectral Elements Alluvial Basin (homogenous)

Progetto S4 – Wien, EGU, 20th April 2009 HYPOCENTER M 0 [N m] L x W [km] Strike [deg] Dip [deg] Rake [deg] Depth of upper points [km] Vr [km/s] rise time τ [s] °N, °E 5700 m depth x Seismic source: h9.40 M W 6 Model M7 of Hernandez et al., 2004; available at the web-site slip pattern source time function

Progetto S4 – Wien, EGU, 20th April 2009 Movie of the Velocity Fault Normal comp.

Progetto S4 – Wien, EGU, 20th April 2009 PGDs and PGVs: comparison with GMPEs GBP GBB GBP GBB Alluvial Bedrock Cauzzi & Faccioli (2008) Akkar & Bommer (2007)

Progetto S4 – Wien, EGU, 20th April 2009 Velocity time histories: comparison with earthquake recordings GBPGBB

Progetto S4 – Wien, EGU, 20th April %-damping acceleration response spectra Distinctive features of ground motion recorded at GBP with remarkable ground motion amplification at long periods (T >~0.6 s). This anomalous behaviour, not observed at GBB, is well reproduced by the 3D numerical simulations.

Progetto S4 – Wien, EGU, 20th April 2009 Spectral ratios at GBP over GBB Simple numerical approaches based on 1D wave propagation turn out to be inadequate to reproduce the large amplification factors at long periods (T>~1 s), while a good agreement is found between the 3D numerical simulations and the observations.

Progetto S4 – Wien, EGU, 20th April 2009Conclusions - Thanks to the quality of the geophysical and geotechnical data available it is possible to constrain a 3D model of the Gubbio alluvial basin -Even if the 3D model here presented should be considered as a preliminary attempt, the agreement between the records of the sequence of the 1997 Umbria-Marche earthquake and the 3D numerical simulation seems to capture important features like the maximum amplification, the duration and the frequency content of the signals. - distinctive features observed at GBP with remarkable ground motion amplification at long period cannot be predicted by simple numerical approaches based on 1D wave propagation in layered media, while 3D numerical simulations turn out to be capable of reproducing the large amplification levels at long periods (T>~1 s). This clearly points out the need of advanced numerical analyses of wave propagation to reproduce such effects with a sufficient degree of reliability