SITE EFFECTS MODELING BY HYBRID TECHNIQUE: THE CASE STUDY OF SOFIA, BULGARIA 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY UNIVERSITA DEGLI STUDI DI TRIESTE DIPARTIMENTO DI SCIENZE DELLA TERRA BULGARIAN ACADEMY OF SCIENCECENTRAL LABORATORY FOR SEISMIC MECHANICS AND EARTHQUAKE ENGINEERING BULGARIAN ACADEMY OF SCIENCE IVANKA PASKALEVA PETIA DINEVA CRISTINA LA MURA GIULIANO F. PANZA

Main Topics HYBRID TOOL: a brief introduction; the viscoelastic isomorphism of Bardet model: advantages and implementation in the hybrid technique; simulation study: effect of the Bardet model, application to Sofia, Bulgaria; conclusions 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

Review….Hybrid Tools Site Effect: difference in earthquake ground motion between two nearby sites! Lateral heterogeneities may give rise to complex wave patterns, so high performance methods have to be developed for simulating seismic wave propagation in complex media. 27 o CONVEGNO GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY In hybrid computational schemes the numerical method is applied in the laterally inhomogeneous part of the medium,whereas the laterally homogeneous part is treated with analytical method. Combination of MSM and BIEM is the hybrid technique used in this work.

Review…..MSM - BIEM… lateral inhomogeneity surface topography water saturation in soils seismic source properties MSM is applied for numerical simulation from the source position to the local region of interest, while the BIEM is used inside the local multilayered geological region. 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY It has the potential to investigate the combined effects of different physical phenomena, like

VISCOELASTIC ISOMORPHISM… In order to describe the poroelastic properties of the layers, the viscoelastic isomorphism of Bardet is used. A two phase solid - fluid system (saturated system) is used its main components are: dry rock (dry soil) solid grain fluid n = Vp/V is the porosity V is the total volume of the solid Vp is the pore volume K g g K f f K dry dry =(1 - n) g bulk modulus and density 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

VISCOELASTIC ISOMORPHISM… Elastic properties of the saturated system sat = dry + n f = (1-n) g + n f density shear modulus = dry = sat = governing equation of motion in the frequency domain are describing the wave propagation in fluid saturated soils Biot (1956) 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

VISCOELASTIC ISOMORPHISM… Q and R are the Biots elastic constants for saturated soils ….BIEM has a limited usage in dynamic poroelasticity due to the multiphase nature of the model…… an equivalent viscoelastic model is proposed (Bardet 1992) saturated soil is considered as a viscoelastic material 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

VISCOELASTIC ISOMORPHISM… Bardet introduced the viscoelastic isomorphism by equating Wave numbers in Biots poroelastic model Wave numbers in viscoelastic model from this operation an equivalence between the poroelastic material and the linear Kelvin - Voigt viscoelastic material is established 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY so a two phase poroelastic dynamic behavior is approximated by the dynamic response of one-phase viscoelastic material

SIMULATION STUDY….. FORMULATION OF THE PROBLEM Let us consider a finite local saturated geological region imbedded in a vertical inhomogeneous half space where a seismic source is buried all soil layers in o are isotropic, anelastic and homogeneous the non-parallel layers in are water saturated 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. The main aim is to obtain synthetic seismograms at sites along the free surface of the region. TWO SUB-PROBLEMS external sub-problem: wave propagation from seismic source to the boundary internal sub-problem: wave propagation in the finite local geological region The two problems are combined through the fulfillment of the compatibility and equilibrium conditions at the boundary 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. 2 and 3 are water saturated, 1 is not saturated, points Ss are the sites where solutions are computed 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. EARTHQUAKE SCENARIO two earthquake scenarios have been used, they do differ for the value of the magnitude M strike angle 21 o dip angle 44 o rake angle 309 o source depth 12km src/rec dist 20km for both Scenario a) M= 3.7 Scenario b) M= o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. EFFECT OF THE BARDET MODEL peaks are shifted to lower frequencies main peak decreases for saturated soils earthquake scenario a) 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. EFFECT OF THE BARDET MODEL earthquake scenario a) 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

SIMULATION STUDY….. EFFECT OF THE BARDET MODEL earthquake scenario a) main peak decreases for saturated soils duration of the signal is reduced in saturated soils 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

one station called REDUTA from the residential district it is located in; here there is the Geo Milev Park together with a medical center, a library and one of the most prestigious schools of Sofia. CASE STUDY: SOFIA 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CASE STUDY: SOFIA 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY another called MLADOST4 for the same reason as the former; in this district the Business Park Sofia has been constructed and the airport is not so far

CASE STUDY: SOFIA The parametric study of site effects in two different water saturated geological profiles, corresponding to two different zones of the city, is made in terms of acceleration response spectral ratio, i.e. RSR = RS from signals along the laterally varying section RS from signals for the bedrock the single - degree - of - freedom damped oscillator used for the computation of the response spectra is with 5% damping the earthquake scenario is b), so M = o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CASE STUDY: SOFIA REDUTA dry soilsaturated soilhorizontal component vertical component 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CASE STUDY: SOFIA MLADOST4 dry soilsaturated soilhorizontal component vertical component 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CASE STUDY: SOFIA different response for different sites: there are site effects due to the mixed influence of lateral inhomogeneity and saturation of soil there is a shift of the peaks toward the higher periods for saturated soils the main peaks of saturated soils decreases with respect to dry soils the fluid acts as stiffener of the material: the pore pressure induced by the seismic load resists the compression and stiffens the rock fluid acts as a viscous damping element: a diffusion process dominated by viscous effects 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY

CONCLUSIONS The hybrid MSM - BIEM technique, combined with the viscoelastic isomorphism of Bardet, is applied for the investigation of the site effects phenomena due to several different physical properties like lateral inhomogeneity and water saturation in soil. The hybrid technique allows to study simultaneously the combined effects, hence a better estimation of the seismic risk is provided. The simulation has been performed for the city of Sofia; Soil mechanical properties and geotechnical parameters are those more common in the central part of the city. THANKS for the ATTENTION!!! 27 o CONVEGNO NAZIONALE GRUPPO NAZIONALE di GEOFISICA della TERRA SOLIDA October 8, 2008 TRIESTE - ITALY