Urban Habitat Constructions under Catastrophic EventsFINAL CONFERENCE. Naples, 16th- 18th September 2010Chair of the Action: Federico Mazzolani, IT, fmm@unina.itCOST Science Officer: Thierry Goger, tgoger@cost.esf.org

COST Action C26

The Mw 6.3, 2009 L’Aquila earthquake: linear and nonlinear site effects.

C. Nunziataa, F. Vaccarib, G.F. Panzab,c

aDipartimento di Scienze della Terra, Univ. Napoli Federico II, Italy; bDipartimento di Geoscienze, Univ. Trieste, Italy; cThe Abdus Salam International Centre for Theoretical Physics, ESP-SAND Group, Trieste, Italy.

CONCLUSIONSThe simple parametric study of the variation of response spectra with the strain-

dependent dynamic properties of soils, made using SHAKE, evidences that nonlineareffects may have affected the ground motion caused by the MW 6.3 L’Aquilaearthquake. Reduced spectral accelerations are due to (1) the amplitude dependentdamping and (2) the shift of resonance frequencies towards lower frequencies due tothe reduction of shear velocity. Nonlinearity may be considerable for soft clay and peatsoils (PGA≥0.1 g) and negligible for stiffer materials for PGA ≥0.2g. Moreover, it cannotbe rightly simulated by the use of higher damping.

REFERENCESCNR-GNDT, 2000. Cattaneo, M., Marcellini A. (eds.), Terremoto dell'Umbria-Marche:Analisi della sismicità recente dell'Appennino umbro-marchigiano. Microzonazione sismicadi Nocera Umbra e Sellano, 228 pp. + CD-ROM.De Luca, G., Marcucci, S., Milana, G., Sanò, T. 2005. Evidence of Low-FrequencyAmplification in the City of L’Aquila, Central Italy, through a Multidisciplinary ApproachIncluding Strong- and Weak-Motion Data, Ambient Noise, and Numerical Modeling. Bull.Seism. Soc. Am. 95, 1469–1481.EMERGEO Working Group 2010. Surface rupture of the 2009 L’Aquila earthquake (Italy).Terra Nova, 22, No. 1, 43–51.Guadagno, F. M., Nunziata, C., Rapolla, A. 1992. Dynamic parameters of volcaniclastic soilsand rocks of Campi Flegrei (Naples, Italy), Volcanic seismology, IAVCEI Proceedings involcanology, Springer-Verlag, 533-546.Panza, G.F., Romanelli, F., Vaccari, F. 2001. Seismic wave propagation in laterallyheterogeneus anelastic media: theory and applications to seismic zonation. Advances inGeophysics, vol. 43. Academic Press, 1–95.Pure and Applied Geophysics, 2004. Topical Volume “Seismic ground motion in large urbanareas”, 161, n 5/6, Panza G. F., Paskaleva I., Nunziata C. (eds.), ISSN 0033-4553,Birkhauser.Schnabel, P. B., Lysmer, J., Seed, H. B. 1972. SHAKE: a computer program for earthquakeresponse analysis of horizontally layered sites. Report No. UCB/EERC 72/12, EarthquakeEngineering Res. Center, Univ. California, Berkeley.Vinale, F. 1988. Caratterizzazione del sottosuolo di un'area campione di Napoli ai fini diuna microzonazione sismica, Rivista Italiana di Geotecnica, 22, 77-100.

Fig. 7. Acceleration response spectra computed at R19 and R38 receivers withSHAKE program by using as input the seismograms computed, with modesummation technique, considering: extended source with unilateral rupture;extended source with-bilateral rupture; point source. The fourth plot refers to thecase in which the PGA of the point-source signal has been conventionally takenequal to 0.1 g.

Fig. 6. Variation of shear modulus and damping withstrain: pozzolana (Guadagno et al., 1992); clay (CNR-GNDT, 2000); peat (Vinale, 1988).

Fig. 4. Spectral amplifications (RSR 2D/1D) along the cross section of the L’Aquila basin (Fig. 3). Response spectra are computed for

5% damping. From the top vertical, radial and transverse components of the computed ground motion.

Fig. 3. SW-NE cross section of the L’Aquila basin (from DeLuca et al., 2005). Legend: 1. Air; 2. Aterno river recentdeposits; 3. Megabreccias; 4. Upper lacustrine clays; 5.lower lacustrine clays; 6. Limestone. Receivers R19 andR38 are 6.75 km and 7.5 km far from the MW 6.3earthquake epicentre, respectively.

Fig. 1 (from http://www.mi.ingv.it/eq/090406/quest.html)

NONLINEAR EFFECTS

An estimate of possible nonlinear site effects can be obtained through a parametric study of response spectra dependence on shear moduli variations with increasing shear strain. Nonlinear site effects are estimated attwo receivers (R19 and R38 in Fig. 3) representative of the L’Aquila basin, with and without Aterno river deposits. SHAKE program (Schnabel et al., 1972) is used to compute seismograms on the top of the soil columns(limestone is assumed as bedrock), using as seismic input the 1D seismogram computed, at the specific receiver, with the modal summation technique, by assuming different sources, that is an extended source withunilateral rupture, an extended source with bilateral rupture, and a point source.

GROUND MOTION MODELING

Simulations of the 2009 L’Aquila earthquake have been performed with the NDSHA approach, that is a hybrid method consisting of modal summation and finite difference methods (Panza et al., 2001). The NDSHA approach has been used, among others, in the framework of the UNESCO-IUGS-IGCP project 414 "Seismic

Ground Motion in Large Urban Areas", to evaluate ground motion of a group of Large Urban Areas and Megacities in the world representative of a broad spectrum of seismic hazard severity (Pageoph, 2004). Modal summation is applied along the bedrock (1D) model that represents the average path between the

assumed source and the local, laterally heterogeneous (2D) structure beneath the area of interest. These signals are numerically propagated through the laterally varying local structure by the finite difference method.

A MW 6.3 earthquake struck on 6 April 2009, at 01.32GMT, the Abruzzi region (central Italy). The damagelevel generally corresponded to intensity ≤ 8 MCS,with few maximum values ≥ 9 MCS generallyassociated to construction vulnerability and, in somecases, to site amplification effects (Fig. 1). Theepicentral area of the main shock corresponds to theupper and middle Aterno river valley (Fig. 2).

Fig. 2. Geological map of L’Aquila with location (blue circle) of RAN

stations and the cross section (red line).

Fig. 8.

A remarkable difference between SHAKE (1D) and 2D (NDSHA) spectra exists, as expected, because of the lateral variations in the subsoil (Fig.7).

Response spectra computed with SHAKE by using the sand (pozzolana) soil nonlinear properties show nonlinear site effects (amplitude reduction and frequency shift to lower frequencies) more evident at

receiver R19 and mostly for a point source (PGA=0.2 g). Taking PGA equal to 0.1 g, for point source seismograms, strongly reduces such nonlinear effects.

A parametric study is performed at receiver R19by assuming the seismic input corresponding tothe point source and by attributing to the recentdeposits of Aterno river the nonlinear soilproperties of clay and peat. A noticeablenonlinear effect appears even in the point-sourcesignal with PGA conventionally taken equal to 0.1g (Fig. 8).

Strain dependent shear modulus (G/Gmax) and damping (D/Dmin) of sand (pozzolana), clay and peatare assigned to the Aterno river recent deposits, and of sand (pozzolana) to megabreccias and thelacustrine deposits (Fig. 6). Pozzolana sample is a cemented sand (hydrothermal hardening process)and can be considered as a stiff soil.

In the light of the good agreement betweensynthetic and recorded accelerograms nearbyAQV station, with maximum PGA (0.6 g), aparametric study of the expected groundmotion has been done along a geological crosssection through the town of L’Aquila (Figs. 2-3)by assuming extended and point sources.

The inadequacy of H/V spectral ratios is clearly seen even when they are computed from the

synthetics (Fig. 5).

Fig. 5. H/V spectral ratios along the cross section

Spectral amplifications of about 10 are computed in correspondence of the Aterno river alluvial

sediments (Fig. 4) and for a wide frequency range (0.5-5 Hz), corresponding to resonance

frequencies of several structure typologies. This result is very different from the maximum peaks of H/V spectral ratios at 0.6 Hz computed from

strong and weak earthquakes and noise along the same cross section (De Luca et al., 2005).