Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale, Università degli Studi dell Aquila, Italy 2

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1 Comparison between shallow geology, resonance frequency and buildings damage due to the April 6, 2009 Mw 6.1 and the February 2, 1703 Mw 6.7 earthquakes at L Aquila historical downtown M. Tallini 1, F. Durante 1, L. Macerola 1, M. Nocentini 2 1 Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale, Università degli Studi dell Aquila, Italy 2 Dipartimento di Scienze, Università degli Studi Roma Tre, Italy Introduction. For L Aquila historical downtown, the comparison between the areal distribution of the shallow geology, the resonance frequency (f 1 ) and the buildings damage distribution caused by the recent April 6, 2009 (Mw 6.1) and the more severe February 2, 1703 (Mw 6.7) earthquakes is presented. This comparison allows to understand deeply the seismic site characterization of a high seismic hazard urban area of Central Apennines, such as L Aquila historical downtown (i.e. the urban area encircled by the mediaeval walls). Geological setting and seismostratigraphy. The L Aquila historical downtown stratigraphy is composed downward by: i) 2-20 m thick of Holocene-Upper Pleistocene soft soil (Red silt Fm.); ii) m thick of calcareous breccia (Middle Pleistocene) which represents the relief on which the L Aquila historical downtown lays; iii) 200 m thick of pelite and sand (Lower Pleistocene); iv) Meso-Cenozoic carbonate bedrock (Del Monaco et al., 2013; Gruppo di Lavoro MS AQ, 2010; Nocentini, 2016; Tallini et al., 2016a, 2016b).The HVNSR analysis of about 300 microtremor recordings identifies two resonance frequencies, f 0 ( Hz) and f 1 (3-10 Hz). 1D and 2D numerical simulations evidences that the origin of f 0 and f 1 is caused by two main geophysical impedance contrasts. f 0 is due to the superposition of pelite and sand 401

GNGTS 2016 Sessione 2.2 (Madonna della Strada Synthem, Nocentini, 2016) onto the Meso-Cenozoic carbonate bedrock at 200-300 m below ground level.

2 GNGTS 2016 Sessione 2.2 (Madonna della Strada Synthem, Nocentini, 2016) onto the Meso-Cenozoic carbonate bedrock at m below ground level. f1 is caused by shallower impedance contrast due to 3-20 m thick of Red silt onto calcareous L Aquila breccia (Colle Macchione-L Aquila Synthem of Upper Middle Pleistocene, Marine Isotope Stages MIS - 6-8) (Nocentini, 2016; Tallini et al., 2016a). The Red silt corresponds to a colluvium and karst deposits made of reddish to dark brown clayey silts with sparse sub-angular clasts. These deposits show thickness up to 20 m and are generally interpreted as re-worked and colluviated paleosol (Alfisol) mantling the epikarst developed onto the L Aquila breccia in a wet and warm interglacial stage (Last Interglacial, Eemian, MIS 5e) (Nocentini et al., 2016). The L Aquila breccia is composed by clast- to matrix-supported, massive or stratified, both well cemented and incoherent debris flow and rock avalanche breccias and megabreccias, with highly heterometric (up to m3), poorly sorted, angular to sub-angular carbonate blocks in a whitish calcareous sandy silty matrix. Whitish to greyish calcareous clayey silts levels and lenses are interlayered at different elevations (Nocentini, 2016; Nocentini et al., 2016). While the seismic significance of f0 was already analyzed in detail (Bordoni et al., 2014; Del Monaco et al., 2013; Gruppo di Lavoro MS AQ, 2010), then our study was focused on the relationship between the shallow subsoil model, the medium resonance frequency (f1) and the buildings damage distribution due to the recent April 6, 2009 (Mw: 6.1) and the February 2, 1703 (Mw: 6.7) earthquakes. Results and discussion. We elaborated via ArcGis the map of the bottom surface of the Red silt, the maps of the f1 contour lines and the building damage distribution due to the above mentioned earthquakes (Clementi and Piroddi, 1986; Tertulliani et al., 2011). The contour line pattern of Red silt matches enough with that of f1 (Figs. 1 and 2). To analyze deeply the relationship between the Red silt thickness and f1 we select about ten sites which the subsoil model is well known (Tab. 1). We apply for the sites reported in Fig. 1 and Tab. 1 the following equation: f1=vs/4hcal Fig. 1 Comparison of the Red silt thickness and the damaged buildings due to the earthquake of February 2, 1703, Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to the heavy buildings damage or buildings collapse, respectively; the letters refer to the sites of Tab

Sessione 2.2 GNGTS 2016 Tab. 1 Comparison between the Red silt thickness obtained by borehole investigation (Hbor) and the Red silt thickness calculated with the equation f1=vs/4h (Hcal).

3 Sessione 2.2 GNGTS 2016 Tab. 1 Comparison between the Red silt thickness obtained by borehole investigation (Hbor) and the Red silt thickness calculated with the equation f1=vs/4h (Hcal). The site location is reported in Fig. 1. site f1 (Hz) Hbor (m) Hcal (m) Hbor-Hcal (m) TE DU BE CA BI SB CE CS DA CF RO where, for each site, f1 is the above mentioned resonance frequency, Vs is the shear wave velocity of the Red silt (around 325 m/s as obtained from down hole investigations, Del Monaco et al., 2013) and Hcal is the thickness of the Red silt. For each site, we note a good agreement between Hcal and the red soil thickness found in the borehole (Hbor). So f1 is caused by the seismic impedance contrast due to the superposition of the Red silt onto L Aquila breccia which has a Vs around 800 m/s. Moreover, to understand in-depth if the seismostratigraphy had a role on the buildings damage due to the two mentioned earthquakes, we compare the areal distribution of f1 with that of buildings damage (very heavy damage and building collapse) (Figs. 1 and 2). We note that the 80% of the heavy damaged buildings are placed in sites characterized by f1 in the range of 3-10 Hz (Fig. 3). But, considering the April 6, 2009 earthquake, the majority of the buildings of L Aquila historical downtown have a resonance frequency also in the range of 3-10 Hz (F. Di Fabio, pers. com.). The sites of L Aquila historical downtown, where the subsoil resonance Fig. 2 Comparison of the subsoil resonance frequency (f1) and the damaged buildings due to the earthquake of February 2, 1703, Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to severe buildings damage or collapse, respectively. 403

4 Fig. 3 L Aquila historical downtown buildings damage caused by 1703 (A) and 2009 (B) earthquake vs f1 resonance frequency. Data of buildings damage due to the 1703 and 2009 earthquakes are from Clementi and Piroddi (1986) and Tertulliani et al. (2011), respectively. Blue and red colors refer to severe buildings damage or collapse, respectively. frequency (f1) is comparable in the range of 3-10 Hz to the resonance frequency of the building we note the presence of heavy damaged buildings (Fig. 2 and Fig. 3). So we can hypothesize the coupling between the resonance frequency of subsoil (f 1 ) and that of the damaged building because they have the same range of values (3-10 Hz). In the case of the February 2, 1703 earthquake, we note a similar areal distribution of the heavy damaged buildings of the April 6, 2009 earthquake, hypothesizing a quite similar cause of the building damaging for the February 2, 1703 earthquake. Conclusions. The main conclusions are summarized as follows: the resonance frequency f 1 is caused by the seismic impedance contrast due to the superposition of the Red silt onto L Aquila breccia (Tab. 1); the damaged/collapsed buildings caused by 1703 and 2009 earthquakes are scattered in areas with subsoil resonance frequencies in the range of 3-10 Hz (Figs. 1 and 2); taking into account the April 6, 2009 earthquake, the 80% of the heavy damaged buildings are placed in sites characterized by f 1 in the range of 3-10 Hz (Fig. 3); the majority of the buildings of L Aquila historical downtown have a resonance frequency also in the range of 3-10 Hz, so the coupling between the resonance frequency (f 1 ) of subsoil and that of the buildings could be hypothesized because they have the same range of values (3-10 Hz). The ongoing study will investigate this proposed possible coupling phenomena between the resonance frequency of the shallow subsoil model and that of the buildings of L Aquila historical downtown through GIS zonal statistics analysis and the indirect evaluation of the resonance frequency of the buildings damaged by the February 2, 1703 earthquake. 404

5 References Bordoni P., Del Monaco F., Milana G., Tallini M., Haines J.; 2014: The Seismic Response at High Frequency in Central L Aquila: A Comparison between Spectral Ratios of 2D Modeling and Observations of the 2009 Aftershocks. Bulletin of the Seismological Society of America, 104(3), , doi: / Clementi A., Piroddi E.; 1986: L Aquila. Laterza, Roma-Bari. Del Monaco F., Tallini M., De Rose C., Durante F.; 2013: HVNSR survey in historical downtown L Aquila (central Italy): site resonance properties vs. subsoil model. Engineering Geology, 158, Gruppo di Lavoro MS AQ; 2010: Microzonazione sismica per la ricostruzione dell area aquilana. Regione Abruzzo Dipartimento della Protezione Civile, L Aquila, 3 vol., Cd-rom. Nocentini M., Tallini M., Cosentino D., Gliozzi E., Asti R., Durante F., Macerola L.; 2016: Plio-Quaternary geological map of the L Aquila-Scoppito Basin (central Italy). Rend. Online Soc. Geol. It., 40(1), 329. Nocentini M.; 2016: Integrated analysis for intermontane basins studies: tectono-stratigraphic and paleoclimatic evolution of the L Aquila Basin. PhD thesis in Environmental and Resources Geology, University of Roma Tre (XXVII cycle). Tallini M., Durante F., Macerola L., Nocentini M.; 2016a: Does a seismic double-resonance affect L Aquila historical downtown buildings? Rend. Online Soc. Geol. It., 40(1), 685. Tallini M., Durante F., Macerola L., Nocentini M.; 2016b: Up-to-date L Aquila town subsoil model for seismic site and hazard evaluation: boreholes and single station microtremor data to investigate the Pettino Fault. Rend. Online Soc. Geol. It., 40(1), 686. Tertulliani A., Arcoraci L., Berardi M., Bernardini F., Camassi R., Castellano C., Del Mese S., Ercolani E., Graziani L., Leschiutta I., Rossi A., Vecchi M.; 2011: An application of EMS98 in a medium-sized city: the case of L Aquila (Central Italy) after the April 6, 2009 Mw 6.3 earthquake. Bulletin of Earthquake Engineering, 9,

Paper. Paola BORDONI, Giuliano MILANA, John HAINES, Marco TALLINI, Francesco DEL MONACO

Paper. Paola BORDONI, Giuliano MILANA, John HAINES, Marco TALLINI, Francesco DEL MONACO Paper THE ROLE OF LATERAL HETEROGENEITIES AND REVERSAL OF VELOCITY AT L AQUILA TOWN (CENTRAL APPENNINES, ITALY) FROM A COMPARISON BETWEEN 2D MODELING AND OBSERVATIONS FROM THE 2009 EARTHQUAKES SEQUENCE