Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

PROBABILISTIC SEISMIC HAZARD ASSESSMENT OF THE EUROPEAN EXTREMELY LARGE TELESCOPE

("E-ELT") PROJECT (CHILE)

M. Corigliano1 , C.G. Lai2, L. Scandella3, E. Spacone4, G. Camata5, C. Cantagallo6, D. Spallarossa7 and P. Ghiretti8

ABSTRACT The European Extremely Large Telescope (E-ELT) is a planned ground-based extremely large telescope to be built by the European Southern Observatory (ESO) as an integrated part of the Paranal Observatory in Chile. The design includes a 42m-class reflecting telescope, it will be the largest optical/near-infrared telescope in the world and will gather 13 times more light than the largest optical telescopes existing today. A probabilistic seismic hazard assessment (PSHA) at the site was carried out with the scope of defining the seismic input for the conceptual design of seismic isolation system. The PSHA study was conducted for free-field conditions assuming flat topographic surface on rock site ideally representing outcropping bedrock conditions, and it is aimed at defining the Uniform Hazard Spectra for different return periods. The standard Cornell and a zone-free approach have been used for hazard computations, the zone-free method has been considered to overcome the subjective definition of seismogenic zoning inherent in the Cornell approach. The seismotectonic setting of the area has been examined and a composite earthquake catalogue has been compiled. Epistemic uncertainty in the seismic hazard was

1Civil Engineer, Italy, PV 27100 2Associate Professor, Dept. of Civil Engineering and Architecture, University of Pavia, Italy, PV 27100 3 Researcher, Geotechnics and Engineering Seismology Unit, Eucentre, Pavia, PV 27100 4 Professor, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy. E-mail address: espacone@unich.it 5 Assistant Professor, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy. E-mail address: g.camata@unich.it 6 PhD, Post-Doc, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy. E-mail address: cristina.cantagallo@unich.it 7Associate Professor, Dept. of Earth Science, Environmental and Life, University of Genoa, Italy, GE 16132 8 Civil Engineer, European Southern Observatory (ESO) Corigliano M, Lai CG, Scandella L, Spacone E, Camata G, Cantagallo C, Spallarossa D, Ghiretti P. Probabilistic Seismic Hazard Assessment of the European Extremely Large Telescope ("E-ELT") Project (Chile). Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

addressed within a logic-tree framework. The results of PSHA are given in terms of UHS between 0 and 3 s for 4 return periods (36-, 475-, 975-, and 1898-yr).

Probabilistic Seismic Hazard Assessment Of The European Extremely Large Telescope ("E-ELT") Project (Chile)

M. Corigliano 1 C.G. Lai2, L. Scandella3, E. Spacone4, G. Camata5, C. Cantagallo6, D. Spallarossa7 and P. Ghiretti8

ABSTRACT The European Extremely Large Telescope (E-ELT) is a planned ground-based extremely large

telescope to be built by the European Southern Observatory (ESO) as an integrated part of the Paranal Observatory in Chile. The design includes a 42m-class reflecting telescope, it will be the largest optical/near-infrared telescope in the world and will gather 13 times more light than the largest optical telescopes existing today.

A probabilistic seismic hazard assessment (PSHA) at the site was carried out with the scope of defining the seismic input for the conceptual design of seismic isolation system. The PSHA study was conducted for free-field conditions assuming flat topographic surface on rock site ideally representing outcropping bedrock conditions, and it is aimed at defining the Uniform Hazard Spectra for different return periods. The standard Cornell and a zone-free approach have been used for hazard computations after the compilation of a composite earthquake catalogue. Epistemic uncertainty in the seismic hazard was addressed within a logic-tree framework.

Introduction Chile due to its location along the convergent margin between Nazca and South American plates presents an high level of seismicity which represents one of the most important factors contributing to natural hazards in this region. This article describes a state-of-the-art probabilistic seismic hazard analysis (PSHA) aimed at producing uniform hazard spectra (UHS) for the E-ELT Telescope located in Ventarrones

1 Civil Engineer, Italy, PV 27100

2Associate Professor, Dept. of Civil Engineering and Architecture, University of Pavia, Italy, PV 27100 3 Researcher, Geotechnics and Engineering Seismology Unit, Eucentre, Pavia, PV 27100 4 Professor, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy, PE 65127 5 Assistant Professor, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy,PE 65127 6 PhD, Post-Doc, Dipartimento di Ingegneria e Geologia, University G. DAnnunzio of Chieti-Pescara, Italy, PE 65127 7Associate Professor, Dept. of Earth Science, Environmental and Life, University of Genoa, Italy, GE 16132 8 Civil Engineer, European Southern Observatory (ESO) Corigliano M, Lai CG, Scandella L, Spacone E, Camata G, Cantagallo C, Spallarossa D, Ghiretti P. Probabilistic Seismic Hazard Assessment of the European Extremely Large Telescope ("E-ELT") Project (Chile). Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

(Chile) (Latitude -243990N, Longitude -702075E). Two different computation techniques have been adopted in the study: the method developed by Cornell (1968), which is the most widely utilized approach for probabilistic hazard assessment, and the zone-free method by Woo (1996) translated into the computer code Kerfract. The two approaches essentially differ in the definition of the seismic sources and their recurrence characteristics. The Cornells method is a zone-dependent approach in which seismotectonic and geological data are used along with an earthquake catalog to identify seismogenic zones within which earthquakes occur at rates defined by a recurrence relationship and are assumed to have the same probability of occurrence at any location (homogeneous sources). The zone-free method by Woo (1996) is an alternative approach that overcomes the subjective definition of seismogenic zoning inherent in the Cornells approach. The method does not require the definition of seismogenic zones, seismic sources are used but not defined based on seismotectonic or geological criteria. A grid of point sources is defined about the site of interest with the activity rate of each point derived from the earthquake catalog. The original Kerfract code has been modified introducing a regional depth distribution to account for the subduction of the investigated area. Recent works where the zone-free method by Woo (1996) is used as an alternative to Cornells approach in PSHA include the studies by Zuccolo et al. (2013) for Italy, Bozzoni et al. (2011) for the Eastern Caribbean region and Menon et al. (2010) for the Indian State of Tamil Nadu. In the following, the former procedure will be referred to as the Cornell approach, whereas the latter will be referred to as the zone-free approach. The seismotectonic setting of the area around Ventarrones has been examined to identify seismogenic zones reflecting the major causes of seismicity in the region. Epistemic uncertainty in hazard definition has been addressed within a logic-tree framework considering four different parameters: method of calculation, GMPE, catalogue completeness and maximum magnitude. The results of PSHA are given in terms of UHS computed for the horizontal component of ground motion, assuming rock and level site conditions, for 4 return periods (36-, 475-, 975-, and 1898-yr) and structural periods between 0 and 3 s.

Seismotectonic Setting and Composite Earthquake Catalog This section gives a brief overview of two key elements for PSHA, namely the seismotectonic setting and the definition of the composite earthquake catalogue. The Andean Cordillera is the classic example of a mountain chain formed during the subduction of an oceanic slab under a continental plate. The present-day Andes are principally the result of an orogenic process that started in the Late Oligocene after a major reorganization of the oceanic plates in the eastern Pacific (e.g. Tebbens and Cande, 1997). More than ten events with magnitude equal or greater than magnitude 8 have taken place along the South America margin during the twentieth century alone. Among these earthquakes is the 1960 events, the largest earthquake ever recorded since the beginning of the instrumental seismology. Such extreme seismic activity is a result of the subduction of the Nazca plate under the south America plate at a present rate of 80 mm/yr (DeMets et al., 1994). In general, seismogenic zones in Chile are basically well established: large shallow (0-50 km) thrust earthquakes along the coast (interplate events); large deeper (70-100 km) tensional as well as compressional events (intraplate or inslab events) within the subducting Nazca Plate; and very shallow seismicity (0-20 km) in a few places, such as the cordillera region in Central Chile and the southern extremity of the continent.

Deeper seismicity (150 to 650 km) occurs beneath Bolivia and northwestern Argentina. The earthquake catalog represents the starting point for both the Cornell and the zone-free approaches. A composite, homogeneous, updated and well-defined earthquake catalog has been compiled for the area around Ventarrones. The reference catalog for the Andean region has been developed by CERESIS (Centro Regional de Seismology para Amrica del Sur) which covers the period from 1471 to 1991, it represents the first unified catalog and contains a collection of historical and recent events of South America. It includes two sub-catalogs, one of the "hypocenters", that mentions only the values of magnitude, and one of the "intensities", which, although less numerous, provides both the values of magnitude and intensity (http://www.ceresis.org/new/es/index.html). This catalog has been updated to include the period from 1991 to 2009. Data have been retrieved from the most accredited databases: the National Seismological Service of the University of Chile (SSN), the National Earthquake Information Center (NEIC), which includes different database (i.e. PDE, NOAA, SISRA, Centennial), the International Seismological Centre (ISC), Advanced National Seismic System (ANSS), and National Geophysical Data Center (NGDC). Duplicate events have been eliminated by hand from the newly compiled catalog. Because one or more of the different magnitude scales (MS, mb, ML, MW) were assigned to a single event in the various accessed databases, it has been necessary to homogenize the measures of magnitude in the composite catalog into moment magnitude MW. Empirical relationships have been adopted to convert MS, mb, and ML to MW. For MS and mb it has been possible to purposely derive such empirical relationships thanks to the availability in the composite catalog of independently estimated pairs MS-MW and mb-MW, respectively. A magnitudeintensity relationship has been purposely developed in this study, based on 116 earthquakes from 1936 to 2008 with independent measures of I0 and moment magnitude (Mw) identified from different sources. The conversion relationships used are reported in Table 1. Table 1. Adopted Conversions between MW and the Other Magnitude Scales.

Magnitude Type Conversion MS (developed in this study) MW=0.6934 MS +2.2316 r

2=0.9314 =0.15

mb (developed in this study) MW=1.402 mb -1.866 r2=0.704 =0.25

ML (Kanamori, 1983) MW ML

Io (developed in this study) MW=0.299 Io +4.685 r2=0.744

The composite catalogue constructed for the area of Ventarrones considers all events with a maximum magnitude greater than 4. After the conversion to moment magnitude the composite catalogue includes 4044 events with Moment Magnitude (MW) between 3.38 and 8.13. In order to consider events with engineering interest, only events with MW 4.5 have been considered for the seismic hazard assessment. Finally, the composite catalogue defined for the area of Ventarrones includes 3081 earthquakes occurred between 1615 and 2009.

PSHA by the Cornell Method

Seismogenic zonation The study area has been divided into 3 seismogenic provinces to properly account for the complex tectonic setting of the region, as shown in Figure 1. The geometry of each zone was established based on the understanding of the active tectonics and on the location of historical and instrumental earthquakes. More in detail, such a partition was based on the following criteria: seismotectonic models adopted in previous seismic hazard studies; geological and geophysical information; patterns of clusters of recent earthquakes epicenters.

Figure 1. Earthquake catalog and seismotectonic provinces used in this study. Two vertical

cross section are also shown. In particular, as in the investigated area the shallow crustal seismic activity (depth < 20 km) is negligible, the seismogenic model presented in this study is mainly based on the seismotectonic features of the Nazca slab (e.g. seismic activity rate, slab dip, style of faulting). In the Province 1 almost all of the earthquakes are located along a narrow band of concentrated seismicity related to the subducted Nazca plate, defining the Wadati-Benioff Zone (WBZ). The upper limit of the WBZ is particularly sharply defined. The WBZ dips with an angle of 17-18

up to about 100 km in depth. Delouis et al (1996) defined in this area a Locked Zone, a seismically coupled plate interface from 20 to 50 km in depth, characterized by underthrusting and localized reverse faulting earthquakes. Underthrusting with a nodal plane dipping slightly towards the east is characteristic of the upper part of the Locked Zone between 20 and 35 km in depth. Focal mechanisms in the range 35-50 km in depth are rather of reverse type, with nodal planes dipping in a steeper way towards the east. The tectonic regime of the crust seems to have been mostly extensional for a long time (since Miocene) with repeated ruptures under EW extension in the very recent time. The tectonic pattern of Province 2 is similar to that of the Province 3, the seismicity of this area is characterized by higher activity rates for intermediate intensity earthquakes. The focal mechanism shows predominantly normal faulting and are characterized by a variability in the azimuth of the nodal planes. From a study on stress patterns and moment release of intermediate depth seismicity, Chen et al (2001) found out that there are variations in slab dip along the subducting Nazca Plate and in this zone the WBZ dips with an angle of 27. In this Province majority of the seismic events were between 100 and 150 km depth. The province 3 is located between 23.7 and 27 S and is defined on the basis of the transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane at about 80 km in depth. Delouis et al (1996) found that the stress axes in the Locked Zone are oriented in the convergence direction (75-80E). In this zone focal mechanisms show predominantly normal faulting, but some strike-slip faulting is observed. In this province the majority of the seismic events were between 70 and 100 km depth. Processing of earthquake catalogue

Declustering After the compilation of the earthquake catalog, declustering has been performed to remove aftershocks and foreshocks to make the catalog consistent with the Poisson earthquake occurrence model adopted in the classical Cornell approach. The declustering algorithm developed by Gardner and Knopoff (1974) for southern California has been used in the analyses. This algorithm assumes that time and spatial distribution of foreshocks and aftershocks are dependent on the magnitude of the main event. After the declustering the composite catalogue includes 769 events. Completeness A second important step in processing an earthquake catalog to make it suitable for a PSHA is the definition of the time windows for which the catalog is considered to be complete (i.e., completeness periods) for different magnitude bins (M=0.5) and each seismogenic zone. In fact, accounts for historical earthquakes are usually more complete for larger earthquakes than for smaller events. Small earthquakes can go undetected for a variety of physical and demographical reasons. Two different methods, namely the visual cumulative method (Tinti and Mulargia, 1985) and the method by Stepp (1973) have been used to calculate the completeness periods. Completeness analyses have been performed using the declustered catalog for the whole area under study and for each individual seismogenic zone. Table 2 and Table 3 summarize the completeness period obtained with the Visual Cumulative method and the Stepp method respectively, considering the three seismogenic zone and also the whole area without the

subdivision in seismogenic zone. Recurrence Characteristics and Maximum Magnitudes The completeness periods listed in Table 2 and Table 3 have been used to compute the GutenbergRichter magnitude-frequency recurrence relationships for each seismogenic zone. Recurrence relationship have been computed after dividing the earthquake magnitude in different magnitude range of amplitude equal to 0.5. The Gutenberg-Richter recurrence relationships, based on the results of the two completeness period methods (viz. Visual Cumulative and Stepps method) have been estimated. In the present study two value of the maximum magnitude have been considered: the maximum historical magnitude and the previous value increased of 0.5. Table 4 and Table 5 summarize the Gutenberg-Richter parameters using Visual Cumulative and Stepps method respectively. Table 2. Completeness period estimated with the Visual Cumulative Method

MW Zone 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25

1 1966 1979 1976 1966 1968 1927 1922 1902 2 1989 1984 1982 1968 1951 1928 1916 - 3 1989 1979 1984 1947 1934 1927 - -

All 1967 1979 1976 1966 1954 1923 1916 1902 Table 3. Completeness period estimated with the Stepps Method

MW Zone 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25

1 1966 1979 1976 1966 1968 1927 1922 1902 2 1989 1984 1982 1968 1951 1928 1916 - 3 1989 1979 1984 1947 1934 1927 - -

All 1967 1979 1976 1966 1954 1923 1916 1902 Table 4. Gutenberg-Richter parameters considering Visual Cumulative method

Table 5. Gutenberg-Richter parameters considering Stepps method

zone a b a @ Mmin @ Mmin Mmin Mmax1 Mmax21 4.124 -0.704 0.778 5.992 9.496 -1.622 5.992 4.5 8 8.52 5.271 -0.889 1.046 11.117 12.136 -2.048 11.117 4.5 7.6 8.13 4.483 -0.806 0.654 4.504 10.322 -1.856 4.504 4.5 7.43 7.93

zone a b a @ Mmin @ Mmin Mmin Mmax1 Mmax21 4.125 -0.727 0.673 4.707 9.498 -1.673 4.707 4.5 8 8.52 5.549 -0.951 1.031 10.744 12.777 -2.19 10.744 4.5 7.6 8.13 4.494 -0.814 0.628 4.248 10.349 -1.874 4.248 4.5 7.43 7.93

PSHA by the Zone-Free Approach The zone-free method proposed by Woo (1996) has been applied as an alternative approach to the Cornell method in the seismic hazard analysis. This method removes uncertainties inherent in the definition of seismogenic zones, the hazard solely reflects the characteristics of the earthquake catalog. The zone-free approach is also called the kernel estimation method because it implements spatial smoothing of seismicity. A magnitude dependent probabilistic smoothing procedure is applied directly to earthquake epicenters of the catalog to construct the source model. Therefore, the kernel-smoothed epicenters depict the spatial non-uniformity of seismicity in contrast to the rigid zonation in the Cornell approach, which assumes that seismicity is uniformly distributed within each seismogenic zone. A grid of point sources (nodes) is defined about the site of interest with relative activity rates determined according to the earthquake catalog. The contribution of each earthquake to the seismicity of the region is smeared over a distance that is magnitude dependent. Instead of defining the activity rate of each source using a recurrence relationship, such as the GutenbergRichter equation, individual rates for each magnitude bin are calculated from the density and proximity of the cataloged events lying within that magnitude bin. To do so the Woo (1996) method uses concepts from fractal geometry. The magnitude-distance dependent relationship is given by the kernel function K, which is a multivariate probability density function, expressed by the following equation:

= 1 +

(1)

where r is the epicentral distance and n is an exponent, which increases with the proximity of epicenters. Its value is typically between 1.5 and 2, and it has only a moderate influence on the computed results. H is the bandwidth for normalizing epicentral distances and is a function of magnitude. It represents the average minimum distance between epicenters of the same magnitude and takes the following exponential form: = (2) where c and d are constants to be determined on the basis of the location of the epicenters of the catalog, and MW is the moment magnitude. A least-square fit is conducted in order to obtain the two parameters c and d, for the current case c=0.140 and d=0.806. Once the activity rate of each magnitude class is defined, a GMPE is applied and the hazard is computed by summing over each point source, as in the Cornell approach. The kernel function allows the influence of magnitude to vary with distance. This accounts for the observation that smaller events exhibit a greater amount of spatial clustering than larger events, suggesting that small events are more likely than large events to occur in places where they have occurred in the past (Woo, 1996). By summing over all events, the cumulative activity density rate is computed for each magnitude class from the minimum magnitude of engineering interest (here taken to be 4.5) to the maximum magnitude of the catalog. Uncertainties in magnitude and epicenter location and an effective historical period of observation need to be introduced into the method. Since the earthquake catalog compiled for the current study is derived from various sources, the uncertainty in moment magnitude measure cannot be defined appropriately. The uncertainty of

earthquake magnitude in the present earthquake catalogue is estimated based on the uncertainties of the magnitude conversion relationship developed for the Northern part of Chile. Particularly, the uncertainties have been computed as the standard deviation of the magnitude conversion relationship. Table 1 summarizes the values used for the different conversion of magnitude type (MS, mb and I). A value of 0.10 was used for the events which has a direct estimate of the moment magnitude including the reduced number of events for which ML has been considered equivalent to MW. For the historical period, an error of 0.5 has been assumed because magnitudes are estimated from macroseismic intensities. The assessment of the uncertainties for the epicentral location reflects the improvements over the years in detecting earthquake epicenters. The historical periods was subdivided into two part: from 1900 to 1969 to which a value of 15 km was considered, while for events prior to 1899 a reasonable value of 30km was assumed, given that the degree of uncertainty in determining the epicentres of historical events from felt reports, is definitely larger. For the instrumental period (after 1970) a value of 5km was assumed. Another parameter requested by the zone-free approach is the effective historical period of observation. In this study, for each events based on their location and magnitude the corresponding values of the completeness period computed using the Stepps method was associated.

Figure 2. Depth associated to each point of the KERFRACT computation grid centred on

Ventarrones. Finally, the original KERFRACT code cannot be directly used for subduction zones because a fixed depth is associated with all nodes. In the processing, depth is only used for the application of the ground motion prediction equation (i.e. the computation of the distance between the source

and the site) and not for the definition of the kernel smoothing function. Therefore, the KERFRACT code has been modified to account for a representative depth at each node of the grid. For each node in the grid the representative depth is determined on the basis of the average depths of the events located within a fixed radius taken equal to 50 km in this study. The grid distribution used is shown in Figure 2. A similar approach has been used in Bozzoni et al. (2011) to address the same problem.

Seismic hazard estimation Epistemic uncertainties in the hazard assessment have been addressed in a logic-tree framework by considering the following parameters: a) the computational method for the hazard, b) estimation methods of catalogue completeness periods, c) GMPE and d) the maximum magnitudes. The logic tree is therefore composed by 2 main branches: one refers to the Cornell method (with weight 0.70), and the other refers to the zone-free approach (with weight 0.30). Equal weights have been assigned as regards to the methods adopted to estimate the completeness periods. Use of the VICU or the Stepps method lead to similar values of the completeness period. The maximum cut-off magnitude is based on the maximum historical earthquake (MHE) in each source zone of the earthquake catalogue. As an alternative, the MHE increased by 0.5 units has been considered. Equal weights have been assigned to the two different maximum magnitudes because there is no reason to prefer one alternative over the other. The selection of appropriate GMPEs has a greater impact than the expert judgment in assigning relative weights to the branches of the logic tree. The GMPE for subduction zones used for the study are: Atkinson and Boore (2003-2008) (AB08); Lin and Lee (2008) (LL08); Youngs et al. (1997); Zhao et al (2006). The choice of the GMPEs have been carried out based on the comparison between recorded data, which for brevity are not included in the paper. The following weights have been assigned: 0.4 for Zhao et al. (2006); 0.2 for Lin and Lee (2008); 0.15 for Young et al. (1997); 0.25 for Atkinson and Boore (2003/2008). The logic tree for the horizontal component is composed of a total of 20 branches, the final results of the PSHA have been obtained as a weighted mean.

Figure 3. Horizontal component UHS for 36, 475, 950 and 1898 years return periods for

Ventarrones (Chile).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Period (sec)

Spec

tral

Acc

eler

atio

n (g

)

RP=36 yearsRP=475 yearsRP=950 yearsRP=1898 years

Seismic hazard results are presented in terms uniform hazard spectra between 0 and 3 second for the horizontal component calculated for rock ground conditions and flat topographic surface representing outcropping ground conditions, five percent structural damping of the critical, four return periods (i.e.36, 475, 950 and 1898 years). The mean UHS up to 3 s for the four return periods considered computed for Ventarrones are shown in Figure 3.

Conclusive remarks A through Probabilistic Seismic Hazard Analysis (PSHA) of Ventarrones has been conducted with the aim of determining the seismic input in terms of horizontal uniform hazard acceleration spectra pertaining to reference return periods of 36, 475, 950 and 1898 years for bedrock outcropping motion and on level ground.

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