GROUND MOTIONS SCENARIO IN THE CATANIA AREA FOR A ...

IC/98/18

United Nations Educational Scientific and Cultural Organizationand

International Atomic Energy Agency

THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

GROUND MOTIONS SCENARIO IN THE CATANIA AREAFOR A MAGNITUDE 7.0 EARTHQUAKE ON THE HYBLEAN FAULT

F. Romanelli, F. VaccariDipartimento di Scienze delta Terra, Universita degli Studi,

via E. Weiss 4, 34127 Trieste, Italyand

CNR-Gruppo Nazionale per la Difesa dai Terremoti,via Nizza 128, 00198 Rome, Italy

and

G.F. PanzaDipartimento di Scienze delta Terra, Universita degli Studi,

via E. Weiss 4, 34127 Trieste, Italyand

The Abdus Salam International Centre for Theoretical Physics, SAND Group,Trieste, Italy.

MIRAMARE - TRIESTE

February 1998

Abstract

A realistic definition of seismic input for the Catania area is obtained using advanced

modelling techniques that allow us the computation of synthetic seismograms, containing

body waves and surface waves. With the modal summation technique, extended to later-

ally heterogeneous anelastic structural models, we create a database of synthetic signals

which can be used for the study of the local response in a set of selected sites located

within the Catania area. We propose a ground shaking scenario corresponding to an

earthquake of the same size of the destructive event that occurred on January 11, 1693.

Making use of the simplified geotechnical map for the Catania area, we produce maps of

the expected ground motion over the entire area, and, using the detailed geological and

geotechnical information along a selected cross section, we study the site response in a

very realistic case.

1. Introduction

The main problem associated with the study of seismic hazard is to determine

the seismic ground motion at a given site, due to an earthquake, with a given

intensity and epicentral distance from the site. The ideal solution for such a

problem could be to use a wide database of recorded strong motions and to group

those accelerograms that have the same source, path and site effects. In practice

however, such a database is not available. Actually, the number of available

recorded signals is relatively low and the installation of site arrays in each zone

with a high level of seismicity is an operation too expensive. An alternative way

is based on computer codes, developed from a detailed knowledge of the seismic

source process and of the propagation of seismic waves, that can simulate the

ground motion associated with the given earthquake scenario. In such a way,

synthetic signals, to be used as seismic input in a subsequent engineering

analysis, can be produced at a very low cost/benefit ratio.

The realistic definition of seismic input can be performed by means of

advanced modelling codes based on the modal summation technique (Panza,

1985; Florsch et al., 1991). These codes and their extension to laterally

heterogeneous structures (Vaccari et al., 1989, Romanelli et al., 1996, Romanelli

et al., 1997a) allow us to accurately calculate synthetic signals, complete of body

waves and of surface waves, corresponding to different source and anelastic

structural models, taking into account the effect of local geological conditions.

Our innovative and efficient methodology is applied to the Catania (Sicily)

area where a pilot project of GNDT (Gruppo Nazionale per la Difesa dai

Terremoti) is in progress for the reduction of the seismic hazard at a sub-regional

and urban scale. More than 500,000 people are living in such an urban area and

the application of advanced seismological methods for the definition of

groundshaking scenarios can play a crucial role in the reduction of possible

losses.

The main task is to define a scenario corresponding to an earthquake of the

same size of the destructive event that occurred on January 11, 1693. From the

analysis of the felt intensities {up to XI) it has been possible to estimate a

magnitude ranging from 7.0 to 7.8 (e.g. Boschi et al., 1995; Decanini et al., 1993).

It is a very difficult task to determine the source characteristics for such a

historical event as the macroseismic data are the only information available. The

poor control over the hypocentral coordinates does not permit to use the

macroseismic data for the inversion of the source mechanism (Panza et al., 1991),

but with these data it is possible to perform an analysis to test the validity of the

source mechanism models that can be formulated on the base of seismotectonics.

The observed intensities can be converted into accelerations (e.g. Decanini et al.,

1995) or displacements (e.g. Panza et al., 1997) and they can be compared with the

synthetic data. Applying such a procedure to the 1693 event (Romanelli et al.,

1997b, Romanelli et al., 1998), a good agreement with the macroseismic data is

obtained for a seismic source located on the Northern Segment of the off-shore

Hyblean fault, which is considered the most important seismogenic structure of

the zone. The knowledge about the source-process is not sufficient to warrant the

inclusion of a source with a detailed time and space structure. Rather, to

minimize the number of free parameters, we utilize a point-source model

properly scaled for a magnitude 7.0 using the Gusev spectral scaling law (1983) as

reported in Aki (1987). The used focal mechanism parameters of the source,

located approximately in the center (latitude: 37.44°; longitude: 15.23°) of the

Northern Segment of the Hyblean fault, are: strike equal to 352°, dip equal to 80°,

rake equal to 180°, focal depth equal to 10 km and seismic moment equal to

3.2-1019 Nm.

The propagation of the seismic waves in the Catania area is modelled using the

geotechnical informations collected within GNDT (GNDT, 1997; Pastore and

Turello, this issue).

2. Maps of ground motion

In Figure 1 the simplified geotechnical zonation map for the Catania area,

together with the 13 simplified cross sections considered in the analysis, are

shown. For further information concerning this map and the selected boreholes

see Pastore and Turello (this issue).

Along each section shown in Figure 1, a set of sites is considered and the site

locations are chosen both in the proximity of the boreholes, and at the edges of

the section. For all the sites the relevant coordinates in the source reference

system are shown in Table 1.

The elastic and anelastic parameters of the regional model, used for the

computation at each site of the 1-D signals, calculated with the modal summation

technique (Panza, 1985; Florsch et al, 1991) with a cut-off frequency of 10 Hz, are

shown in Figure 2 (Costa et al., 1993). The 2-D models associated with each cross

section are built up putting in welded contact (from 2 to 4) different 1-D models:

the regional 1-D model is chosen as the bedrock model and the geotechnical

information related with the selected boreholes are used for the local 1-D models.

The synthetic signals are calculated with the modal summation technique for

laterally heterogeneous models (Vaccari et al., 1989; Romanelli et al., 1996;

Romanelli et al., 1997a), with a cut-off frequency of 10 Hz. The map shown in

Figure 1 defines the borders between the local models, i.e. the distances between

the vertical interfaces separating the different 1-D models.

The synthetic seismograms can be used as a database of waveforms from which

one can extract representative parameters of the ground motion, like for instance

Peak Ground Displacement (PGD), Peak Ground Velocity (PGV), Peak Ground

Acceleration (PGA) and Id, an estimator of the destructive power of ground

motion, defined by Cosenza and Manfredi (1995) as:

}a2(t)dt_o

(PGV) (PGA)Q. /n*nT T\ /-n^—i * ^ ^ '

where the integral of the acceleration squared, a2(t), is called Housner Power. The

results we obtain in each considered site are summarized in Table 1.

In Figure 3 and Figure 4 the velocity and the acceleration time series calculated,

for the transverse component of motion in correspondence of all the sites, are

shown respectively. Each portion of records is 20 s long and is normalized to the

peak value, respectively of PGV and of PGA, for the entire region. Figure 3 and

Figure 4 show clearly how the source and propagation effects can combine

originating a big variety of signals; if laterally heterogeneous models and

azimuthal dependencies are included in the analysis, the parameters describing

the ground motion are strongly site-dependent. For the estimation of the local

site effects we evaluate the ratio between the response spectra (RSR) calculated, at

a given site, for the 2-D and the 1-D signals, in such a way removing the source

and path effects. Each site shown in Figure 1 is characterized by a given set of

resonant frequencies and a corresponding level of amplification.

3. Selected cross-sections and discussion

The model corresponding to section S3 is formed by three 1-D models in

welded contact, separated by vertical interfaces; the three laterally homogeneous

models correspond to the bedrock model and to the two local models constructed

using the data coming from the selected boreholes (see Figure 1 and Table 1). The

combination of the radiation pattern and of the path effect makes S3 the section

with the highest peak ground motion parameters. In Figure 5 the acceleration

time series, and the corresponding response spectra ratios, calculated at the four

sites situated along S3 are shown. Sites 1 and 2, as well as sites 3 and 4, belong to

the same local model, but, as one can see from Figure 5, the level of amplification

of the resonant frequencies is not similar for the two sites. This fact seems to

indicate that the site effect depends on the layering of the local geological

structure and upon the relative position of the site with respect to the lateral

hetero geneities.

In Figure 6 the acceleration time series calculated at the two sites situated along

S10, and the corresponding response spectra ratios, are shown. Since for section

S10 very detailed geotechnical information is available (GNDT, 1997 and Turello

and Pastore, this issue), we choose to examine it in detail, with the purpose of

comparing the results obtained with a simplified laterally heterogeneous model

with those obtained for a realistic model of a sedimentary basin.

The detailed 2-D model used for the calculation of the synthetic seismograms

along S10 is built up using forty-eight 1-D models in welded contact, for a total

extension of approximately 13 km. The geotechnical cross section and its detailed

model are shown in Figure 7 with the elastic parameters of each geotechnical

unit; the Q values vary in the range from 50 to 300, depending upon the unit

considered. The structural model, used to propagate the seismic waves from the

source to the 2-D section, is again assumed to be the bedrock model of Figure 2.

The synthetic signals relative to the transverse component of motion are

calculated, with a cut-off frequency of 3 Hz. The choice of this upper frequency

limit is fully justified by the results shown by Somerville (1996), who supplies the

ground motions for the scenario for a magnitude 7.0 earthquake on the northern

Hayward fault. The signals are calculated for a set of sites, one for each local

model, along the section. Due to the size of the 2-D model and to the low seismic

velocities of some geotechnical units, the application of purely numerical

techniques to the entire section is very difficult, while the modal summation

technique allows us to calculate the signals with a relatively small request of CPU

time and memory on a standard workstation. For an HP715 workstation the CPU

minutes for the computation of the necessary spectral quantities (Florsch et al.,

1991; Romanelli et al., 1996) are about 12N, where N is the number of local

structures. The subsequent generation of N synthetic seismograms, one for each

local structure, does not require more than N minutes. Therefore synthetic

seismograms can be quickly computed for any variation of the source parameters

and for different path lengths associated with each local 1-D structure.

One possibility, to study the behaviour of the site effects along the entire cross-

section, is to evaluate the ratios between the 2-D and 1-D peak values of the

ground motion at the same site, the latter obtained applying the 1-D modal

summation technique to the bedrock model. As an example, in Figure 7a the

maximum of the RSR of the 2-D and 1-D signals (solid line) and the values of the

ratio between the maximum value of the acceleration (Amax) for the 2-D signal

and the amax found for the 1-D signal (dashed line), versus the epicentral

distance are shown. One way to remove the possible bias that can be introduced

by the adoption of a scaled point-source model in the calculations, is to combine

the curves shown in Figure 7a, that carry the information about the local effects,

with probabilistic attenuation curves. In Figure 7b and Figure 7c the curves of the

Sabetta-Pugliese attenuation relations (Sabetta and Pugliese, 1987) for the peak

horizontal acceleration and horizontal velocity are shown respectively. These

relations are valid for the Italian territory and they contain a variable that, if

switched on, takes into account the local geology in a probabilistic way. In Figure

7b the Sabetta-Pugliese curves for horizontal acceleration in the presence of

shallow soil (SPs) can be compared with the curve ("new") obtained scaling the

values of Amax2D/AmaxlD, plotted in Figure 7a, with the values corresponding

to the Sabetta-Pugliese relation for the peak horizontal acceleration on stiff soil

(SP). In Figure 7c the result obtained applying the same procedure of Figure 7b to

velocity is shown.

If we change the strike of the source so that a maximum of the radiation

pattern takes place in the direction of the cross-section, we obtain the results

shown in Figure 8.

Figure 9 shows the variation of the spectral accelerations (Sa) at four selected

frequencies along the profile: Figure 9a is obtained using the signals calculated

with a strike-section angle equal to 80.5°, while Figure 9b is obtained for a value

of the strike-section angle equal to 180°. In Figures 9c-9d the ratios Sa(2D)/Sa(lD)

7

are shown. The main difference between the curves in Figures 9a-9b is the scaling

factor, while the local soil effects are well evidenced in Figures 9c-9d, only for

frequencies above 1 Hz, corresponding to wavelengths that are comparable to the

dimension of the lateral heterogeneities.

The results that are obtained using the detailed model for section S10 can be

easily compared with those coming from the simplified model. In Figure 10 the

RSR are shown for three sites: 1 and 2 are the sites listed in Table 1, while 3 refers

to a site distant 14.7 km from the seismic source. Curves (a), referring to the

simplified model, show generally narrower and higher peaks, while curves (b),

referring to the detailed model, show broader peaks in correspondence of the

resonant frequencies.

4. Conclusions

The modal summation technique for laterally heterogeneous media allows us

to calculate, in a very efficient way, synthetic signals, comprehensive of body and

surface waves, for realistic anelastic structural models. A database of synthetic

seismograms represents a scientifically and economically valid tool for seismic

microzonation. The calculated seismograms, with a broad-band frequency

content, can in fact be used for the determination of any parameter of engineering

interest that describes the ground motion and the site response for different

geological settings.

Making use of the simplified geotechnical map for the Catania area, we are able

to produce maps describing the expected ground motion over the entire area, and,

using the detailed geological and geotechnical information along a selected cross

section, we study the site response in a very realistic case. With the wide set of

seismic signals we construct a ground motion scenario for an earthquake like the

destructive event that occurred on January 11, 1693 on the Hyblean fault. The

results show that, in order to perform an accurate estimate of the site effects, it is

8

necessary to make a parametric study that takes into account the complex

combination of the source and propagation parameters.

Acknowledgements

We acknowledge Dr. Vera Pessina for the help given us to handle the GIS files

necessary for drawing Figure 1.

We acknowledge financial support from GNDT (contracts 94.01703.PF54,

95.00608.PF54, 96.02986.PF54, 97.00540.PF54), MURST (40% and 60% funds), EEC

Contract ENV-CT94-0491, ENV-CT94-0513, ENV4-CT96-0491.

This work is a contribution to the UNESCO IGCP project 414: Realistic Modelling

of Seismic Input for Megacities and Large Urban Areas.

References

Aki, K, 1987, Strong motion seismology, in M. Erdik and M. Toksoz (eds) Strong

ground motion seismology, NATO ASI Series, Series C: Mathematical and

Physical Sciences, D. Reidel Publishing Company, Dordrecht, Vol. 204, pp. 3-39.

Boschi, E., Ferrari, G., Gasperini, P., Guidoboni, E., Smriglio, G., and G. Valensise,

1995, Catalogo dei Forti Terremoti in Italia dal 461 a.C. al 1980, ING-SGA,

Roma, CD-ROM.

Cosenza, E., and Manfredi, G., 1995, La definizione di un coefficiente di struttura

basato su criteri di danno. In: Atti del 7° Convegno Anidis 2, Siena, Italy 25-28

September 1995: 579-588.

Costa, G., Panza, G. F., Suhadolc, P., and Vaccari, F., 1993, Zoning of the Italian

Territory in Terms of Expected Peak Ground Acceleration Derived from

Complete Synthetic Seismograms, /. Appl Geophys 30, 149-160,

Decanini, L., Gavarini, C , and Oliveto, G., 1993, Rivalutazione dei terremoti

storici della Sicilia Sud-Orientale. \n\AHi del 6° Convegno Anidis 3, Perugia,

Italy 13-15 October 1993:1101-1110.

Decanini, L., Gavarini, C, and Mollaioli, F., 1995, Proposta di Definizione delle

Relazioni tra Intensita' Macrosismica e Parametri del Moto del Suolo. ln:Atti

del 7° Convegno Anidis 1, Siena, Italy 25-28 September 1995: 63-72.

Florsch, N., Fah, D., Suhadolc, P., and Panza, G.F., 1991, Complete Synthetic

Seismograms for High-Frequency Multimode SH-Waves, PAGEOPH 136 , 529-

560.

10

GNDT, 1997, Progetto Catania - Caratterizzazione geotecnica del territorio

comunale di Catania a fini sismici, GNDT, Milano, 1997.

Gusev, A. A., 1983, Descriptive statistical model of earthquake source radiation

and its application to an estimation of short period strong motion, Geophys. J.

R. Astron. Soc. 74, 787-800.

Panza, G. F., 1985, Synthetic Seismograms: the Rayleigh Waves Modal

Summation, /. Geophys. 58, 125-145.

Panza, G. F., Craglietto, A., and Suhadolc, P., 1991, Source Geometry of Historical

Events Retrieved by Synthetical Isoseismals, Tectonophysics 193, 173-184.

Panza, G. F, Vaccari, F., and Cazzaro, R., 1997, Correlation between macroseismic

intensities and seismic ground motion parameters, Annali di Geofisica 15,

1371-1382.

Pastore, V., and Turello, R., 1998, Geotechnical earthquake engineering

characterization of the Catania soils through a new database, this issue.

Romanelli, F., Bing, Z., Vaccari, F., and Panza, G.F., 1996, Analytical Computation

of Reflection and Transmission Coupling Coefficients for Love Waves,

Geophys. J. Int. 125 , 132-138.

Romanelli, F., Bekkevold, J., and Panza, G. F., 1997a, Analytical Computation of

Coupling Coefficients in Non-Poissonian Media, Geophys. J. Int. 129, 205-208.

Romanelli, F., Vaccari F., and Panza, G. F., 1997b, Modellazione realistica del

moto del terreno per la riduzione della pericolosita sismica nella citta di

11

Catania. In: Atti 8° Convegno Nazionale Anidis 1, Taormina, Italy 21-24

September 1997: 65-72.

Romanelli, F., Vaccari, F., and Panza, G. F., 1998, Realistic Modelling of ground

motion: techniques for site resposnse estimation. In: Proceedings of 6th U.S.

National Conference on Earthquake Engineering, Seattle, U.S.A. 31 May - 4

June 1998/ in press.

Sabetta, F., and Pugliese, A., 1987, Attenuation of peak horizontal acceleration

and velocity from Italian strong-motion records, Bull. Seism. Soc. Am. 77 5,

1491-1512.

Somerville, P., 1996, Ground Motions, in Scenario for a magnitude 7.0

earthquake on the Hayward fault, EERI, Oakland, pp. 35-42.

Vaccari, F., Gregersen,S., Furlan, M., and Panza, G. F., 1989, Synthetic

Seismograms in Laterally Heterogeneous, Anelastic Media by Modal

Summation of P-SV Waves, Geophys. J. Int. 99 , 285-295.

L2

Table 1.Selection of parameters of the ground motion extracted from the time series calculated for the sites

shown in Figure 1. The angle between the fault strike and the direction of the section (strike-sectionangle) is measured counterclockwise from the strike direction. For each section the site numberincreases with increasing epicentral distance. The numbers in parentheses are the boreholes selectedfor the modelling as numbered in the geotechnical report (GNDT,1997)

Section

(boreholes)

SI(1250)

S2(1038)

S3(402)(95)

S4(1027)(358)(1234)

S5(1247)(166)(1237)

S6(1088)

(6)

S7(241)(1327)

S8{1385}(1334)

S9(1303)(1358)

S10(1369)

S l l(1298)(1264)

S12(1261)

S13(1067)(1274)

Strike-Sectionangle(deg)

39.4

42.5

47.9

50.5

52.7

63.0

67.0

71.8

77.6

80.5

85.5

90.5

96.5

Site

121231234123451234123412341234123121234123123

Epicentraldistance

(km)

14.615.215.015.316.014.414.815.517.514.114.717.218.320.113.914.915.417.514.214.715.518.313.914.716.218.413.515.117.018.513.216.523.013.123.613.017.819.925.313.117.924.313.615.316.1

PGD

(cm)ID9.69.09.18.98.39.38.88.47.09.18.87.16.45.59.48.18.15.77.05.88.45.18.46.15.54.55.44.94.13.73.83.02.63.01.41.41.00.90.50.20.10.12.01.71.5

2D11.510.711.010.48.8

12.811.58.07.510.710.18.77.28.1

12.210.97.87.610.18.58.56.013.814.57.25.5

15.58.33.83.56.04.73.24.41.72.01.41.20.70.30.20.12.92.51.7

PGV

(cm/s)ID39.233.435.333.929.336.633.631.222.436.132.522.519.214.532.832.128.920.726.324.322.214.522.621.817.612.418.516.812.79.914.59.03.910.52.85.42.82.11.10.50.30.17.66.05.2

2D43.334.446.338.631.863.143.325.726.541.932.928.320.514.257.033.920.523.649.928.715.518.646.146.618.412.163

28.17.210.233.214.66.418.93.68.64.03.81.41.20.50.216.57.84.4

PGA

(g/10)I D5.64.84.14.94.25.54.04.32.85.75.02.32.31.34.84.94.12.63.33.82.71.53.53.42.61.62.72.61.51.12.51.30.41.60.21.00.40.20.10.10.00.01.30.80.7

2D9.57.911.69.18.711.29.73.85.17.97.63.72.71.87.06.33.92.67.15.23.93.510.16.22.61.66.86.01.21.65.33.61.03.90.71.90.80.40.20.20.10.02.81.91.1

Id

ID1.01.11.20.91.01.11.41.00.91.01.01.20.91.01.21.21.01.01.31.11.21.11.31.01.00.91.51.01.11.11.11.10.81.51.01.10.90.90.81.90.80.81.01.11.1

2D4.75.74.65.54.87.47.94.12.45.05.92.62.02.32.74.86.12.24.56.94.92.23.24.03.54.43.44.84.25.14.62.91.97.83.85.23.05.45.25.12.94.74.14.02.3

13

Legend

I Thick lavas (> 10m1 in the first 30 m)I Thin lavas (< 10m1 in the first 30 m)

| Clays

j Alluvial materials

i PrimosoleI Palagonia Hills

Boreholes

S13

Figure 1. Simplified geotechnical zonation map for the Catania area and the 2-D cross sectionsconsidered in the analysis.

0 1.5 2.5 0 0 350 1100

a.

Q

10

20

30

40

] }

• "

Density (g/cm3) Velocity (km/s) Q

Figure 2, Elastic and anelastic parameters of the regional model used for the calculation of thesynthetic seismograms. Thin lines refer to S-waves, thick lines refer to P-waves.

15

Legend

Thick lavas (> 10min the first 30 m)

Thin lavas (< 10min the first 30 m)

Clays

Alluvial materials

i PrimosolePalagonia Hills

Boreholes

Figure 3. Simplified geotechnical zonation map for the Catania area and the 2-D velocities timeseries calculated at the sites. Each signal is scaled to the maximum value of FGV over the entirearea. The signals with a peak value lower than 1 cm/s are not shown.

16

Legend

Thick lavas (> 10m: in the first 30 m)I Thin lavas (< 10m1 in the first 30 m)

| Clays

\ Alluvial materials

, Primosole1 Palagonia Hills

Boreholes

Figure 4. Simplified geotechnical zonation map for the Catania area and the 2-D acceleration timeseries calculated at the sites. Each signal is scaled to the maximum value of PGA over the entirearea. The signals with a peak value lower than 0.1 g/10 are not shown.

17

b)

c)

d)

11.2 g/10

-r

^^^j)^p^^^-^<f^y^^

"WTAA^\^V^^^-^-^^^^

time (s) 15

/

1V

0 2 4 6 S 10frequency (Hz)

Vf

0 2 4 6 8 10frequency (Hz)

'S/A


0

a

w 1 -

0-

i) ——-. .. _....0 2 4 6 8 10

frequency (Hz)

Figure 5. Acceleration time series calculated for the transverse component of motion at the four sitessituated along section S3. For each site the 1-D signal, the 2-D signal and the response spectra ratios(2D/1D) are shown, a) site 1; b) site 2; c) site 3; d) site 4.

18

1-a)

2.2g/10

1

aQ n

CM

p

0-

1w\ -—V.


b)

0.7 g/10

time (s) 20

6 -

A

1

0-

j j _V


Figure 6. Acceleration time series calculated for the transverse component of motion at the two sitessituated along section S10. For each site the 1-D signal, the 2-D signal and the response spectraratios (2D/1D) are shown, a) site 1; b) site 2.

19

1000

25 23 21 19epicentral distance (km)

0

90

25 24 23 22 21 20 19

Alluvial depositsAlf p=1.99g/cm3

1 K=450m/s P=210m/s

SandsM p=1.91g/cm3

a=550m/s P=

Quartzous sandsSg p=2.12g/cm3

a=1600m/s p=500m/s

. Yellow claysASg p=2.04 g/cm3

a=1400 m/s P =280 m/s

Grey-blue claysp=2.06 g/cm3 :a=1700 m/s p =650 m/sj

LavasE p=2.45 g/cm3

a=1700m/s P =500 m/s

Figure 7. Cross-section (bottom) and corresponding model for section S10. The distance along thesection is measured in km from the source, while the vertical scale is in m. a) Amax2D/AmaxlDratio (solid line) and maximum of the 2D/1D RSR versus distance (dashed line); b) peak horizontalacceleration versus distance for the SP and SPs relations and for the curve (new) Amax2D/AmaxlDscaled with SP; c) same as b) but peak velocities are considered.

20

10

Q7 -

3CM

11000

-

1 1 I 1

A

i i i '

Ki i

- « «

i i i i

^j • • —

i i i i

(a)

200

(c)

1525 19 17

epicentral distance (km)15 13

\VV\ 90

25 24

Hi23 22

Hn. -*

21

^ ^

20 19 18 1/ 16

PP15 14 1

Alluvial depositsp=1.99g/cn\3a=450m/s P =210 m/s

Sands j.^— Grey-blue claysM p=1.91 g/cm3 j ! p a Aa p=2.06 g / ^

a=550m/s p = 2 8 0 m / s J ^ ^ 1700

Quartzous sandsSg p=2.12g/cm3

a=1600m/s p=500m/s

. Yellow claysASg p=2.04 g/cm3

a=1400m/s p=280m/s

! _ , Lavas' ^ 3 E p=2.45 g/cm3

a=1700 m/s 3 =500 m/s

Figure 8. Same as in Figure 7 but the strike-section angle is 180°, very close to a maximum of theradiation pattern for the focal mechanism used in the analysis.

21

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Figure 9. Spectral accelerations at four selected frequencies (0.2 Hz, 0.5 Hz, 1.0 Hz, 2.5 Hz) versusepicentral distance, a) Strike-section angle equal to 80.5°. b) Strike-section angle equal to 180°.Ratios between the spectral accelerations of the 2-D signals and the corresponding spectralaccelerations of the 1-D signals, for the four selected frequencies (0,2 Hz, 0.5 Hz, 1.0 Hz, 2.5 Hz),versus epicentral distance, c) Strike-section angle equal to 80.5°. d) Strike-section angle equal to180°.

22

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Figure 10. Comparison of the RSR obtained for the simplified (a) and for the detailed (b) laterallyheterogeneous models of section S10.1 and 2 are the sites described in Table 1, while 3 refers to a siteon SIO at 14.7 km from the source.

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