chapter 2 numerical modelling of the december 13, 1990, m ......catania enea-enel accelerometric...

Chapter 2

Numerical modelling of the December 13, 1990, M=5.8 Eastern Sicily earthquake

G. Laurenzano1,2 & E. Priolo1 1Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy 2

Abstract

This study aims at evaluating the effectiveness of some numerical techniques used for ground motion modelling. We focus our analysis on the site of the Catania ENEA-ENEL accelerometric station, which recorded the M ≅ 5.8 Eastern Sicily earthquake on December 13, 1990. The recorded seismogram features an anomalously high amplitude, which was interpreted as a possible effect of crustal structure or site response. In the first part of this study, we compare the recorded seismograms to those computed numerically by the 2-D Spectral Element Method (SPEM), a technique which solves the 2-D full-wave propagation through a complex geological structure. The agreement is very good. Next, we show that any plane layer representation, which simplifies the same structure, does not provide a comparable agreement. In this case, synthetic seismograms are computed by the wavenumber integration method. Finally, we

earthquake record, and 3) the seismogram simulated by the SPEM. Again, the overall agreement is very good. However, microtremor HVSR identify well the fundamental mode of vibration near 1.5 Hz, but miss a peak at higher frequencies. This study has several outcomes. Firstly, it demonstrates that the whole approach based on the SPEM and used in a previous study to simulate the ground shaking for a destructive scenario earthquake provides reliable results. Secondly, it shows that the model used to represent the crustal structure beneath

SEISMIC PREVENTION OF DAMAGE 19

University of Trieste, Dept. of Civil Engineering, Trieste, Italy

compare the H/V spectral ratios (HVSR) obtained from: 1) seismic noise, 2) the

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this area is realistic. Indeed, simplified models do not predict the site response adequately. Finally, it explains the high amplitude displayed by the Catania station during the 1990 earthquake as a combined effect of site and structure-path.

1 Introduction

A number of national projects have recently been carried out and are still ongoing with the aim of estimating and reducing the seismic risk of Eastern Sicily (Faccioli et al [1]; Decanini et al [2]; Maugeri [3]). The municipal area of Catania is certainly one of the Italian cities more exposed to seismic risk, and this explains why two of the above projects have been entirely directed to this city (Faccioli et al [1]; Maugeri [3]) and surrounding areas (Decanini et al [2]). Within all the above projects, several studies have been performed with the purpose of predicting the ground motion and possibly building-up ground shaking scenarios for the study areas, as well as estimating the site response at selected sites. Two levels of scenario earthquake were initially addressed for Catania, i.e. level I, damaging, and level II, destructive earthquakes (Sirovich et al [4]; Azzaro et al [5]). However, most studies focused on developing scenarios for the destructive M>7 earthquake. The approaches used to solve the scenario studies range from “empirical methods” (Pessina [6], [7]) to methods based on numerical simulations of the seismic wavefield radiated from the source (Langer et al [8], [9]; Priolo [10], [11]; Romanelli et al [12], [13]; Zollo et al [14], [15]). The latter improve significantly the detail of the analysis, however the obtained results should be validated with recorded data. Demonstrating the effectiveness of the numerical technique used in the scenario study by Priolo [10], [11] is one of the aims of this work.

The identification of the major seismogenic structures of Eastern Sicily is still the subject of debate. Despite a few alternative hypotheses (Sirovich et al [4]; Barbano et al [16]), most authors agree on identifying the only structure able to generate earthquakes with a magnitude as large as 7 in the Ibleo-Maltese fault system, which is located some tens of kilometres offshore of the eastern Sicily coast (Bianca et al [17]; Azzaro et al [5], [18]). Unfortunately for what concerns seismological studies, the seismicity of this structure is rather anomalous, since it features a very low number of weak to medium earthquakes. Consequently, there are only a few events that can be used to validate numerical simulations. The December 13, 1990, M = 5.8 earthquake is actually the only medium size event to have occurred along the northern segments of the Ibleo-Maltese system which has been recorded instrumentally. This earthquake is associated to a rupture of the transcurrent segment of the Ibleo-Maltese fault, and it was recorded by the ENEA-ENEL accelerometric network (Fig. 1).

Another reason of interest for modelling this earthquake is the fact that the seismogram recorded by the Catania ENEA-ENEL station (CAT in Fig. 1) features anomalously large ground accelerations. For example, the ground acceleration recorded at Catania has amplitudes 2-3 times larger than that recorded at Sortino, although both stations are at about the same epicentral

20 SEISMIC PREVENTION OF DAMAGE


distance (25-30 km). These anomalies are attributed (e.g.: Di Bona et al [19]) to both local site effects and the presence of strong crustal heterogeneities. The use of a method ― the 2-D Chebyshev spectral element method ― which solves the seismic full-wave propagation through a complex geological structure, is thus of maximum interest to verify this kind of hypothesis.

Figure 1: Base map of the study area, showing geography, transect position (black line), and the accelerometric stations (triangles) which recorded the December 13, 1990, earthquake. The Catania station location is emphasized by the circle. The acceleration seismograms (North-South component) recorded by each station are shown. The dashed line shows the trace of the Ibleo-Maltese fault adopted in this study. The earthquake is associated to the transcurrent fault segment.

The outline of this paper is as follows. First, we describe the 2-D spectral

element simulation of the Dec. 13, 1990, M = 5.8 Eastern Sicily earthquake, and compare the results of the simulations to the data recorded instrumentally. Then, we try to evaluate the importance of the 2-D structure, i.e. whether the recorded data can be reproduced adequately using a plane layer structure. In this part, we



model the earthquake using the wavenumber integration method (Herrmann [20], [21]). In the third and last part we evaluate the influence of the site of the Catania station looking at the horizontal-to-vertical (H/V) spectral ratios (HVSR). Here, we use also the spectral ratios estimated from records of environmental seismic noise.

2 The December 13, 1990 M = 5.8 Eastern Sicily earthquake

The M = 5.8 earthquake that struck Eastern Sicily on December 13, 1990, motivated several geologic, seismologic, and geodetic studies, a number of which are reported in a special issue of the Annali di Geofisica (Basili et al [22]). The source was located about 10 km offshore, near the city of Augusta (Siracusa), at a hypocentral depth of about 20 km (Amato et al [23],). Among the several fault mechanism solutions available, we adopt in this study that given by Giardini et al [24], which corresponds to a nearly pure strike-slip with left lateral motion. For what concerns seismic moment and corner frequency of the main

whole frequency band by assuming a simple ω-2spectral model. Rather, two different sets of values provide the same flat level of acceleration spectrum, i.e., fc = 0.6 Hz, Mo = 3.7×1024 dyna cm, fc = 1.3 Hz and Mo = 0.8×1024 dyna cm, where the lower corner frequency provides a better fit of the low frequency spectral amplitudes.

3 2-D Modelling of the earthquake

The numerical simulations are performed by the 2-D Chebyshev spectral element method (SPEM). The SPEM is a high-order finite element technique, which is particularly suitable to compute numerically accurate solutions of the full wave equations in heterogeneous media. One of the attractive features of this method is that it uses irregular meshes and therefore can reproduce accurately complex geometries. More details about SPEM and its application to the simulation of earthquakes, and in particular to the development of the ground motion scenario in the Catania area, can be found in (Priolo, [10], [11], [25], [26]).

The computational model is defined along transect t06 (Fig. 2). The structure along this transect and the values of the physical parameters used to define it are shown in Fig. 2 and Table 1, respectively. The model structure is consistent with that of the other transects of previous studies (Priolo [10]). A point source model is used, with fault mechanism defined by (φ, δ, λ) = (96°, 85°, 180°), and source depth at zS = 20 km. The source time function is built up as the sum of a deterministic part y(t; fc) and a stochastic contribution r(t; fc). The deterministic part is a combination of two pulses of the form y(t; fc) = α2 t exp(−αt), where α = 2πfc. The two pulses feature distinct corner frequencies fc and fasp, which represent the average source durations along the whole source and the asperity, respectively. fasp is set at fasp ≈ 2.4 fc according to (Somerville et al [27]). The stochastic contribution r(t; fc) affects the high frequency band for f > fasp. Seismic moment and corner frequency of the whole event are set at Mo = 3.7×1024 dyna


event, Di Bona et al [19] found that the earthquake cannot be modelled over the


cm, fc = 0.6 Hz. This corresponds to fasp = 1.4 Hz and an asperity seismic moment of Mo = 0.8×1024 dyna cm. Note that this parameterization gives reasons for both models found by Di Bona et al [19]. The corresponding source time function is shown in Fig. 3. The average slip along the whole fault has been estimated at D = 0.7 m (Somerville et al [27]). Other details are as in Priolo [10].

Figure 2: Computational model along transect t06. The panels show the model structure with different levels of enlargement. The location of the point source is indicated by a star and label “s.IBM”. Units in km. South-East and North-West correspond to positive and negative abscissas, respectively. The vertical scale is exaggerated, and different hor./ver. ratios are used for each panel. Material parameters are described in Table 2.



Table 2 summarizes the seismological parameters that have been assumed for the reference earthquake in this study. Fig. 4 displays seismograms computed at and recorded by the ENEA-ENEL Catania station. The maximum computed frequency is 6 Hz and the total propagation time is 40 s. They compare each other well in terms of the overall shape, polarity of the main arrivals, S-wave amplitude, and seismogram duration.

Table 1: Description of the soil and rock formations used to define the transect

structure. The following parameters are reported: density (ρ), compressional and shear wave velocities (VP, VS), and attenuation (Q).

Description Id ρ (kg/m3) VP (m/s) VS (m/s) Q (s-1) Fine alluvium deposits with sand (M) Alf / M 1870 360 200 12 Alf 1900 340 190 15 Clay and silt interbedded with sand Asg 1950 450 250 18 Sand, coarse gravel and conglomerate SG 2000 810 450 18 Clay interbedded with sand Aa 2000 900 500 30 Pliocenic sediments and alloctonus Spa1 2000 1400 775 45 Spa2 2050 1655 915 50 Spa3 2120 2000 1105 80 Spa4 2150 2300 1310 100 Spa5 2200 2800 1570 100 Vulcanits V2 2630 4100 2335 120 Limestone CC1 2580 4700 2680 120 CC2 2600 5000 2835 150 CC3 2630 5200 2950 150 CC4 2740 6100 3460 200 CC5 2780 6400 3630 200 CC6 2835 7000 3970 300 CC7 2800 6700 3750 300

Figure 3: Source time function used for the 2-D spectral element simulations. See text for more details.



Table 2: Summary of the seismological parameters assumed for the main shock of the December 13, 1990, Eastern Sicily earthquake in this study.

L×W (km)

φ (degrees)

δ (degrees)

λ (degrees)

zS (km)

M M0 (Nm)

fC

(Hz) D

(m)

5×8 95 86 180 20 5.8 3.7 × 1017 0.8 × 1017

0.6 1.4

0.7

Figure 4: Three components velocity and acceleration seismograms recorded by

the Catania ENEA-ENEL accelerometric station (thick lines) and computed by the 2-D spectral element method (thin lines). The ENEA-ENEL seismograms are band-pass filtered at 0.25-6 Hz. The ENEA-ENEL velocity is obtained by time integration of the acceleration records. The origin time of predicted seismograms has been aligned to that of the recorded seismograms. The peak value is explicitly written at the end of the traces.

4 How important is the 2-D structure?

To understand the importance of the effect of the 2-D structure on the propagating wavefield and on ground shaking at the surface, we perform a set of simulations assuming a layered Earth’s structure. The simulations are performed using the wavenumber integration method (Herrmann [20], [21]). The method solves the seismic full-wave equation through a 3-D layered medium.

We use three different layered structures which approximate progressively to the 2-D structure (Fig. 5). The first model is the mean regional model of Sicily which was agreed between the participants to the modelling activity within the first Catania Project (Vaccari [28]). This model was built up from the inversion of surface waves of teleseisms. The second structure uses the deep structure (z > 2 km) of the mean regional model and approximates to the shallow structure (z < 2 km) of the 2-D structural model specifically built-up for this study. The third model approximates to the whole structure of the 2-D model of this study. As the layered structure approximates to the 2-D structure, the models become progressively more complicated and feature more layers. The mean regional



model and the model which approximates to the 2-D model feature mainly three differences: 1) the depth of the Moho, which is at about 32 km and 36 km in the two models, respectively; 2) the velocity contrast at about 6 km and 14 km of depth in the two models, respectively; and, 3) the velocities of the shallowest layers (z < 2 km), which decrease much more rapidly toward the surface in the model which approximates to the 2-D structure.

The mean regional structure provides very simple seismograms, whereas, as the plane layer model approximates to the 2-D structure with a rougher and more detailed representation, the synthetic seismograms fit the recorded ones progressively better, and feature a number of additional phases which can be clearly recognized in the recorded seismograms (see for instance the phase marked by the dashed line in Fig. 6). The fitting improves for both the amplitude (of the overall seismogram as well as of the single peaks) and the arrival times of the main phases.

Figure 5: Plane layer models used in this study. Grey line: mean regional structure. Black line: plane layer model derived from the 2-D model specifically built-up for transect t06 in this study. Dashed line: plane layer model which accounts for the deep structure (z > 2 km) of the mean regional model from, and the shallow structure (z < 2 km) of the 2-D model.

The structure of the Catania area, south of the Etna volcano, differs from the

mean regional structure in the following aspects: 1) a sharper velocity contrast is present at about 12-14 km, in what is indicated as the carbonatic basement (Makris et al [29]; Della Vedova et al [30]); 2) the sediment profile, which features an overall thickness of 2 km, a strong velocity contrast at its bottom, and a decreasing gradient toward the surface, is a good average representation of the real structure. Our simulations are not sensitive to the depth of the Moho interface, which we set about 4-5 km shallower than the mean regional model, following the results of the active seismic surveys performed in the area (e.g., Hirn et al [31]). In fact, the signature of this interface does not show up at the distance simulated here. The fact that the 1-D seismograms feature a lower coda



amplitude than the 2-D seismograms suggests that the high attenuation of the shallow structure is realistic only locally and it cannot be assumed for the regional scale.

Though with the limitation of an analysis based on the record of only one event at one station, this study shows that the use of realistic models can improve significantly the modelling effectiveness in a number of cases. Besides that, we note here that, even if we adopt a simple point source, the seismograms calculated using the 2-D model are extremely realistic. This supports the need for methods that accurately model realistic geologic structures.

Figure 6: Three component velocity and acceleration seismograms recorded by

the Catania ENEA-ENEL accelerometric station (thick lines) and computed numerically (thin lines) for different plane layer models. (WIM: Mean) mean regional structure; (WIM: Mean + Priolo 99) deep structure of the mean regional model and shallow structure of the 2-D model; (WIM: Priolo 99) the best plane layer approximation of the 2-D structure; (SPEM) 2-D model. Other details as in Fig. 4.

5 Influence of the Catania ENEA-ENEL site on the seismic response

In addition to the earthquake data, environment microtremors were recorded at the same site of the Catania station (Priolo et al [32]) with the aim of helping in the estimation of the seismic response locally. We used Nakamura’s technique (Nakamura [33]), which provides the main features of the dynamic ground



response (i.e. the fundamental frequency) through calculation of the spectral ratio between the horizontal and vertical components (i.e., H/V ratio) of background microtremors. In Fig. 7 we compare the HVSR obtained from i) the environment seismic noise, ii) the seismograms recorded by the accelerometric station during the December 13, 1990 M = 5.8 Eastern Sicily earthquake, and iii) the seismograms computed by the 2-D spectral element method for the same earthquake. The ratios obtained from the seismic noise measurements easily detect the fundamental mode of vibration at about 1.5 Hz. As Fig. 8 shows, this frequency corresponds to the resonant frequency of either the thin fine alluvium deposit (Alf) or its combination with the underlying layer of light blue clays (Aa). We recall here that the shallow structure of the 2-D model has been built up on the base of the rich geotechnical dataset compiled during the first part of the Catania Project (Faccioli [34]), and therefore it simulates the real structure very closely. The HVSR estimated from seismic noise misses the peak at the higher frequency of 4-5 Hz. This fact may either confirm that Nakamura’s method can be used for identifying only the fundamental mode of vibration, or that the peak at 4-5 Hz in the earthquake records is a feature of the earthquake source. The HVSR determined from the synthetic seismograms are generally noisier ― because of the short seismogram duration, the spectral estimation is performed on only one time window ― but they clearly reproduce the behaviour of the spectral ratios determined from the full ENEA-ENEL recordings for frequencies larger than 1 Hz. The low-frequency peak at 0.4-0.8 Hz of the computed HVSR is likely to be the effect of a thicker portion of sediments (i.e. down to the bottom of Spa1 in Fig. 8, for a total thickness of about 220 m) which has been defined arbitrarily in the model.

Figure 7: HVSR computed for the ENEA-ENEL Catania station. Both panels,

grey curves: ratios determined from the recordings of the December 13, 1990 Eastern Sicily earthquake (full record and coda from 20 to 45 s, left and right panels, respectively). Left panel, black curve: ratio simulated for the same event and using the SPEM 2-D spectral element code. Right panel, black curve: ratio obtained from the seismic noise measurements (Priolo et al [32]).



6 Conclusions

The seismograms computed for the M ≅ 5.8, December 13, 1990 earthquake using the SPEM approach are in good agreement with those recorded by Catania ENEA-ENEL accelerometric station. This validates the whole 2-D approach used to estimate the ground motion for the Catania area. The 2-D model used to represent the crustal structure beneath this area is realistic, while simpler (plane layers) models are not adequate for predicting the ground motion accurately. The large amplitude recorded by the Catania station during the 1990 earthquake can be explained as a combined effect of structure-path and site. In general, this study emphasizes the importance of methods that accurately model the wavefield propagation through realistic geologic structures for predicting the ground motion.

Figure 8: Detail of the shallow structure beneath the ENEA-ENEL station (see also Fig. 2 for the complete structure). This site corresponds to site A15 of the microtremor survey (Priolo et al [32]). The fundamental frequencies of the first three layers are explicitly indicated.

Acknowledgements

This work was partially funded by the National Group for the Defence Against Earthquakes (GNDT), under The Catania Project (contract n. 98.03227.PF54 of the National Council of Research (CNR)), and the project Detailed Scenarios and Actions for Seismic Prevention of Damage in the Urban Area of Catania of the National Institute of Geophysics and Volcanology (INGV). Fig. 1 has been made by GMT software (Wessel et al [35]).

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