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Amplification of ground motion in fault and fracture zones: Observations from the Tremestieri fault, Mt. Etna (Italy)
Journal of Volcanology and Geothermal Research 153 (2006) 167 – 176 www.elsevier.com/locate/jvolgeores
Amplification of ground motion in fault and fracture zones: Observations from the Tremestieri fault, Mt. Etna (Italy) G. Lombardo *, R. Rigano Dipartimento di Scienze Geologiche, University of Catania, Italy Received 1 February 2005; received in revised form 31 August 2005; accepted 31 October 2005 Available online 30 January 2006
Abstract A series of ambient noise measurements have been performed to evaluate their use in detecting the possible influence of tectonic structures on local amplification of seismic waves. The area studied is located on the southeastern flank of Mt. Etna, a few kilometres north of the town of Catania. Using a mobile station, data were collected along three profiles which cross both a fault and an eruptive fracture. In addition, ambient noise was sampled at two permanent stations installed in close proximity to the fault, and at another station located within the city limits of Catania and considered to be a reference site. Analysis using spectral ratio techniques shows there is a tendency towards amplification at the sites located near the fault. In H / V spectral ratios such amplification is observed in the frequency range 4.0–8.0 Hz and the amplitudes of the dominant spectral peaks decrease rapidly over the first few tens of meters from the discontinuity. A similar tendency was observed for the sites located on the eruptive fracture zone. Dominant spectral peaks at 1.8 Hz are also common in the study area. The theoretical transfer function, obtained through 1-D modelling, indicates that such peaks can be related to the local stratigraphy. The mean H / H standard spectral ratio of microtremor evaluated at the permanent station located on the fault also shows an amplification peak at 4.0 Hz. Azimuthal analysis shows that the resonance at 4.0 Hz is directional, with the maximum in the azimuth range N308E–N808E. D 2005 Elsevier B.V. All rights reserved. Keywords: ambient noise; local seismic response; H / V spectral ratio; fault; eruptive fracture; directional resonance
1. Introduction The distribution of seismic energy depends on the complexity of morphologic and structural features of the investigated areas, as well as on the lithology. In experiments in California (Cormier and Spudich, 1984) both active and inactive faults behave as wave-guides for an impinging wave front. Waveform analysis of earthquakes recorded during the 1998 Umbria–Marche * Corresponding author. E-mail address: [email protected] (G. Lombardo). 0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.10.014
seismic sequence (Marra et al., 2000) showed a large local amplification corresponding to the fault zone. More recently, Donati et al. (2001) demonstrated that site response contributed significantly to the level of damage which affected the town of Nocera Umbra during such earthquakes. The highest levels of damage were estimated for two zones, on the soft sediment deposits in the Topino River valley and on the fractured marly sandstone terrains within a 200 m-wide fault zone crossing the town of Nocera Umbra. Spudich and Olsen (2001) show, through numerical simulations along the Calaveras fault (California), that the
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amplification caused by a low velocity zone may enlarge the area of increased hazard within 1 km of some faults. The aim of present paper is to investigate the effects of the tectonic structures located in the southernmost flank of Mt. Etna, at the northern boundary of the town of Catania, on the seismic wavefield (Fig. 1). This area is crossed by an eruptive fracture and by a segment of a NW–SE trending fault. The spectral ratio technique, using either earthquakes or ambient noise, is among the most commonly used methods to estimate ground motion amplification. Both horizontal-to-vertical component ratios and spectral ratios to a reference rock station are used. Many authors have recently demonstrated that the H / V spectral ratios from microtremor measurements are consistent in shape with H / V spectral ratios from earthquake recordings, and with earthquake spectral ratios using a reference bedrock station (Bard, 1999). Although there is not unanimous agreement on the theoretical justification for the use of ambient noise in evaluating site response, the use of H / V spectral ratios of ambient noise (Nakamura, 1989) has been widely adopted to characterise earthquake site response. Recently, investigations on the reliability of ambient noise measurements in microzonation studies have been tested in the SESAME project (see http://sesame-fp5.
Fig. 1. Location of the study area and investigated tectonic structure.
obs.ujf-grenoble.fr/) and results of tests on the implications for the use of H / V spectral ratios are shown in Cara et al. (2003). The method performs best when there is a strong velocity contrast between the bedrock and the soft upper layers. It can be reasonably assumed that a good impedance contrast between the bgougeQ of a fault and the surrounding rocks, as well as between a dyke intruded into an eruptive fracture and the surrounding materials, should exist. We therefore test the possibility of using microtremor measurements to determine local amplification effects in such peculiar situations. In this study ambient noise signals were analysed in order to evaluate whether the presence of a tectonic discontinuity can contribute to locally enhanced site effects. 2. Local geological and structural framework The tectonic setting of Mt. Etna results from the interaction of regional tectonics and local scale volcano-related processes (McGuire and Pullen, 1989). The youngest normal faults of the Etnean area are located along the base of the eastern flank of the volcano. Here, several NNW and NNE-trending fault segments, arranged in a 30 km long system, control the present topography and show steep escarpments with very sharp morphology (Monaco et al., 1997). The study area is located on the southeastern flank of Mt. Etna, a few kilometres north of the town of Catania, where morphological evidence of the Tremestieri fault is visible (Fig. 1). In this zone, an important structural trend is present which is responsible both for several faults and for numerous eruptive events that have developed along fissures (Lo Giudice et al., 1982). This area was selected because it is extensively covered by lava flows, and crossed by both an eruptive fracture and by a segment of Tremestieri fault, which trends NW–SE. Features of the outcropping lithotypes are particularly suitable for our study since the impedance contrasts between the fairly compact lavas and both the gouge of the fault and the dyke intrusion in the fracture help to enhance the focusing effect of seismic waves. The Tremestieri fault usually releases seismic energy in earthquake swarms; nevertheless, aseismic creep phenomena also occur frequently (Lo Giudice and Rasa`, 1992). During a seismic sequence in 1980, a left-stepping, secondary system of en echelon cracks accompanied the NW–SE-trending ruptures. On the whole, the displacement was right-lateral and oblique (Azzaro, 1999).
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3. Data acquisition and processing Twenty eight measurements of ambient noise were performed at sites located along three profiles which crossed both the fracture and the fault (Fig. 2). Each profile had a sampling site on the fault scarp. Near the fault, sites were spaced 20 m apart, and as the distance from the fault increased, site spacing increased to 50 and 100 m. Time histories of ambient noise were recorded using a three-component 1-Hz Mark L4C 3-D seismometer connected to a 12-bit analog-to-digital converter and a notebook computer. Sampling frequency was 100 Hz; two antialiasing filters cut higher frequencies with a 10 dB/oct slope. At each site, five time series of 120 s length were recorded. The time series were baseline corrected in order to remove spurious offsets and low-frequency trends. After the application of a Hanning window, a Fast Fourier Transform algorithm was applied to obtain spectra in the frequency band 0.5–15 Hz. The lower limit of
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0.5 Hz was chosen as the low-frequency cut-off, being where the standard deviation of the microtremor became equal to the natural noise fluctuation of the 12-bit digitizer. We applied the Nakamura (1989) technique, dividing the vector composition of horizontal component noise spectra by the vertical one. The final H / V results from the geometric mean of the five spectral ratios at each measurement site. A running, smoothing function of 0.1 Hz was also used, both on the spectra of each individual component and on the final ratio, to reduce spectral fluctuations. The stability of the adopted processing method was tested by calculating the arithmetic mean of the horizontal components as well, and the spectral ratios obtained did not show variations. Similarly, negligible differences were found when the H / V ratio was calculated using the mean spectrum of each noise component. In the framework of an agreement between researchers of INGV (Roma) and the Department of Geological Science of the University of Catania, two sites in the
Fig. 2. Geological and structural map of the studied area with location of permanent seismic stations and noise measurement sites.
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study area were instrumented with broadband seismometers coupled to Reftek 72A07 24-bit digitisers. The stations were synchronised by a GPS timing system. Stations cav0 and cav2 were situated on massive lavas next to a segment of the Tremestieri fault (Fig. 2). A third station, located on a Quaternary clay bedrock
outcropping within the city limits of the town of Catania, was chosen as the reference site. The seismometer deployment was planned to collect a significant number of earthquake records in order to compare results from their processing with those derived from the ambient noise measurements. At present,
Fig. 3. Schematic geolithological profiles and plots of H / V microtremors spectral ratios.
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data from only a small set of earthquakes is available, so we will only show findings from noise data. For this purpose, starting from selected 2 min intervals of microtremor, three 25 s segments with stationary signals were chosen for analysis. Each signal was cosinetapered and a trend removal function was applied before the use of amplitude FFT. The spectra were then smoothed using a 0.25 Hz running frequency window. 4. Results Before describing the results it is worth noting that ambient noise measurements were taken at several sites situated on massive lavas overlaying the clayey formation that forms the bedrock in the Catania area, in order to obtain a standard for defining the significant amplification spectral peaks. The standard deviation evaluated for the H / V produces spectral fluctuations of F0.5 units, around mean amplitude values not exceeding 3.0 units. Based on these fluctuations only spectral peaks higher than 3.5 units were taken to be statistically significant. The schematic cross-sections shown in Fig. 3 refer to the short profiles crossing the Tremestieri fault and the eruptive fracture. Along each profile, H / V spectral ratios obtained from the noise measurements collected at sites located at increasing distances on both sides of the fault are shown. Notably, at the sampling sites located in close proximity to the tectonic structure, H / V spectral ratios show a tendency towards amplification in the frequency range 4.0–8.0 Hz. The spectral
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amplitude decreases rapidly just a few tens of meters away from the discontinuity. A similar tendency is observed in several sites located on the eruptive fracture zone (sites 48 in A–AV and 40 in C–CV). Here the H / V spectra have amplification peaks at 4.5–5.0 Hz. Amplification peaks in the frequency range 1.5– 2.0 Hz are also observed at many sampling sites and are particularly evident at all locations along the profile B–BV. At several other sites (e.g. sites 11, 15, 31, 39, 52), significant amplification peaks are observed as well. However, as can be observed in the schematic lithological cross-section (Fig. 3), such effects may be related to specific local conditions, such as the presence of altered lavas and detritus. From the records of permanent stations, both Nakamura H / V and H / H spectral ratios to a reference site were calculated (Figs. 4 and 5 respectively). The H / V spectral ratios obtained from records of the broadband station cav2 confirm the presence of amplification in the same frequency range observed from mobile stations measurements (1.5–2.0 and 4.0–8.0 Hz). Fig. 5a shows the spectral ratio obtained by dividing the spectra of each of the horizontal components of cav2 and cav0 by the corresponding component from the bedrock reference station. It is worth noting that amplification is again observed in the frequency range 4.0–8.0 Hz, the same range found from the measurements made with mobile stations. Especially at station cav2, which is located in the uppermost portion of the fault escarpment, the mean H / H spectral ratio shows a pronounced peak at 4.0 Hz. In addition, the H / H spectral ratios for
Fig. 4. Nakamura H / V spectral ratio at cav2 station. Dashed curves indicate the F1 standard deviation about the geometric mean for ambient noise.
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Fig. 5. Spectral ratio to Catania reference site at cav0 and cav2 stations (a) and to cav0 reference site at cav2 station (b).
station cav2 were calculated using the station cav0 as reference site (Fig. 5b). The same amplification peak at 4.0 Hz is again visible, although it appears less pronounced. 4.1. 1-D modelling Besides the amplification between 4.0 and 8.0 Hz, observed in the H / V spectral ratios from records near the fault and fracture, the H / V spectral ratios in Fig. 3 show that amplification at about 1.8 Hz is also quite common in the study area. Such a peak is evident at almost all sites located along the profile B–BV and at many others (i.e. sites 35, 36, 38, 45, 46, 47, 49). In
order to validate the experimentally observed spectral peaks and to interpret them in terms of resonance effects due to the local stratigraphy, we performed 1D modelling using the Dobry et al. (1976) and the Haskell–Thomson methods. The Dobry et al. (1976) method is known as the bsuccessive use of two layers methodQ. It adopts as a physical model a simple resonant cavity closed at its lower extremity, which generally coincides with the contact between the bbasementQ and the overlying formations. In this case, the bbasementQ has a physical meaning rather than the classical geological one. It refers, in fact, to the physical interface beneath which the seismic input can be considered to be constant.
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Using this method it is possible to calculate the fundamental period of a stratigraphic sequence by dividing it into a series of layer pairs. The fundamental period T of a pair of layers is obtained by b Ta solving the equation: tanð p2 d TTa Þtanð p2 d TTb Þ ¼ qqb H a H a Tb 4Ha 4Hb where Ta ¼ Va and Tb ¼ Vb are the fundamental periods of the layers a and b, and H a , H b , Va , V b , q a and q b are the thicknesses, the shear wave velocities and the densities of the layers a and b, respectively. Moreover, through the Haskell–Thomson method, the theoretical transfer function for vertically incident SH waves was also computed, using as input data the thickness of the layers, the density values, and the velocities of transverse waves taken either from the literature or from field measurement. In particular, the S-wave velocity for the first thirty meters of depth was drawn from available seismic tomographic data and average values of the thickness of different lithotypes were taken from available borehole data. The results of two short tomographic seismic profiles, performed across the fault, are depicted in Fig. 6. They clearly show a shallow area, having shear wave velocities of about 200 and 300 m/s, correlated with the superficial layers and the gouge of the fault. Values of shear wave velocities for greater depth were
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taken from the shallower part of Etnean crustal models given in the literature (Lombardo et al., 1983; Hirn et al., 1991; Cardaci et al., 1993). Although it underestimates observed amplifications, the theoretical transfer function obtained using this modelling (Fig. 7) shows dominant resonant peaks sharing the same frequency range as the measured spectra (1.8– 2.0 Hz). The results obtained through the Dobry method show that the boundary surface beneath the resonant layer is at depth of about 60 m in the ambit of lavas (see values in italic in the lower right corner of Fig. 7). 4.2. Investigation on directional amplification effects The spectral ratios relative to the reference site showed small changes in the amplitude of the amplification peaks when either NS or EW components of the seismic signal were considered (Figs. 4 and 5). We therefore attempted to investigate possible directional effects connected to the presence of the Tremestieri fault, taking into account the ambient noise recorded at station cav2. Following Spudich et al. (1996), we have analysed the H / H spectral ratios as a function of both frequency and direction of motion. The horizontal plane was
Fig. 6. Seismic tomography profiles across the Tremestieri fault. The upper right picture shows an example of recent displacements.
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Fig. 7. Comparison between theoretic transfer function (dark line) and H / V spectral ratio (dashed line). Parameters used for 1-D modelling and result obtained are reported below the diagram.
divided into a set of directions spaced at 108 intervals from 08 (north) to 1808 (south) and the noise records of the horizontal components were rotated through these angles. The contours of the spectral amplification were plotted as a function of frequency and direction of motion. In Fig. 8 the contours of the geometric mean of the standard spectral ratios and the F1 standard deviation are shown. It is evident that directional resonance has a maximum at a frequency of 4.0 Hz, and the effect reaches a maximum when the horizontal components are rotated through the azimuth range N308E–N808E. 5. Discussion and concluding remarks H / V spectral ratios have been computed from ambient noise sampled with portable stations along three
profiles which cross the Tremestieri fault and an eruptive fracture. Amplification of H / V spectral dominant peaks were observed in two different frequency bands, 1.5–2.0 and 4.0–8.0 Hz. At the sites located close to the fault, significant amplification is observed in the frequency range 4.0– 8.0 Hz. It is also quite evident that, although there are a few exceptions (site 43 in profile A–AV), dominant spectral peaks tend to decrease in amplitude as the distance from the discontinuity increases. In the neighbourhood of the Calaveras fault (California), Mooney and Ginzburg (1986) have observed that the P wave velocities changes from about 3000 m/s within the fault to about 5000 m/s at its edges. Such differences are responsible for trapping seismic waves in the fault gouge and the consequent amplification. In the case of our study, information about the shear
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Fig. 8. Contours of the geometric mean of spectral ratios (H / H) as a function of frequency (x-axis) and direction of motion ( y-axis). The bar indicates the H / H amplitude.
wave velocity, available from field surveys and borehole data, show comparable changes. The S-wave velocity ranges from about 210 m/s within the gouge area to about 600 m/s in the shallow surrounding rocks. Rovelli et al. (2002) found, by 2D modelling of a fault area in central Italy, that a reduction of shear wave velocity by 40–50% inside the wedge of the fault itself can imply trapped waves, and consequently significant amplification effects can occur.
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The clear tendency observed of amplification effects localized in the gouge zone of the tectonic structure investigated here appears, therefore, to be explainable in such terms and to be related to focusing of seismic energy. Enhancement of local seismic amplitudes between 4.0 and 8.0 Hz is detected both near the fault and near the eruptive fracture, thereby confirming the important role played by the impedance contrast in enhancing site effects. The same frequency interval is found in both the H / V and the standard H / H spectral ratios computed using noise recorded at the permanent seismic stations. In particular, at station cav2, which is located in the uppermost portion of the fault escarpment, the mean H / H spectral ratio shows a pronounced peak at a frequency of 4.0 Hz. Analysing the H / H spectral ratios as a function of direction of motion, we observed a directionally enhanced resonance at 4.0 Hz, for azimuths ranging from N308E to N808E almost perpendicular to the strike of the fault. This result is quite different from the findings of other authors (Spudich et al., 1996; Cultrera et al., 2003) who have observed directional resonance along the strike of the fault. However, during the 1980 seismic sequence, NW–SE trending ruptures were accompanied by a left-stepping secondary system of en echelon cracks (Azzaro, 1999). Therefore, a correlation with the field evidence of secondary coseismic fractures having strike N458E, linked to the oblique component of motion of the Tremestieri fault, has in our opinion to be taken into account as a possible explanation of such findings. Besides the dominant spectral peaks connected to amplification effects near the fault zone, H / V spectral peaks at 1.8 Hz are widely observed in the studied area. In order to interpret these observations in term of resonance effects linked to the local stratigraphic sequence, we performed 1-D modelling. The theoretical transfer function confirms such resonance frequency. It can be interpreted to be associated with a stratigraphic sequence bounded downwards by an equivalent basement, located among the lavas, at a depth of about 60 m. It is worth noting that such amplification effects are predominant along the profile B–BV and that they partially mask and reduce the other observed amplifications. Along this profile, H / V spectral ratio amplifications in the range 4.0–8.0 Hz are in fact less evident or not present (site 16). It is, however, interesting to observe that this study has shown that important local propagation effects in tectonic structures can be detected not only through the
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analysis of seismic waves generated by earthquakes outside the fault but from records of ambient noise as well. We have not yet attempted to validate our findings using earthquake recordings as the number of events recorded is, at present time, not enough to be significant. Preliminary results from this ongoing research confirm our findings and the earthquake data show more pronounced high frequency local amplifications in the same frequency range. Moreover, we believe that studies of particle motion polarization are needed, as well as investigations of possible relationships between the directional resonance and the local Etnean anisotropy. Acknowledgements The authors wish to thank the anonymous reviewers for constructive comments which contribute to significantly improving the quality of the paper. A special thanks to Peggy Hellweg for helpful suggestions and comments in the drafting of the manuscript. This work was carried out in the framework of the research supported by the INGV-GNDT project: bDetailed scenarios and actions for seismic prevention of damage in the urban area of CataniaQ. References Azzaro, R., 1999. Earthquake surface faulting at Mount Etna volcano (Sicily) and implications for active tectonics. J. Geodyn. 28, 193 – 213. Bard, P.Y., 1999. Microtremor measurements: a tool for site effect estimation? In: Irikura, P.Y., Kudo, Okada, Sasatani (Eds.), The Effects of Surface Geology on Seismic Motion. Balkema, Rotterdam, pp. 1251 – 1279. Cara, F., Di Giulio, G., Rovelli, A., 2003. A study on seismic noise variations at Colfiorito, central Italy: implications for the use of H/V spectral ratios. Geophys. Res. Lett. 30 (18), 1972. doi:10.1029/2003GL017807. Cardaci, C., Coviello, M., Lombardo, G., Patane`, G., Scarpa, R., 1993. Seismic tomography of Etna volcano. J. Volcanol. Geotherm. Res. 56, 357 – 368. Cormier, V.F., Spudich, P., 1984. Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults. Geophys. J. R. Astron. Soc. 79, 135 – 152.
Cultrera, G., Rovelli, A., Mele, G., Azzara, R., Caserta, A., Marra, F., 2003. Azimuth-dependent amplification of weak and strong ground motions within a fault zone (Nocera Umbra, central Italy). J. Geophys. Res. 108. Dobry, R., Oweis, I., Urzua, A., 1976. Simplified procedures for estimating the fundamental period of a soil profile. Bull. Seismol. Soc. Am. 66 (4), 1293 – 1321. Donati, S., Marra, F., Rovelli, A., 2001. Damage and ground shaking in the town of Nocera Umbra during Umbria–Marche, central Italy, earthquakes: the special effect of a fault zone. Bull. Seismol. Soc. Am. 91 (3), 511 – 519. Hirn, A., Nercessian, A., Sapin, M., Ferrucci, F., Wittlinger, G., 1991. Seismic heterogeneity of Mt. Etna: structure and activity. Geophys. J. Int. 105, 139 – 153. Lo Giudice, E., Rasa`, R., 1992. Very shallow earthquakes and brittle deformation in active volcanic areas: the Etnean region as example. Tectonophysics 202, 257 – 268. Lo Giudice, E., Patane`, G., Rasa`, R., Romano, R., 1982. The structural framework of Mt. Etna. Mem. Soc. Geol. Ital. 23, 125 – 158. Lombardo, G., Gresta, S., Patane`, G., Cristofolini, R., 1983. Proposta di un modello di velocita` a due strati per la crosta superficiale della regione etnea, Atti 28 Convegno annuale G.N.G.T.S., Roma, 155–162. Marra, F., Azzara, R., Bellocci, F., Caserta, A., Cultrera, G., Mele, G., Palombo, B., Rovelli, A., Boschi, E., 2000. Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra: central Italy. J. Seismol. 4, 543 – 554. McGuire, W.J., Pullen, A.D., 1989. Location and orientation of eruptive fissures and feeder dykes at Mount Etna; influence of gravitational and regional tectonic stress regimes. J. Volcanol. Geotherm. Res. 38, 325 – 344. Monaco, C., Tapponnier, P., Tortorici, L., Gillot, P.Y., 1997. Late Quaternary slip rates on the Acireale–Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily). Earth Planet. Sci. Lett. 147, 125 – 139. Mooney, W.D., Ginzburg, A., 1986. Seismic measurements of the internal properties of fault zones. Pure Appl. Geophys. 124, 141 – 157. Nakamura, Y., 1989. A method for dynamic characteristics estimation of subsurface using microtremors on the ground surface. Quarterly Rept. RTRI, Jpn., vol. 30, pp. 25 – 33. Rovelli, A., Caserta, A., Marra, F., Ruggiero, V., 2002. Can seismic waves be trapped inside an inactive fault zone? The case study of Nocera Umbra, central Italy. Bull. Seismol. Soc. Am. 92, 2217 – 2232. Spudich, P., Olsen, K.B., 2001. Fault zone amplified waves as a possible seismic hazard along the Calaveras fault in central California. Geophys. Res. Lett. 28 (13), 2533 – 2536. Spudich, P., Hellweg, M., Lee, H.K., 1996. Directional topographic site response at Tarzana observed in aftershocks of the 1994 Northridge, California, earthquake: implications for mainshock motions. Bull. Seismol. Soc. Am. 86, S193 – S208.
Amplification of ground motion in fault and fracture zones: Observations from the Tremestieri fault, Mt. Etna (Italy) G. Lombardo *, R. Rigano Dipartimento di Scienze Geologiche, University of Catania, Italy Received 1 February 2005; received in revised form 31 August 2005; accepted 31 October 2005 Available online 30 January 2006
Abstract A series of ambient noise measurements have been performed to evaluate their use in detecting the possible influence of tectonic structures on local amplification of seismic waves. The area studied is located on the southeastern flank of Mt. Etna, a few kilometres north of the town of Catania. Using a mobile station, data were collected along three profiles which cross both a fault and an eruptive fracture. In addition, ambient noise was sampled at two permanent stations installed in close proximity to the fault, and at another station located within the city limits of Catania and considered to be a reference site. Analysis using spectral ratio techniques shows there is a tendency towards amplification at the sites located near the fault. In H / V spectral ratios such amplification is observed in the frequency range 4.0–8.0 Hz and the amplitudes of the dominant spectral peaks decrease rapidly over the first few tens of meters from the discontinuity. A similar tendency was observed for the sites located on the eruptive fracture zone. Dominant spectral peaks at 1.8 Hz are also common in the study area. The theoretical transfer function, obtained through 1-D modelling, indicates that such peaks can be related to the local stratigraphy. The mean H / H standard spectral ratio of microtremor evaluated at the permanent station located on the fault also shows an amplification peak at 4.0 Hz. Azimuthal analysis shows that the resonance at 4.0 Hz is directional, with the maximum in the azimuth range N308E–N808E. D 2005 Elsevier B.V. All rights reserved. Keywords: ambient noise; local seismic response; H / V spectral ratio; fault; eruptive fracture; directional resonance
1. Introduction The distribution of seismic energy depends on the complexity of morphologic and structural features of the investigated areas, as well as on the lithology. In experiments in California (Cormier and Spudich, 1984) both active and inactive faults behave as wave-guides for an impinging wave front. Waveform analysis of earthquakes recorded during the 1998 Umbria–Marche * Corresponding author. E-mail address: [email protected] (G. Lombardo). 0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.10.014
seismic sequence (Marra et al., 2000) showed a large local amplification corresponding to the fault zone. More recently, Donati et al. (2001) demonstrated that site response contributed significantly to the level of damage which affected the town of Nocera Umbra during such earthquakes. The highest levels of damage were estimated for two zones, on the soft sediment deposits in the Topino River valley and on the fractured marly sandstone terrains within a 200 m-wide fault zone crossing the town of Nocera Umbra. Spudich and Olsen (2001) show, through numerical simulations along the Calaveras fault (California), that the
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amplification caused by a low velocity zone may enlarge the area of increased hazard within 1 km of some faults. The aim of present paper is to investigate the effects of the tectonic structures located in the southernmost flank of Mt. Etna, at the northern boundary of the town of Catania, on the seismic wavefield (Fig. 1). This area is crossed by an eruptive fracture and by a segment of a NW–SE trending fault. The spectral ratio technique, using either earthquakes or ambient noise, is among the most commonly used methods to estimate ground motion amplification. Both horizontal-to-vertical component ratios and spectral ratios to a reference rock station are used. Many authors have recently demonstrated that the H / V spectral ratios from microtremor measurements are consistent in shape with H / V spectral ratios from earthquake recordings, and with earthquake spectral ratios using a reference bedrock station (Bard, 1999). Although there is not unanimous agreement on the theoretical justification for the use of ambient noise in evaluating site response, the use of H / V spectral ratios of ambient noise (Nakamura, 1989) has been widely adopted to characterise earthquake site response. Recently, investigations on the reliability of ambient noise measurements in microzonation studies have been tested in the SESAME project (see http://sesame-fp5.
Fig. 1. Location of the study area and investigated tectonic structure.
obs.ujf-grenoble.fr/) and results of tests on the implications for the use of H / V spectral ratios are shown in Cara et al. (2003). The method performs best when there is a strong velocity contrast between the bedrock and the soft upper layers. It can be reasonably assumed that a good impedance contrast between the bgougeQ of a fault and the surrounding rocks, as well as between a dyke intruded into an eruptive fracture and the surrounding materials, should exist. We therefore test the possibility of using microtremor measurements to determine local amplification effects in such peculiar situations. In this study ambient noise signals were analysed in order to evaluate whether the presence of a tectonic discontinuity can contribute to locally enhanced site effects. 2. Local geological and structural framework The tectonic setting of Mt. Etna results from the interaction of regional tectonics and local scale volcano-related processes (McGuire and Pullen, 1989). The youngest normal faults of the Etnean area are located along the base of the eastern flank of the volcano. Here, several NNW and NNE-trending fault segments, arranged in a 30 km long system, control the present topography and show steep escarpments with very sharp morphology (Monaco et al., 1997). The study area is located on the southeastern flank of Mt. Etna, a few kilometres north of the town of Catania, where morphological evidence of the Tremestieri fault is visible (Fig. 1). In this zone, an important structural trend is present which is responsible both for several faults and for numerous eruptive events that have developed along fissures (Lo Giudice et al., 1982). This area was selected because it is extensively covered by lava flows, and crossed by both an eruptive fracture and by a segment of Tremestieri fault, which trends NW–SE. Features of the outcropping lithotypes are particularly suitable for our study since the impedance contrasts between the fairly compact lavas and both the gouge of the fault and the dyke intrusion in the fracture help to enhance the focusing effect of seismic waves. The Tremestieri fault usually releases seismic energy in earthquake swarms; nevertheless, aseismic creep phenomena also occur frequently (Lo Giudice and Rasa`, 1992). During a seismic sequence in 1980, a left-stepping, secondary system of en echelon cracks accompanied the NW–SE-trending ruptures. On the whole, the displacement was right-lateral and oblique (Azzaro, 1999).
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3. Data acquisition and processing Twenty eight measurements of ambient noise were performed at sites located along three profiles which crossed both the fracture and the fault (Fig. 2). Each profile had a sampling site on the fault scarp. Near the fault, sites were spaced 20 m apart, and as the distance from the fault increased, site spacing increased to 50 and 100 m. Time histories of ambient noise were recorded using a three-component 1-Hz Mark L4C 3-D seismometer connected to a 12-bit analog-to-digital converter and a notebook computer. Sampling frequency was 100 Hz; two antialiasing filters cut higher frequencies with a 10 dB/oct slope. At each site, five time series of 120 s length were recorded. The time series were baseline corrected in order to remove spurious offsets and low-frequency trends. After the application of a Hanning window, a Fast Fourier Transform algorithm was applied to obtain spectra in the frequency band 0.5–15 Hz. The lower limit of
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0.5 Hz was chosen as the low-frequency cut-off, being where the standard deviation of the microtremor became equal to the natural noise fluctuation of the 12-bit digitizer. We applied the Nakamura (1989) technique, dividing the vector composition of horizontal component noise spectra by the vertical one. The final H / V results from the geometric mean of the five spectral ratios at each measurement site. A running, smoothing function of 0.1 Hz was also used, both on the spectra of each individual component and on the final ratio, to reduce spectral fluctuations. The stability of the adopted processing method was tested by calculating the arithmetic mean of the horizontal components as well, and the spectral ratios obtained did not show variations. Similarly, negligible differences were found when the H / V ratio was calculated using the mean spectrum of each noise component. In the framework of an agreement between researchers of INGV (Roma) and the Department of Geological Science of the University of Catania, two sites in the
Fig. 2. Geological and structural map of the studied area with location of permanent seismic stations and noise measurement sites.
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study area were instrumented with broadband seismometers coupled to Reftek 72A07 24-bit digitisers. The stations were synchronised by a GPS timing system. Stations cav0 and cav2 were situated on massive lavas next to a segment of the Tremestieri fault (Fig. 2). A third station, located on a Quaternary clay bedrock
outcropping within the city limits of the town of Catania, was chosen as the reference site. The seismometer deployment was planned to collect a significant number of earthquake records in order to compare results from their processing with those derived from the ambient noise measurements. At present,
Fig. 3. Schematic geolithological profiles and plots of H / V microtremors spectral ratios.
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data from only a small set of earthquakes is available, so we will only show findings from noise data. For this purpose, starting from selected 2 min intervals of microtremor, three 25 s segments with stationary signals were chosen for analysis. Each signal was cosinetapered and a trend removal function was applied before the use of amplitude FFT. The spectra were then smoothed using a 0.25 Hz running frequency window. 4. Results Before describing the results it is worth noting that ambient noise measurements were taken at several sites situated on massive lavas overlaying the clayey formation that forms the bedrock in the Catania area, in order to obtain a standard for defining the significant amplification spectral peaks. The standard deviation evaluated for the H / V produces spectral fluctuations of F0.5 units, around mean amplitude values not exceeding 3.0 units. Based on these fluctuations only spectral peaks higher than 3.5 units were taken to be statistically significant. The schematic cross-sections shown in Fig. 3 refer to the short profiles crossing the Tremestieri fault and the eruptive fracture. Along each profile, H / V spectral ratios obtained from the noise measurements collected at sites located at increasing distances on both sides of the fault are shown. Notably, at the sampling sites located in close proximity to the tectonic structure, H / V spectral ratios show a tendency towards amplification in the frequency range 4.0–8.0 Hz. The spectral
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amplitude decreases rapidly just a few tens of meters away from the discontinuity. A similar tendency is observed in several sites located on the eruptive fracture zone (sites 48 in A–AV and 40 in C–CV). Here the H / V spectra have amplification peaks at 4.5–5.0 Hz. Amplification peaks in the frequency range 1.5– 2.0 Hz are also observed at many sampling sites and are particularly evident at all locations along the profile B–BV. At several other sites (e.g. sites 11, 15, 31, 39, 52), significant amplification peaks are observed as well. However, as can be observed in the schematic lithological cross-section (Fig. 3), such effects may be related to specific local conditions, such as the presence of altered lavas and detritus. From the records of permanent stations, both Nakamura H / V and H / H spectral ratios to a reference site were calculated (Figs. 4 and 5 respectively). The H / V spectral ratios obtained from records of the broadband station cav2 confirm the presence of amplification in the same frequency range observed from mobile stations measurements (1.5–2.0 and 4.0–8.0 Hz). Fig. 5a shows the spectral ratio obtained by dividing the spectra of each of the horizontal components of cav2 and cav0 by the corresponding component from the bedrock reference station. It is worth noting that amplification is again observed in the frequency range 4.0–8.0 Hz, the same range found from the measurements made with mobile stations. Especially at station cav2, which is located in the uppermost portion of the fault escarpment, the mean H / H spectral ratio shows a pronounced peak at 4.0 Hz. In addition, the H / H spectral ratios for
Fig. 4. Nakamura H / V spectral ratio at cav2 station. Dashed curves indicate the F1 standard deviation about the geometric mean for ambient noise.
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Fig. 5. Spectral ratio to Catania reference site at cav0 and cav2 stations (a) and to cav0 reference site at cav2 station (b).
station cav2 were calculated using the station cav0 as reference site (Fig. 5b). The same amplification peak at 4.0 Hz is again visible, although it appears less pronounced. 4.1. 1-D modelling Besides the amplification between 4.0 and 8.0 Hz, observed in the H / V spectral ratios from records near the fault and fracture, the H / V spectral ratios in Fig. 3 show that amplification at about 1.8 Hz is also quite common in the study area. Such a peak is evident at almost all sites located along the profile B–BV and at many others (i.e. sites 35, 36, 38, 45, 46, 47, 49). In
order to validate the experimentally observed spectral peaks and to interpret them in terms of resonance effects due to the local stratigraphy, we performed 1D modelling using the Dobry et al. (1976) and the Haskell–Thomson methods. The Dobry et al. (1976) method is known as the bsuccessive use of two layers methodQ. It adopts as a physical model a simple resonant cavity closed at its lower extremity, which generally coincides with the contact between the bbasementQ and the overlying formations. In this case, the bbasementQ has a physical meaning rather than the classical geological one. It refers, in fact, to the physical interface beneath which the seismic input can be considered to be constant.
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Using this method it is possible to calculate the fundamental period of a stratigraphic sequence by dividing it into a series of layer pairs. The fundamental period T of a pair of layers is obtained by b Ta solving the equation: tanð p2 d TTa Þtanð p2 d TTb Þ ¼ qqb H a H a Tb 4Ha 4Hb where Ta ¼ Va and Tb ¼ Vb are the fundamental periods of the layers a and b, and H a , H b , Va , V b , q a and q b are the thicknesses, the shear wave velocities and the densities of the layers a and b, respectively. Moreover, through the Haskell–Thomson method, the theoretical transfer function for vertically incident SH waves was also computed, using as input data the thickness of the layers, the density values, and the velocities of transverse waves taken either from the literature or from field measurement. In particular, the S-wave velocity for the first thirty meters of depth was drawn from available seismic tomographic data and average values of the thickness of different lithotypes were taken from available borehole data. The results of two short tomographic seismic profiles, performed across the fault, are depicted in Fig. 6. They clearly show a shallow area, having shear wave velocities of about 200 and 300 m/s, correlated with the superficial layers and the gouge of the fault. Values of shear wave velocities for greater depth were
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taken from the shallower part of Etnean crustal models given in the literature (Lombardo et al., 1983; Hirn et al., 1991; Cardaci et al., 1993). Although it underestimates observed amplifications, the theoretical transfer function obtained using this modelling (Fig. 7) shows dominant resonant peaks sharing the same frequency range as the measured spectra (1.8– 2.0 Hz). The results obtained through the Dobry method show that the boundary surface beneath the resonant layer is at depth of about 60 m in the ambit of lavas (see values in italic in the lower right corner of Fig. 7). 4.2. Investigation on directional amplification effects The spectral ratios relative to the reference site showed small changes in the amplitude of the amplification peaks when either NS or EW components of the seismic signal were considered (Figs. 4 and 5). We therefore attempted to investigate possible directional effects connected to the presence of the Tremestieri fault, taking into account the ambient noise recorded at station cav2. Following Spudich et al. (1996), we have analysed the H / H spectral ratios as a function of both frequency and direction of motion. The horizontal plane was
Fig. 6. Seismic tomography profiles across the Tremestieri fault. The upper right picture shows an example of recent displacements.
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Fig. 7. Comparison between theoretic transfer function (dark line) and H / V spectral ratio (dashed line). Parameters used for 1-D modelling and result obtained are reported below the diagram.
divided into a set of directions spaced at 108 intervals from 08 (north) to 1808 (south) and the noise records of the horizontal components were rotated through these angles. The contours of the spectral amplification were plotted as a function of frequency and direction of motion. In Fig. 8 the contours of the geometric mean of the standard spectral ratios and the F1 standard deviation are shown. It is evident that directional resonance has a maximum at a frequency of 4.0 Hz, and the effect reaches a maximum when the horizontal components are rotated through the azimuth range N308E–N808E. 5. Discussion and concluding remarks H / V spectral ratios have been computed from ambient noise sampled with portable stations along three
profiles which cross the Tremestieri fault and an eruptive fracture. Amplification of H / V spectral dominant peaks were observed in two different frequency bands, 1.5–2.0 and 4.0–8.0 Hz. At the sites located close to the fault, significant amplification is observed in the frequency range 4.0– 8.0 Hz. It is also quite evident that, although there are a few exceptions (site 43 in profile A–AV), dominant spectral peaks tend to decrease in amplitude as the distance from the discontinuity increases. In the neighbourhood of the Calaveras fault (California), Mooney and Ginzburg (1986) have observed that the P wave velocities changes from about 3000 m/s within the fault to about 5000 m/s at its edges. Such differences are responsible for trapping seismic waves in the fault gouge and the consequent amplification. In the case of our study, information about the shear
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Fig. 8. Contours of the geometric mean of spectral ratios (H / H) as a function of frequency (x-axis) and direction of motion ( y-axis). The bar indicates the H / H amplitude.
wave velocity, available from field surveys and borehole data, show comparable changes. The S-wave velocity ranges from about 210 m/s within the gouge area to about 600 m/s in the shallow surrounding rocks. Rovelli et al. (2002) found, by 2D modelling of a fault area in central Italy, that a reduction of shear wave velocity by 40–50% inside the wedge of the fault itself can imply trapped waves, and consequently significant amplification effects can occur.
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The clear tendency observed of amplification effects localized in the gouge zone of the tectonic structure investigated here appears, therefore, to be explainable in such terms and to be related to focusing of seismic energy. Enhancement of local seismic amplitudes between 4.0 and 8.0 Hz is detected both near the fault and near the eruptive fracture, thereby confirming the important role played by the impedance contrast in enhancing site effects. The same frequency interval is found in both the H / V and the standard H / H spectral ratios computed using noise recorded at the permanent seismic stations. In particular, at station cav2, which is located in the uppermost portion of the fault escarpment, the mean H / H spectral ratio shows a pronounced peak at a frequency of 4.0 Hz. Analysing the H / H spectral ratios as a function of direction of motion, we observed a directionally enhanced resonance at 4.0 Hz, for azimuths ranging from N308E to N808E almost perpendicular to the strike of the fault. This result is quite different from the findings of other authors (Spudich et al., 1996; Cultrera et al., 2003) who have observed directional resonance along the strike of the fault. However, during the 1980 seismic sequence, NW–SE trending ruptures were accompanied by a left-stepping secondary system of en echelon cracks (Azzaro, 1999). Therefore, a correlation with the field evidence of secondary coseismic fractures having strike N458E, linked to the oblique component of motion of the Tremestieri fault, has in our opinion to be taken into account as a possible explanation of such findings. Besides the dominant spectral peaks connected to amplification effects near the fault zone, H / V spectral peaks at 1.8 Hz are widely observed in the studied area. In order to interpret these observations in term of resonance effects linked to the local stratigraphic sequence, we performed 1-D modelling. The theoretical transfer function confirms such resonance frequency. It can be interpreted to be associated with a stratigraphic sequence bounded downwards by an equivalent basement, located among the lavas, at a depth of about 60 m. It is worth noting that such amplification effects are predominant along the profile B–BV and that they partially mask and reduce the other observed amplifications. Along this profile, H / V spectral ratio amplifications in the range 4.0–8.0 Hz are in fact less evident or not present (site 16). It is, however, interesting to observe that this study has shown that important local propagation effects in tectonic structures can be detected not only through the
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analysis of seismic waves generated by earthquakes outside the fault but from records of ambient noise as well. We have not yet attempted to validate our findings using earthquake recordings as the number of events recorded is, at present time, not enough to be significant. Preliminary results from this ongoing research confirm our findings and the earthquake data show more pronounced high frequency local amplifications in the same frequency range. Moreover, we believe that studies of particle motion polarization are needed, as well as investigations of possible relationships between the directional resonance and the local Etnean anisotropy. Acknowledgements The authors wish to thank the anonymous reviewers for constructive comments which contribute to significantly improving the quality of the paper. A special thanks to Peggy Hellweg for helpful suggestions and comments in the drafting of the manuscript. This work was carried out in the framework of the research supported by the INGV-GNDT project: bDetailed scenarios and actions for seismic prevention of damage in the urban area of CataniaQ. References Azzaro, R., 1999. Earthquake surface faulting at Mount Etna volcano (Sicily) and implications for active tectonics. J. Geodyn. 28, 193 – 213. Bard, P.Y., 1999. Microtremor measurements: a tool for site effect estimation? In: Irikura, P.Y., Kudo, Okada, Sasatani (Eds.), The Effects of Surface Geology on Seismic Motion. Balkema, Rotterdam, pp. 1251 – 1279. Cara, F., Di Giulio, G., Rovelli, A., 2003. A study on seismic noise variations at Colfiorito, central Italy: implications for the use of H/V spectral ratios. Geophys. Res. Lett. 30 (18), 1972. doi:10.1029/2003GL017807. Cardaci, C., Coviello, M., Lombardo, G., Patane`, G., Scarpa, R., 1993. Seismic tomography of Etna volcano. J. Volcanol. Geotherm. Res. 56, 357 – 368. Cormier, V.F., Spudich, P., 1984. Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults. Geophys. J. R. Astron. Soc. 79, 135 – 152.
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