Shear-wave velocities in caracas inferred from inversion of phase velocities and ellipticities of rayleigh waves

C. Cornou¹, H. Cadet², V. Rocabado3, M. Schmitz³*, H. Rendón³, M. Causse¹, M. Wathelet¹

1 Laboratoire de Géophysique Interne et Tectonophysique, IRD, CNRS, UJF, Grenoble, France, e-mail: cecile.cornou@obs.ijf-grenoble

2 ITSAK, Thessaloniki, Greece, e-mail: kdhelo@gmail.com

3 FUNVISIS, Caracas, Venezuela, *corresponding autor, e-mail: mschmitz@funvisis.gob.ve

ABSTRACT

Reliable knowledge of soil mechanical properties, especially shear wave velocities, is important to estimate useful parameters in earthquake engineering (Vs30, response spectra, etc.) and to allow reliable numerical prediction of strong ground motion at frequencies of interest in earthquake engineering. Shear-wave velocity structure can be inferred either from borehole measurements (e.g. cross-hole or down-hole measurements) or from active or passive techniques using body- or surface-waves, respectively. Recent studies have also shown that ellipticity of Rayleigh waves can be extracted from microtremor measurements and subsequently jointly inverted with phase velocities in order to get reliable shear wave velocity profile down to seismic bedrock. In this paper, we present shear-wave profiles derived from joint inversion of phase velocity and ellipticity of Rayleigh waves obtained from microtremor array measurements at five different sites in Caracas. Phase velocities and ellipticities are extracted by using microtremor measurements. Derived shear-wave velocity profiles are consistent with available geotechnical and geophysical information.

Keywords: Microtremor arrays, Shear-wave profile, Ellipticity of Rayleigh waves, Joint inversion, Caracas.

RESUMEN

El conocimiento de las propiedades mecánicas del suelo, en específico de velocidades de propagación de las ondas de corte, resulta importante para estimar parámetros útiles en la ingeniería sísmica (Vs30, espectros de respuesta, entre otros), y para permitir predicciones numéricas confiables de los movimientos fuertes del terreno en los rangos de frecuencia de interés para la ingeniería sísmica. La estructura de la velocidad de propagación de las ondas de corte puede inferirse de mediciones de pozo (mediciones cross-hole o down-hole) o de técnicas de aplicación de ondas superficiales activas o pasivas. Además estudios recientes han demostrado que se puede extraer la elipticidad de ondas de Rayleigh de mediciones de microtremores, y que estos datos pueden ser invertidos conjuntamente con las velocidades de fase con el fin de obtener un perfil confiable de velocidades de ondas de corte hasta el tope del basamento sísmico. En este trabajo presentamos perfiles de velocidad de ondas de corte obtenidos de la inversión conjunta de las velocidades de la fase y de la elipticidad de las ondas de Rayleigh obtenidos en cinco sitios en Caracas. Las velocidades de fase y las elipticidades han sido extraídos de las mediciones de microtremores. Los perfiles de velocidad de las ondas de corte resultantes están consistentes con la información geotécnica y geofísica disponible.

Palabras clave: Arreglos de microtremores, Perfiles de velocidad de ondas de corte, Elipticidad de ondas de Rayleigh, Inversión conjunta, Caracas.

Recibido: octubre de 2009 Recibido en forma final revisado: julio de 2011

INTRODUCTION

During its history, Caracas has undergone several destructive earthquakes. The most recent one, the July 1967 Caracas earthquake, a magnitude 6.6 earthquake which occurred about 25 km northwest of Caracas (Suárez & Nábĕlek, 1990) caused damage to numerous buildings and the collapse of 4 multi-story buildings (Briceño et al. 1978). Since then, numerous studies have been performed in order to better assess building characteristics, seismic response and ground shaking characteristics (Seed at al. 1970; Papageorgiou & Kim, 1991; Abeki et al. 1998; Schmitz et al. 2002; Yamazaki et al. 2005). Especially, geological and geotechnical (about 170 drill holes down to bedrock) and geophysical surveys (seismic refraction, gravimetric measurements, H/V measurements) have allowed to derive a subsurface velocity model suitable for ground motion simulation (Weston, 1969; Kantak et al. 2005; Sánchez et al. 2005; Rocabado et al. 2006; Amarís et al. 2009). First 2D and 3D simulations of strong ground motion have thus outlined large 2D-3D site effects such as focusing effects and generation of surface waves diffracted at valley edges and in narrowing areas of the basin (Semblat et al. 2002; Delavaud, 2007). As outlined in Delavaud (2007) however, simulations are now missing a subsurface shear-wave velocity model enough detailed to enable reliable ground motion prediction up to frequencies of interest for earthquake engineering purposes.

Detailed shear-wave structure can be derived either from borehole measurements (cross-hole or down-hole measurements) or from active or passive techniques, using body- or surface-waves, respectively (Aki, 1957). Being non-invasive, passive surface-wave techniques are very useful to extract shear-wave velocities in urban environment. During spring 2006, microtremor array measurements have thus been carried out at five different locations in Cacaras, which have also been instrumented during five months in early 2006 for recording earthquakes. Phase velocities of Rayleigh waves were extracted by applying both SPAC and FK techniques (Wathelet et al. 2008), while Rayleigh waves ellipticities were measured by applying a newly developed technique within the framework of the on-going NERIES European project (NERIES, Deliverable D4, 2008). Estimation of ellipticity of Rayleigh waves is indeed very useful to retrieve information on the dispersive characteristics of Rayleigh waves (Fäh et al. 2003; Arai & Tokimatsu, 2004) in the low frequency range that is not easily investigable by using microtremor array measurement due to limited array apertures. Phase velocities and ellipticities of Rayleigh waves are then jointly inverted to get the shear-wave velocities over a large depth range (Arai & Tokimatsu, 2005). Reliability of derived shear-wave velocity profiles are then compared to available geophysical knowledge (borehole and SPT measurements).

AMBIENT SEISMIC NOISE MEASUREMENT: DATA, PROCESSING, SHEAR-WAVE VELOCITIES

Microtremor measurements have been performed by using seismological stations from the French mobile network (SISMOB) composed of Minititan3XT for the acquisition unit and Le3D-5s velocimeters having a cut-off frequency of 0.2 Hz. Sites location are indicated in figure 1 and array layouts in figure 2. Microtremors were recorded during thirty minutes to one hour by using array of different apertures in order to measure phase velocities over a wide range of frequencies. Dispersion curves of Rayleigh waves were estimated by using the FK and SPAC techniques as implemented in the SESARRAY package (http://www.geopsy.org; Wathelet et al. 2008). Minimum and maximum measured wavelengths as well as minimum inter-station distance and array aperture are indicated in table 1. Whenever dispersion curves could not be retrieved down to the resonance frequency of the site, we have used the Time Frequency Analysis (TFA) technique (NERIES, Devliverable D4, 2008) to extract the left flank of ellipticity curves of fundamental Rayleigh wave mode. This technique is based on the use of the modified Morlet wavelet for extracting time windows that consist predominantly of Rayleigh waves. This technique was applied to the deepest sites: Enfermeria, Parque del Este and San Ignacio (Location in figure 1). Then dispersion curves and ellipticity of Rayleigh waves –when used– were jointly inverted by using the Conditional Neighborhood Algorithm (Wathelet, 2008). In the inversion, parameterization of the model space consisted in a small number of layers (2 to 4) overlaying a homogeneous bedrock. Such simple parameterization has been shown to be suitable for reliable estimates (Savvaidis et al. 2009; Renalier et al. 2009). Joint inversion of phase velocities and ellipticities was done by considering an equal weight in the misfit computation for both data type. Figure shows, for each array, the set of shear-wave profiles having a misfit lower than one Sigma, i.e. explaining the data within its uncertainty bounds.

Bedrock depths derived from microtremor array measurements are consistent with known bedrock depth at 4 sites: Enfermeria (300 m), Hacienda La Vega (75 m), Parque del Este (110 m), UCV (55 m). As already known, such techniques are however not suitable to precisely estimate bedrock velocity (Cornou et al. 2009). Regarding shear-wave velocities averaged over the uppermost 10, 20 and 30 meters, velocities were computed by averaging the extreme average velocities extracted from the envelope of the set of average velocity profiles.

Average velocities are very consistent with borehole measurements in Enfermeria and Parque del Este (Inparques) sites, and they are within the range of the standard deviations (Table 2). For the San Ignacio site, discrepancy between average shear-wave velocity derived from borehole and microtremor array measurement is probably due to the fact that the minimum measured wavelength is too large (43 m) to allow reliable estimate at shallow depth (Cornou et al., 2009). Comparison between average shear-wave velocity derived from microtremor array measurement and SPT correlation (Cadet, 2008) shows SPT-derived velocities systematically lower than velocities derived from microtremor array measurements, except in San Ignacio and Hacienda La Vega sites.

CONCLUSIONS

We have shown that microtremor array measurements can be very useful to estimate shear-wave velocity profiles in Caracas. Since shear-wave velocities found in surficial layers (from 200 to 500 m/s) are much lower than the homogeneous shear-wave velocity (650 m/s) used in numerical modelling, such measurements should be repeated at various sites in Caracas valley in order to build a detailed subsurface shear-wave velocity model. Important variations are observed for the shear wave velocities down to bedrock, where velocities between 500 and 800 m/s prevail. As investigated sites in this paper have been also instrumented for several months in order to record earthquakes, the next step is to compare 1D amplification predicted by the 1D shear-wave velocity profiles with actual amplification in order to quantify the part of amplification due to 2D/3D site effects and, hereafter, to define an amplification correction function to apply to 1D transfer functions for accounting such 2D-3D site effects.

ACKNOWLEDGEMENTS

This work was supported by FONACIT/ECOS-Nord project Nr. 2004000347 and IRD. We aknowledge Myriam Kristekova and Donat Fäh for providing us the HVTFA algorithm. We also thank all participants of the microtremor measurements.

REFERENCES

1. Abeki, N., Seo K., Matsuda, I., Enomoto T., Watanabe, D., Schmitz, M., Rendón, H., Sánchez, A. (1998). “Microtremor observations in Caracas city, Venezuela”. In: Irikura et al., (ed.). The Effects of Surface Geology on Seismic Motion, Rotterdam, AA Balkema, 619-624.        [ Links ]

2. Aki, K. (1957). Space and time spectra of stationary stochastic waves, with special reference to microtremors. Bull. Earthq. Res. Inst. 35, 415-456.        [ Links ]

3. Amaris, E., Sánchez, J., Rocabado, V., Moncada, J., Schmitz, M., González, M. (2009). Espesores y características de los sedimentos profundos. Sub-capítulo 3.4, Informe Técnico Final, Volumen 1 Caracas, Proyecto de microzonificación sísmica en las ciudades Caracas y Barquisimeto (FONACIT 200400738), FUNVISIS FUN-035a, 2007, Inédito, p. 354-384.        [ Links ]

4. Arai, H. & Tokimatsu, K. (2004). S-wave velocity profiling by inversion of microtremor H/V spectrum. Bull. Seism. Soc. Am., 94, 53–63.        [ Links ]

5. Briceño, F., Sanabria, J., Azpúrua, P., Planchart, M., Castellanos, S., Olivares, A., Lustgarten, P., Kelemen, J., García, J., González de J., C., Carrillo, P., Pérez, H., Seed, H., Whitman, R., Murphy, V., Linehan, D., Turcotte, T., Steinbrugge, K., Espinosa, A., Algermissen, S., Arcia, J., Puig, J., Schmidt, L., González, J.V., Martínez, J., Knudson, C., Cran, C., Preshel, M., Holoma, S., Gómez, J., Luchsinger, J., Silva, M., Fortoul, C., Lamar, S., Grases, J., Vignieri, L., Valladares, E., Suárez, J., Gómez, G., Azpúrua, J., Paparoni, M., Ramos, C., Romero, A., Delgado, J., Azopardo, P., Grinsteins, V., Isaacura, J., Castellanos, H., Vargas, J. (1978). Segunda fase del estudio del sismo ocurrido en Caracas el 29 de julio de 1967. Comisión Presidencial para el Estudio del Sismo, Ministerio de Obras Públicas, 2 volúmenes, 1281 pp. (FUNVISIS, editor, Caracas).        [ Links ]

6. Cadet, H. (2008). Exchange program to Caracas, Venezuela, Mission Report. FONACIT/ECOS-Nord project Nr. 2004000347, FUNVISIS, internal report, 52 pp.        [ Links ]

7. Cornou, C., Ohrnberger, M., Boore, D. M., Kudo, K., Bard, P.-Y. (2009). Using ambient noise array techniques for site characterisation: results from an international benchmark, in Proc. 3rd Int. Symp. on the Effects of Surface Geology on Seismic Motion, Grenoble, 30 August - 01 September, 2006, vol. 2, LCPC Editions, 92 pages, in press.        [ Links ]

8. Delavaud, E. (2007). Simulation numérique de la propagation d’ondes en milieux géologiques complexes: application à l’évaluation de la réponse sismique du bassin de Caracas. PhD thesis, IPGP, France, pp. 155.        [ Links ]

9. FÄh, D., Kind, F., Giardini, D. (2003). Inversion of local Swave velocity structures from average H/V ratios, and their use for the estimation of site-effects. J. Seism. 7, 449–467.        [ Links ]

10. Geopsy. (2008). SESARRAY package, http://www.geopsy.org, page was last updated on: 2011-01-05.        [ Links ]

11. Kantak, P., Schmitz, M., Audemard, F. (2005). Sediment thickness and a west-east geologic cross section in the Caracas Valley. Rev. Fac. Ing. UCV, 20 (4), 43-56.        [ Links ]

12. Neries Deliverable D4. (2008). Using Ellipticity Information for Site Characterisation, EC project number 026130, 54 pages, page was last updated on: 2011-03- 10.        [ Links ]

13. Papageorgiou, A. S. & Kim, J. (1991). Study of the propagation and amplification of seismic waves in Caracas valley with reference to the 29 July 1967 earthquake: SH waves. Bull. Seism. Soc. Am., 81, 2214-2233.        [ Links ]

14. Renalier, F., Jongmans, D., Wathelet, M., Cornou, C., Endrun, B., Ohrnberger, M., Savvaidis, A. (2009). Influence of parameterisation on inversion of surface wave dispersion curves and definition of a strategy of inversion. EGU 2009-7799, Vienna, Austria.        [ Links ]

15. Rocabado, V., Schmitz, M., Rendón, H., Vilotte, J.-P., Audemard, F., Sobiesiak, M., Ampuero, J.-P., Alvarado, L. (2006). Modelado numérico de la respuesta sísmica 2D del valle de Caracas. Rev. Fac. Ing. UCV, 21 (4), 81-93.        [ Links ]

16. Savvaidis, A., Ohrnberger, M., Wathelet, M., Cornou, C., Bard, P-Y. and Theodoulidis, N. (2009). Variability Analysis of Shallow Shear Wave Velocity Profiles Obtained from Dispersion Curve Inversion considering Multiple Model Parametrizations, SSA meeting, Poster# 54, Monterey, USA.        [ Links ]

17. Sánchez, J., Schmitz, M., Cano, V. (2005). Mediciones sísmicas profundas en Caracas para la determinación del espesor de sedimentos y velocidades sísmicas. Boletín Técnico IMME, 43 (2), 49-67.        [ Links ]

18. Seed, HB., Idriss, IM., Dezfulian, H. (1970). Relationships between soil conditions and building damage in the Caracas earthquake of July 29, 1967. EERC-Report 70-2, Berkeley, California, 40 pp.        [ Links ]

19. Schmitz, M., Enomoto, R., Ampuero, J.-P., Rocabado, V., Kantak, P., Sánchez, J., Rendón, H., González, J., Abeki, N., Villote, J.-P.,  Navarro, M., Delgado, J. (2002). Seismic microzonation study in Chacao district, Caracas, Venezuela. 12th European Conference on Earthquake Engineering, London, paper#808.        [ Links ]

20. Semblat, J. F., Duval, A. M., Dangla, P. (2002). Seismic site effects in a deep alluvial basin: numerical analysis by the boundary element method. Computers and Geotechnics, 29, 573-585.        [ Links ]

21. Suárez, G. & NábĔlek, J. (1990). The 1967 Caracas earthquake: fault geometry direction of rupture propagation and seismotectonic implications. J. Geophys. Res., 95, (B11), 17 459-17 474.        [ Links ]

22. Wathelet, M. (2008). An improved neighborhood algorithm: parameter conditions and dynamic scaling. Geophys. Res. Lett., 35, L09301, doi:10.1029/2008GL033256.        [ Links ]

23. Wathelet, M., Jongmans, D., Ohrnberger, M., Bonnefoy-Claudet, S. (2008). Array performances for ambient vibrations on a shallow structure and consequences over Vs inversion. J. Seism., 12, 1-19.        [ Links ]

24. Weston Geophysical Engineers International Inc, E. (1969). Investigaciones Sísmicas en el Valle de Caracas y en el Litoral Central (bajo la planificación y supervisión de la Comisión Presidencial para el Estudio del Sismo), Caracas. 22 pp.        [ Links ]

25. Yamazaki, Y., Audemard, F., Altez, R., Hernández, J., Orihuela, N., Safina, S., Schmitz, M., Tanaka, I., Kagawa, H., and Jica Study Team-Earthquake Disaster Group. (2005). Estimation of the seismic intensity in Caracas during the 1812 earthquake using seismic microzonation methodology. Revista Geográfica Venezolana, Número Especial 2005, 199-216.        [ Links ]