Seismic site classification of the Costa Rican Strong-Motion Network based on VS30 measurements and site fundamental period Luis A. Pinzón1, Diego A. Hidalgo Leiva2, Aaron Moya-Fernández2, Victor Schmidt-Díaz2, Luis G. Pujades3. 1 Research Division, Universidad Católica Santa María La Antigua, Panama City, Panama 2 Earthquake Engineering Laboratory, Universidad de Costa Rica, San Jose, Costa Rica 3 Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya, Barcelona, Spain Keywords: Site classification; site fundamental period; shear-wave velocity; Costa Rica; HVSR; Vs30 Palabras clave: Clasificación de sitio; período fundamental de sitio; velocidad promedio de la onda de corte; Costa Rica; relaciones horizontales/ verticales; Vs30 ISSN 1794-6190 e-ISSN 2339-3459 https://doi.org/10.15446/esrj.v25n4.93927 EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 25, No. 4 (December, 2021): 383-389 SE IS M O L O G Y Record Manuscript received: 26/02/2021 Accepted for publication: 01/10/2021 ABSTRACT In this paper, the soil profile of the Costa Rican Strong-Motion Network (CRSMN) stations is classified based on actual measurements and seismic regulations. The soil classification of the Costa Rican Seismic Code based on the average shear-wave velocity of the top 30 m (VS30) is used as a reference. The site fundamental period (Tf) is included as a pa- rameter to complement the existing characterization. For this, VS30 measurements from 52 accelerometric stations are related to the site fundamental period obtained through horizontal-to-vertical spectral ratios (HVSR) using ground motion records from the Costa Rican Strong-Motion Database. The H/V ratios are estimated with 5% damped accele- ration response spectra and with traditional Fourier amplitude spectra from the S-wave window. From the relation be- tween VS30 and Tf, different ranges of Tf are assigned to the existing soil profile classification and a graph with three-lines and four-areas is proposed to classify the stations of the CRSMN. These lines are plotted from the intersection between values assigned to each site class. The assigned classification at each station will be the one corresponding to the area where the pair of values (VS30 – Tf) falls. With this proposal, both parameters take relevance and are compensated, reducing the differences due to possible errors in measurements or interpretations. Clasificación de sitio de la Red de Acelerógrafos de Costa Rica basada en mediciones de VS30 y en el período fundamental RESUMEN En este artículo, se propone una nueva clasificación de sitio para la Red de Acelerógrafos de Costa Rica (RACR). Se utiliza como referencia la clasificación de sitio del Código Sísmico de Costa Rica basada en la velocidad promedio de la onda de corte de los 30 m superiores (VS30). El período fundamental del sitio (Tf) se incluye como parámetro para com- plementar la caracterización existente. Para ello, las mediciones de VS30 de 52 estaciones acelerométricas se relacionan con el período fundamental del sitio, este obtenido a partir de relaciones espectrales H/V utilizando registros de la Base de Datos de Movimiento Fuerte de Costa Rica. Las relaciones H/V se estiman con espectros de respuesta de aceleración con 5% de amortiguamiento y con espectros de amplitud de Fourier tradicionales obtenidos de la ventana de ondas S. A partir de la relación entre VS30 y Tf, fueron asignados diferentes rangos de Tf a la clasificación de perfil de suelo exis- tente y se propone un gráfico con tres líneas y cuatro áreas para clasificar las estaciones de la RACR. Estas líneas fueron graficadas a partir de la intersección de los valores asignados a cada clase de sitio. La clasificación de cada estación será la correspondiente a la zona donde se encuentre el conjunto de valores VS30 y Tf. Con esta propuesta, ambos parámetros toman relevancia y se compensan, reduciendo las diferencias por posibles errores en las mediciones o interpretaciones. How to cite item: Pinzon, L. A., Hidalgo-Leiva, D. A, Moya-Fernandez, A., Schmidt-Diaz, V., & Pujades, L. G. (2021). Seismic site classification of the Costa Rican Strong- Motion Network based on V S30 measurements and site fundamental period. Earth Sciences Research Journal, 25(4), 383-389. https://doi.org/10.15446/esrj. v25n4.93927 https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.93927 https://doi.org/10.15446/esrj.v25n4.93927 384 Luis A. Pinzón, Diego A. Hidalgo Leiva, Aaron Moya-Fernández, Victor Schmidt-Díaz, Luis G. Pujades. Introduction Costa Rica is considered one of the countries with the greater seismicity of the region. Its complex tectonic framework, subduction zone and active volcanism in the continental zone, has motivated to develop more studies to reduce the seismic risk of the country (Protti and McNally 1994; Quintero and Güendel 2000; Moya-Fernández et al. 2020). In seismic risk studies, three features must be taken into account: the hazard, vulnerability and damage value. A correct definition of the seismic hazard is essential to obtain reliable results. Seismic sources, site characterization and ground motion prediction models must be defined (Douglas 2017; Pinzón et al. 2019a). Several parameters have been used as proxy to define the seismic site conditions based on the topography, geology, site fundamental period (or frequency) and shear-wave velocity of the soil profile. Among these, the most used is the average shear-wave velocity in the top 30 m of the soil profile (VS30). In United States, the NEHRP (BSSC 2003) and the ASCE 7-16 (ASCE 2017) proposed six site classes based on VS30, from A (hard rock, VS30 > 1500 m/s) to F (soil with special condition). The Costa Rican Seismic Code (CRSC) (CFIA 2016) uses an equivalent classification, based on VS30, varying only nomenclature (S1 to S4) (Dobry et al. 2000). The VS30 is estimated using the following expression: (1) where di is the thickness of each soil layer in the profile until it reaches 30 m deep, VSi is the shear-wave velocity of each layer in m/s and N is the number of layers until it reaches the 30 m. Recent studies show some limitations and problems of using VS30 to classify sites (Steidl 2000; di Alessandro et al. 2012; Pinzón et al. 2019a). Some of these limitations are the limited disposal of near-surface shear-wave data at strong-motion sites, the complexity of data acquisition, and the fact that these classifications do not consider the effect of the thickness of soft sediments. As an alternative, classifications based on the site fundamental period (Tf) have been used (Zhao et al. 2006; Ghasemi et al. 2009; di Alessandro et al. 2012; Pinzón et al. 2019a). The site fundamental period is a parameter that characterizes the dynamics conditions of the soil and has been used as a proxy for soil amplification in ground-motion prediction models (di Alessandro et al. 2012). There are different methods to estimate the fundamental period, but the most used are those based on the horizontal-to-vertical spectral ratios (HVSR). One is the conventional HVSR with the Fourier amplitude spectra from the S-wave window. This approach has been used to determine different dynamic characteristics of the ground through micrometer measurements (Nakamura 1989; Alfaro et al. 2001; Caselles et al. 2010) and strong-motions from earthquakes (Nagashima et al. 2014). The ratio is defined by the following expression: (2) where HF and VF are the horizontal and vertical Fourier amplitude spectra of ground-motion determined with the S-wave window, respectively. Another of the most recently used technique is the one based on the proposal of Zhao et al. (2006). With this approach, a site classification for Japan was determined from all the seismic records available on each station (Zhao et al. 2006). The method is based on the estimation of the spectral ratio H/V from the acceleration response spectra with a 5% of damping for the horizontal components (SAH) and the vertical (SAV) (see Equation 3). (3) Zhao et al. (2006) found that the use of averages among all the HVSR obtained at each station eliminates extreme peaks or anomalies. Besides, they observed that these averages are not strongly affected if data is segregated by hypocentral distance, magnitude or depth. In Costa Rica, the strong-motion network (CRSMN) operates since 1983 and is administrated by the Earthquake Engineering Laboratory at the University of Costa Rica (LIS-UCR for its acronym in Spanish). The LIS- UCR is in charge of recording, processing and storing all acceleration records for academic and research purposes. The network has more than 150 active accelerometric stations around the country. Further details about the network can be found in Moya-Fernández et al. (2020). VS30 measurements on each station is essential in order to classify the site conditions of this network based on existing regulations. The main issue is that only a third of the stations have this kind of measures. The lack of data due to the high cost of measurements and other issues, introduces uncertainty in the estimation of site conditions that directly affect the seismic hazard studies carried out in the country. In recent studies, several relations between the site fundamental period, the topography and the Vs30 were found (Wald and Allen 2007; Cadet et al. 2011; Pinzón et al. 2019b). These relations cannot be used to classify Costa Rican stations since they depend on the geomorphology of the region. The main objective of this study is to classify the sites of the accelerometric stations of the CRSMN, considering the current seismic regulations and data limitation. To do this, a relation between the site fundamental period and the VS30 is defined to be able to classify stations that do not have VS30 measurements. Average shear-wave velocity of the top 30 m (VS30) In the last two years, VS30 have been measured in 52 accelerometric stations (33% of the network) with the aim of improving the existing classification. To obtain VS30 values, the stations of the CRSMN were scanned using the multichannel analysis of surface waves (MASW) geophysical method. The MASW is one of the most used seismic recognition methods where the elastic condition (stiffness) of the soil is evaluated. In this method, the velocity of the surface-waves is measured at different frequencies, obtaining the depth variation of the shear-wave velocity (VS) from the measured soil profile. The variation of velocity at different frequencies is mainly attributed to the stratification of the S-waves velocities whose values are obtained with a numerical inversion system. In Figure 1, the shear-wave velocity profile from the accelerometric stations Puntarenas-Cóbano (PCOB) and Puntarenas-Paquera (PPQR) are presented. These stations present VS30 values of 407.5 and 214.1 m/s respectively. Figure 1. Shear-wave velocity profiles from stations (a) PCOB and (b) PPQR. 385Seismic site classification of the Costa Rican Strong-Motion Network based on VS30 measurements and site fundamental period The fundamental period through horizontal-to-vertical spectral ratios (HVSR) Because almost 70% of the stations do not have measured values of VS30, it was decided to determine the fundamental period of each site (station) through horizontal-to-vertical spectral ratios (HVSR) and fill the gap produced by the lack of information. To estimate site fundamental periods, the average of the HVSR obtained from all the records available at each station is calculated with HVSRSA(5%) and HVSRFourier. Then, the average of both fundamental periods (from both methods) is estimated, obtaining a unique value per station. Records with high acceleration values were discarded (PGA > 0.30 g), preventing fundamental period values from high deformations that could lead to a nonlinear response of the soil (Pinzón et al. 2019a). In Figure 2, the results obtained from the HVSR of the accelerometric stations PCOB and PPQR are exposed. The HVSR are presented normalized by the maximum value. Site fundamental periods of 0.25 and 0.63 s were obtained in PCOB and PPQR stations respectively. HVSRFourier presents several peaks while the HVSRSA(5%) has a smoother shape. In both cases, the fundamental period was properly captured. This method was applied to the entire network, obtaining the site fundamental period at the 157 stations. A comparison of the results obtained with the HVSRSA(5%) and HVSRFourier for all the stations is shown in Figure 3. Figure 2. Horizontal-to-vertical spectral ratios normalized by the maximum value and the site fundamental period from stations (a) PCOB and (b) PPQR. Figure 3. Comparison between the site fundamental periods obtained with HVSRSA(5%) and HVSRFourier. VS30 and site fundamental period relation The VS30 values measured were related to their site fundamental period. Results from the relation are observed in Figure 4. As seen in the figure, the relation follows a bilinear trend with a constant value of VS30 (260 m/s) in sites with a fundamental period higher than 0.5 s. Similar results were obtained in the United States from Central and Eastern North America (CENA) sites (Hassani and Atkinson 2016). Figure 4b shows the relation in linear scale. It is observed that the models fit the measured data reasonably well. The dispersion obtained is similar to that obtained in other studies (see Figure 9 from Hassani and Atkinson, 2016, and Figure 8 from Ghofrani and Atkinson, 2014). The bilinear model is defined by the following equations: (4) (5) As seen in the results, some sites with a null period have VS30 values that are not indicative of a rock outcrop. It is important to note that, in those cases, a Tf = 0 s was assigned since the fundamental period was not identifiable, presenting a flat H/V with amplitudes < 2. For fundamental periods of the soil deposit larger than 0.5 s, the graph shows that sites with similar VS30 can have significantly different Tf values. This outcome, observed also in other studies (Hassani and Atkinson, 2016), is quite significant. This behavior points a significant drawback of the VS30 approach for seismic site classification. For instance, in those cases, the considered 30 m might not be sufficient to characterize the vibration period of the soil deposit and, therefore, VS30 might not provide a reasonable proxy of the seismic response of the site. On Table 1, we have made a proposal relation between the VS30 used on the CRSC and the soil fundamental period. Sites with fundamental periods lower than 0.15 s are considered rock sites (S1), between 0.15 and 0.35 s stiff soils (S2), 0.35 and 0.75 s soft soils (S3) and sites with fundamental period higher than 0.75 s are very soft soils (S4). To assign this classification, only Equation 4 was used (see the dashed line of Fig. 4). Equation 5 was neglected since sites with very soft soils (S4) could not be classified in terms of VS30. Although, in terms of safety (design), this proposal can be beneficial since sites with periods larger than 0.50 s will result in a lower classification compared to the VS30 approach, it is important to note that the use of Equation 4 for periods larger than 0.50 s is arbitrary and not supported with data. Probably, as mentioned before, the considered 30 m might not be sufficient to identify the fundamental period of the soil deposit. Nevertheless, as can be seen in Zhao et al. (2006) and di Alessandro et al. (2012), a period of 0.60 s has been used as a 386 Luis A. Pinzón, Diego A. Hidalgo Leiva, Aaron Moya-Fernández, Victor Schmidt-Díaz, Luis G. Pujades. Figure 4. Measured Vs30 values at the station of the Costa Rican Strong-Motion Network plotted against site fundamental period values obtained from HVSR. (a) The bilinear model (continuous line – Equation 4 for periods < 0.5 s and Equation 5 for periods ≥ 0.5 s) and the proposed model based on the Costa Rican Seismic Code (dashed line – Equation 4) are depicted. (b) Plot in linear scale. limit to classify soils S4 (Equivalent to NEHRP soil E), so our proposal at 0.75 s, is not very different from those already proposed. A graphic with three-lines and four-areas is proposed to classify stations with both VS30 and site fundamental period values (see Fig. 5). Each area (triangle) represents a site class based on the CRSC. The lines are plotted from the intersection between values assigned to each site class (see Table 1). The assigned classification at each station will be the one corresponding to the area where the pair of values (VS30 – Tf) falls. Although this definition is arbitrary, we believe is a reasonable limit according to the available information we have presented in this paper. With this proposal, both parameters take relevance and are compensated, reducing the differences due to possible errors in measurements or interpretations. In this way, we can guarantee that sites will have similar geodynamic characteristics, and consequently possible error sources in the use of accelerograms are reduced (e.g., in the development of ground motion prediction equations or the dynamic analysis of structures). Using this classification, stations PCOB and PPQR are classified as S2 and S3 respectively. Figure 5. Site classification proposal based in VS30 and site fundamental period values. CRSMN site characterization From the 157 stations of the network, 52 were classified with the proposal described above and the other 105 were categorized only with their site fundamental period using Table 1 ranges. In Figure 6, a map of Costa Rica with the geographic distribution of the accelerometric stations and their assigned site class is shown. A total of 12 stations were classified as rock sites, 52 as stiff soils, 67 as soft soils and 26 as very soft soil (see Fig. 7). A complete list with the assigned site class of each station is available at the LIS-UCR website (http:// www.crsmd.lis.ucr.ac.cr/?id=Estaciones). In Figure 7b, the mean value of the HVSR per site class, estimated from ratios of all the stations is exposed. The shapes of the mean HVSR values correspond to the expected. The rocky sites have a flat shape with amplification Table 1. Site classification of the Costa Rican Seismic Code, NEHRP equivalence, VS30 ranges and site fundamental periods proposed. Site class based on Costa Rican Seismic Code (CRSC) (CFIA 2016) NEHRP (BSSC 2003) equivalence Shear-wave velocity of top 30 m Site fundamental period S1 (rock) A+B Vs30 ≥ 750 m/s Tf < 0.15 s S2 (stiff soil) C 360 < Vs30 ≤ 750 m/s 0.15 ≤ Tf < 0.35 s S3 (soft soil) D 180 < Vs30 ≤ 360 m/s 0.35 ≤ Tf < 0.75 s S4 (very soft soil) E Vs30 ≤ 180 m/s Tf ≥ 0.75 s http://www.crsmd.lis.ucr.ac.cr/?id=Estaciones http://www.crsmd.lis.ucr.ac.cr/?id=Estaciones 387Seismic site classification of the Costa Rican Strong-Motion Network based on VS30 measurements and site fundamental period −86˚ −85˚ −84˚ −83˚ 8˚ 9˚ 10˚ 11˚ 75 km Nicaragua P a n a m a Soil type S1 (Rock) S2 (Hard soil) S3 (Soft soil) S4 (Very soft soil) 10 km Figure 6. Map with the geographical distribution of the accelerometric stations with the assigned soil class. 388 Luis A. Pinzón, Diego A. Hidalgo Leiva, Aaron Moya-Fernández, Victor Schmidt-Díaz, Luis G. Pujades. values less than two, while sites S2, S3 and S4 have amplification values around three in the range of periods assigned on each site class (see Table 1). The H/V spectral ratio for S4 soils has a peak zone from 0.9 to 1.5 seconds that is reasonably consistent with the definition of this site class being dominant periods larger than 0.75 seconds. Figure 7. (a) Number of stations per site class and (b) Mean HVSR from all the stations per site class. Conclusions In this study, a site classification has been assigned to the stations of the Costa Rican Strong-Motion Network. A total of 157 stations were classified based on VS30 measurements and the site fundamental period values obtained from HVSR of recorded ground-motions. This classification arises from the relationship between the Tf and the VS30 values measured in 52 stations. Because the current classification is based on the VS30, the proposed one has the advantage that stations that do not have VS30 measurements can be classified using the Tf calculated from the earthquake acceleration records. With this classification, since it considers the fundamental period, we guarantee that sites assigned in the same class have an equivalent dynamic response. This fact plays an important role in the assessment of structures when ground-motion records are used for deterministic and/or probabilistic time-history analyses, decreasing uncertainty and improving the results for a specific region. The influence of this classification on the definition of new ground motion prediction model, compared to the existing/traditional ones remains a significant and interesting issue. Although we are aware of the limitations of the VS30 as a parameter to classify sites, based on the results obtained in this study, we were forced to use it in this study since it is the basis of the current regulations. In this sense, we would like to recommend to those responsible of the seismic regulations to reconsider using VS30 as a measure to classify sites. From our perspective, this parameter has clear deficiencies, which do not allow us to accurately classify soft soil sites (T ≥ 0.5 s), as is observed in Figure 4. Therefore, we believe that it is necessary to use measures that guarantee a reliable site classification, grouping sites with similar geodynamic characteristics, as is the case of the use of the site fundamental period as proxy. Acknowledgements This research was partially funded by the National Emergency and Risk Prevention Law N° 8933 from Costa Rica and the UCREA funds from the University of Costa Rica through the project referenced as B9780. 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