The extent of damage occurring at a specific location, apart from the magnitude of the earthquake and epicentral distance, significantly depends on local soil conditions (Reiter, 1990). These effects play an important role in the amplification of seismic ground motion at individual locations and must be determined for each site. It has been observed that ground motions recorded on loose soil (e.g., alluvial basins) are significantly higher than those recorded on rock outcrops. The main characteristics of the local soil are the average seismic shear wave velocity up to a depth of 30 meters (VS30) and the resonant frequency of the soil, which are determined through geophysical surveys using methods such as MASW and HVSR.
MASW (Multichannel Analysis of Surface Waves) is a non-destructive seismic method used to assess the thickness of soil layers, shear wave velocities (VS30), Poisson’s ratio, and soil density (Xia et al., 1999). It is based on the analysis of surface seismic waves generated by an artificial source (e.g., hammer, explosion). The method is fast and simple because such seismic sources can generate sufficiently strong surface seismic waves, which are then measured using an array of geophones (Figure 1). A typical MASW measurement system consists of a seismograph, 24 geophones (usually with a frequency of 4.5 Hz), and data collection and storage devices (Figure 2). The resolution of the recording depends on the spacing between geophones, while the depth of recording depends on the length of the profile and the strength and type of the wave source. Depending on specific measurement requirements, various geophone arrangements are possible, such as linear, L-layout, triangular, etc. Surface waves are dispersive, meaning that the phase velocity of the wave depends on the wavelength or frequency. This allows obtaining a dispersion curve by measuring the travel times of individual wave phases. Through inverse modeling from the dispersion curve, it is possible to obtain a 1D or 2D profile of the S-wave velocity (Figure 3; Foti et al., 2018).
Figure 1. MASW measurement using the Geometrics Atom wireless geophysical instruments
Figure 2. Schematic diagram of the MASW method measurements (Strelec et al., 2016)
MASW measurements are conducted using the wireless Geometrics Atom system, procured as part of the CRONOS project. Unlike a traditional wired system, it is not limited by cable length, allowing for arbitrary spacing between geophones and potentially greater depth of recording. Additionally, the geophones used have lower frequencies (2 Hz), which contributes to better quality recordings for deeper profiles.
Figure 3. Example of a 1D shear wave velocity profile obtained using the MASW method
The Horizontal-to-Vertical Spectral Ratio (HVSR) is a non-destructive seismic method that utilizes measurements of microseismic noise to obtain the fundamental resonant frequency of the ground, a key parameter used in earthquake engineering. HVSR is calculated as the ratio of the amplitude of the Fourier spectrum of the horizontal and vertical components of microseismic noise (Nakamura, 1989). Microseismic noise is the continuous shaking of the ground surface caused by natural sources (wind, ocean waves, distant earthquakes, etc.) and anthropogenic sources (industry, infrastructure, etc.). HVSR measurements are performed using a three-component velocimeter/accelerometer securely attached to the ground at the measurement location (Figure 4). The frequency at which the maximum amplitude occurs, or the frequency of the greatest amplification, is determined from the HVSR curves (Figure 5). This frequency is called the resonant frequency of the ground (f0) and is used for soil classification and determining the depth to the bedrock (H800, depth to the rock with shear wave velocities (VS) greater than 800 m/s), as well as estimating the average shear wave velocity up to a depth of 30 m (VS30) using empirical relationships (Stanko and Markušić, 2020) that connect H800 and VS30 with f0. HVSR measurements are conducted using Moho Tromino instruments, of which seven were procured as part of the CRONOS project.
Figure 4. left: the Moho Tromino instrument; right: HVSR measurement using the Moho Tromino instrument
Figure 5. Example of an HVSR curve measured using the new Tromino instrument
In addition to assessing the natural frequency of local soil and the depth to the bedrock, the HVSR method is also applied in the seismic monitoring of vulnerable buildings. In other words, the method and measurements can be performed both outside and inside buildings to estimate their own frequencies. Therefore, the HVSR method allows for the identification of potentially seismically hazardous areas in terms of soil-building resonance, which can be implemented in spatial planning and earthquake-resistant construction.
These mentioned methods are utilized within Component 2 of the CRONOS project to assess local soil conditions (including changes in local and regional soil properties) at selected instrument locations (accelerometers, seismographs). This will enable the development of local and regional maps for soil characterization based on VS30, according to the soil classification standard of Eurocode 8. These maps provide information on potential amplification or attenuation of ground motion and can be used for nonlinear amplification models based on empirical and stochastic modeling. Characterizing local soil conditions is necessary for probabilistic seismic hazard assessment on a real surface.
Prepared by Bruno Mravlja and Iva Lončar
LITERATURE:
Foti, S., Hollender, F., Garofalo, F., Albarello, D., Asten, M., Bard, P.Y., Comina, C., Cornou, C., Cox, B., Di Giulio, G. and Forbriger, T. (2018) ‘Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project’, Bulletin of Earthquake Engineering, 16(6), pp. 2367–2420. Available at: https://doi.org/10.1007/s10518-017-0206-7.
Nakamura, Y. (1989) ‘A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface’, Railway Technical Research Institute, Quarterly Reports, 30(1).
Reiter, L. (1990). Earthquake hazard analysis (254 p). Columbia University Press. References – Scientific Research Publishing. Available at: https://www.scirp.org/(S(vtj3fa45qm1ean45%20vvffcz55))/reference/referencespapers.aspx?referenceid=10 18276 (Accessed: 10 January 2022).
Stanko, D. and Markušić, S. (2020) ‘An empirical relationship between resonance frequency, bedrock depth and VS30 for Croatia based on HVSR forward modelling’, Natural Hazards, 103(3), pp. 3715–3743. Available at: https://doi.org/10.1007/s11069-020-04152-z.
Strelec, S., Stanko, D. and Gazdek, M. (2016) ‘Empirical correlation between the shear-wave velocity and the dynamic probing heavy test : case study Varaždin, Croatia’, Acta Geotechnica Slovenica, 13(1), pp. 3–15.
Xia, J., Miller, R.D. and Park, C.B. (1999) ‘Estimation of near‐surface shear‐wave velocity by inversion of Rayleigh waves’, GEOPHYSICS, 64(3), pp. 691–700. Available at: https://doi.org/10.1190/1.1444578.
