As part of Component 2 of the CRONOS project, geophysical measurements were carried out (Figure 1) in order to determine the main characteristics of the local soil. The mean seismic speed of transverse waves from the surface to a depth of 30 m (V_S30) and the fundamental ground frequency (f₀) were determined using the MASW and HVSR methods.

Figure 1. Map of the locations of all measurements made

MASW

Figure 2. MASW measurement system

MASW measurements are used to estimate soil layer thickness, shear wave velocity (V_S30), and soil density (Xia et al., 1999). From the recording of induced seismic waves from the field (Figure 3), a dispersion curve (Figure 4) is obtained, and as a final result, the dispersion curve is inverted to find which model of seismic wave velocities in soil or rock layers corresponds to it (Figure 5; Foti et al. ,2018). On the example of loose soil, the obtained V_S30 value is 310 m/s.

Figure 3. Recording of seismic waves from the field

Figure 4. Dispersion diagram

Figure 5. Velocity profile of loose soil

HVSR

Figure 6. Tromino

The HVSR method passively measures ambient vibrations, i.e. microseismic disturbance using the Tromino device (Figure 6). By analyzing the ratio of the horizontal and vertical components of the motion (Nakamura, 1989), the fundamental frequency of the ground (f₀) and the depth to the bedrock (H₈₀₀) are determined, and V_S30 is estimated using empirical relations (Stanko and Markušić, 2020). After removing the noises from the field recording (Figure 7), a graph of the amplitude spectrum (Figure 8) and the HVSR curve (Figure 9) are obtained, from which the basic frequency of the ground (the highest peak on the curve) is then read. In this example, the fundamental frequency of loose soil is 4 Hz.

Figure 7. Measurement stability

Figure 8. Amplitude spectrum

Figure 9. HVSR curve

Figure 10. HVSR curve for the location in Vrlika

You can read more about the implementation and description of the measurements on the project’s blog.

References:

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. 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).

 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. https://doi.org/10.1007/s11069-020-04152-z.

 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. https://doi.org/10.1190/1.1444578.

After the procedure of selecting borehole locations for the installation of borehole accelerographic systems, which was based on the results of field prospecting and geophysical research, drilling was carried out. In the period from September 25 to October 6, 2023, Ph.D. Tvrtko Korbar, associate of Component 2 of the CRONOS project, geologically determined the cores of geotechnical wells in Imotski, Sinj and Vrlika, as follows:

1. Imotski

The uppermost surface layer, up to 1 m thick, is recent soil, composed of poorly compacted silt. After that, mostly loose gravels and sands were drilled to a depth of about 10 meters, along with some horizons richer in silt. Deeper, all the way to the bottom of the well (40 m), there is light gray poorly consolidated silt, with an increased proportion of clay towards the bottom of the well. Carbonate material dominates throughout the core. The Quaternary age of all the mentioned deposits is assumed.

Figure 1. The core of the borehole in Imotski with a total length of 40 m.

2. Sinj

The top 2 m thick layer is predominantly loose silt. Deeper there is siltous clay, brown in the upper part, and light gray to dark gray, even black (bituminous) in the lower part, where the interlayer of peat has been pierced. At 23.5 m there is a half-meter thick layer of weakly cemented limestone with mollusk shells. At the very bottom of the well (25 m) there is gray siltous compacted clay. All determined deposits are probably of Quaternary age, while the deposits at the bottom of the borehole possibly belong to the peak Neogene. The layers are horizontal.

Figure 2. The core of the borehole in Sinj with a total length of 25 m.

3. Vrlika

The uppermost meter consists of recent soil in the form of compacted brown earth with mostly small carbonate rock rubble (potentially anthropogenic). Up to 14 m we find deposits of reddish to yellowish-brown compacted siltous clay, which is of Quaternary age. All the way to the bottom of the borehole (17 m) there is gray deformed gypsum, with fragments and/or fillings of grey siltous clays. Gypsum with admixtures is of Permo-Triassic age and forms the bedrock in this location, as well as in the greater part of the Vrličko polje.

Figure 3. The core of the borehole in Vrlika with a total length of 17 m.

Seismic microzonation is the determination of local soil characteristics that show an estimate of how strong the ground motion will be, that is, “amplified” depending on the parts of an area. It is displayed as a set of maps. A series of geophysical measurements using the MASW method were made in the Sinj area. The results of geophysical measurements enable soil characterization according to the Vs30 soil classification described in the Eurocode 8 standard. Microseismic disturbance measurements were also made and analyzed by the HVSR method to estimate the natural frequency of the local soil and the depth of the bedrock.

Microzonation maps (preliminary) for the Vs30 classification of local soil based on Eurocode 8, and maps of basic frequencies f0 and depth to bedrock H800 (soil layer when the speed of transverse waves exceeds Vs>800 m/s) were made in the area of the city of Sinj. Based on these parameters, an amplification assessment map for an earthquake of magnitude M6.2, with an epicentral distance of 50 km, was derived. Microzonation maps clearly delineate areas prone to higher amplification of seismic waves, especially in parts of the city that are located on alluvial and proluvial deposits.