This electronic supplement provides supplementary figures referenced in the main text showing: 1) the effect of the source time function duration on the MFT diagrams of synthetic accelerograms (Figure S1) and for all the dataset in the distance-period domain (Figure S2), 2) a MFT analysis of surface and borehole accelerometrics recordings to evaluate the influence of soils on SmSM waves (Figure S3) and the period-distance domain spectral amplitudes obtained by examining all the surface stations (Figure S4) and 3) the average period-velocity transfer function for different NEHRP soil classes (Figure S5).
Figure S1. MFT analysis on an accelerometric synthetic signal for a station located at a distance of 108 km and an azimuth of 45°, using two different source time function durations (STFD). STFD=1 s, radial (a) and transverse (b) components; STFD=0.5 s, radial (c) and transverse (d) components. Contoured color scale is in dB; red=100 dB, blue=0 dB. Black symbols show the maximum coherence of the signal. Shortening the STFD the spectral amplitudes remain consistent but the energy of the signal shifts towards lower periods.
Figure S2. Normalized spectral amplitude as a function of distance and period for radial and transverse components of synthetics calculated using a STFD of 0.5 s. Sg=S direct waves, S*=either upgoing S wave from a source in the lower crust or an S wave bottoming in the lower crust below the Conrad, SmSM=critical and postcritical reflected SmS, FM=surface waves fundamental mode. The comparison with Figure 2b (synthetic calculated using STFD of 1 s) clearly shows that shortening the STFD the SmSM spectral amplitudes shifts towards lower periods.
Figure S3. Accelerograms and corresponding MFT analysis on KiK-net borehole and surface WKY07 accelerometric seismic station in the period range 0.25-2 s (SmSM domain). The station is located at a distance of about 150 km and an azimuth of 216°, on a soil site classified as C with the borehole seismometer at a depth of 100 m. Borehole radial (a) and transverse (c) components; surface radial (b) and transverse (d) components. Contoured color scale is in dB; red=100 dB, blue=0 dB. Black symbols show the maximum coherence of the signal. Rayleigh and Love wave fundamental and higher modes superimposed to the MFT diagrams (white curves) are calculated using Herrmann and Ammon (2002) and assuming the simple flat-layered model of Figure 2a. From borehole to surface the move out toward lower periods of the SmSM maximum spectral amplitude along the same velocity waveguide for higher modes can be observed together with an amplitude enhancement (see the normalized a, b, c and d accelerograms). The change in the velocity from borehole to the surface of the dominant period is negligible and there is no meaningful energy entrapment of surface waves at group velocities lower than those measured for SmSM or at higher periods typical of surface waves fundamental mode.
Figure S4. Normalized spectral amplitude as a function of distance and period for radial and transverse components together for KiK-net stations at surface. Sg=S direct waves, S*=either upgoing S wave from a source in the lower crust or an S wave bottoming in the lower crust below the Conrad, SmSM=critical and postcritical reflected SmS, FM=surface waves fundamental mode. The comparison with Figure 6 (borehole normalized spectral amplitude as a function of distance and period for radial and transverse components) clearly shows an enhancement in amplitude of the SmSM domain.
Figure S5. Period-velocity average transfer functions for different NEHRP soil classes (A+B, C, and D, from top to bottom) and radial (left) and transverse (right) ground motion components. Transfer function is calculated from surface-borehole spectral amplitude ratio after multiple filter analysis of accelerometric data. The 3D image is obtained by evaluating spectral ratios of 104 station pairs. 47 pairs for both A+B, and C classes, and 10 pairs for D class.
Herrmann, R. B., and Ammon, C. J., 2002. Computer Programs in Seismology version 3.30: Surface waves, receiver functions, and crustal structure, St. Louis University, Missouri. Available at http://www.eas.slu.edu/eqc/eqc_cps/CPS/CPS330/cps330c.pdf.
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