Fig. 1.
2D resonance modes along a cross-section of a sediment-filled valley. a Fundamental longitudinal (L0) and transverse (T0) mode shapes of a valley slice (ideally corresponding to the yellow volume in b). Red colors indicate the maximum displacement, which has a horizontal component only in the longitudinal mode and both a horizontal and a vertical component in the transverse mode. b Resonance peaks corresponding to the L0 and T0 modes, recorded along the longitudinal and transverse directions of a valley. c Amplitude spectra of motion recorded along different directions with rotations between 0 and 90 degrees with respect to the longitudinal direction of the valley. The grey line indicates the position of the peak identifying L0