Detecting 1-D and 2-D ground resonances with a single-station approach

The vibration modes of the ground have been described both in the 1D and 2D case. The 1D resonance is found on geological structures whose aspect ratio is low, that is on layers with a lateral width much larger than their thickness. A typical example is that of a horizontal soft sediment layer overlying hard bedrock. In this case, the 1D resonance frequency, traditionally detected by means of the microtremor H/V technique, depends on the bedrock depth and on the shear wave velocity of the resonating cover layer. The H/V technique is thus used both to map the resonance frequencies in seismic microzonation studies and for stratigraphic imaging. When 2D resonance occurs, generally on deep and narrow valleys, the whole sedimentary infill vibrates at the same frequency and stratigraphic imaging can no longer be performed by means of the 1D resonance equation. Understanding the 1D or 2D resonance nature of a site is therefore mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment. In this paper we suggest a procedure to address this issue using single station approaches, which are much more common compared to the multi-station synchronized approach presented by research teams in earlier descriptions of the 2D resonances. We apply the procedure to the Bolzano sedimentary basin in Northern Italy, which lies at the junction of 3 valleys, for which we observed respectively 1D-only, 1D and 2D, and 2D-only resonances. We conclude by proposing a workflow scheme to conduct experimental measurements and data analysis in order to assess the 1D or 2D resonance nature of a site using a single station approach.