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

Abstract

The vibration modes of the ground have been described both in the 1-D and 2-D case. The 1-D 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 1-D resonance frequency, traditionally detected by means of the microtremor H/V (horizontal to vertical spectral ratio) 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 2-D 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 1-D resonance equation. Understanding the 1-D or 2-D 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 multistation synchronized approach presented by research teams in earlier descriptions of the 2-D resonances. We apply the procedure to the Bolzano sedimentary basin in Northern Italy, which lies at the junction of three valleys, for which we observed respectively 1-D-only, 1-D and 2-D, and 2-D-only resonances. We conclude by proposing a workflow scheme to conduct experimental measurements and data analysis in order to assess the 1-D or 2-D resonance nature of a site using a single-station approach.