Ambient vibrations of a deep maar resonator

We introduce non-invasive seismic methods for identifying and characterizing maar sedimentary infills. We conducted a dense ambient vibration survey and employed state-of-the-art 3D resonance analysis techniques to map the lateral extent and depths of the sedimentary layers, a challenge with traditional geophysical methods due to unfavorable aspect ratios (depth > lateral dimension). The ambient vibrations of maars are predominantly driven by normal mode motions due to 3D resonance of crater infills. Dense station coverage enabled detailed images of the normal mode shapes revealing infill symmetries. The resonance results in extreme ground motion amplification, with factors reaching up to 30 on the vertical component, challenging conventional beliefs about site effects and methodologies based on widely used horizontal-to-vertical spectral ratios. These results are supported by numerical simulations of the maar's seismic response. The observed response is so specific that it can be used to identify partly eroded maar structures in the field.