Identification of Impedance Contrast Interfaces through Microtremor HVSR Migration at a Dense Array—Application to Tongzhou, Beijing sub-center

SUMMARY The peak frequencies of the microtremor horizontal-to-vertical spectral ratio (HVSR) are empirically used to estimate the locations of strong impedance contrast interfaces in sedimentary basins. This implies that it is important to properly select HVSR peak frequencies and calibrate empirical formulas relating peak frequencies to strong contrasts using borehole data. However, the criteria proposed by SESAME may not apply universally across all regions or HVSR curves—particularly those with multiple peaks. Moreover, empirical relationships between resonance frequencies and strong impedance interfaces exhibit regional dependencies. Herein, we propose a practical workflow for peak frequency selection that combines automatic identification with manual evaluation: we first use an algorithm to identify candidate peaks automatically, and then manually verify and adjust the selections by comparing adjacent stations along cross-sections based on expert judgment, under the assumption that subsurface structures exhibit gradual lateral variations such that the HVSR curves show smooth spatial variation across adjacent stations. Applied to 919 sites in Tongzhou (Beijing sub-center) dense array, this method enabled peak frequency selection at all operational stations. Furthermore, rather than employing empirical relationship between peak frequency and the interface depth with strong contrast, we migrate the HVSR curves from the frequency to depth domain based on the pre-existing 3D S-wave velocity model beneath the array (which is obtained by inverting multimode surface waves extracted from the same ambient noise dataset via the array technique), to highlight the location of the interfaces. Two interfaces with strong impedance contrast are then identified in Tongzhou. The shallow one corresponds to a soft sedimentary layer with a thickness of 50–100 m. The deeper one corresponds to the Quaternary and Tertiary sedimentary layers, with thicknesses varying between 100 and 600 m across different tectonic units.