HVSR MEASUREMENTS IN COMPLEX SEDIMENTARY ENVIRONMENT AND HIGHLY STRUCTURED RESONATOR TOPOGRAPHY – COMPARISONS WITH SEISMIC REFLECTION PROFILES AND GEOPHYSICAL BOREHOLE LOGS

Abstract

Over the last two decades, horizontal-to-vertical spectral ratio (HVSR) measurements from microtremor recordings have gained popularity for seismic microzonation and assessment of earthquake site characteristics such as fundamental frequency (or period). More recently, procedures have been described where empirical relationships are developed between the fundamental frequency and sediment thickness at regional sites where shear wave velocity depth functions are well understood and a simple 2-layer-model is a good approximation of the subsurface structure. In contrast however, in complex glacial stratigraphy, sediment types commonly vary drastically from very soft glaciomarine clay to overconsolidated till. We observe that these changes can lead to strong impedance contrasts and hence, resonating horizons well above bedrock can be resolved. Without a-priori knowledge, sediment thickness could be significantly under-estimated. We examine the frequency spectra of microtremor recordings in both simple and complex sedimentary settings at locations along high-resolution shear wave seismic reflection profiles and at continuously cored boreholes with shear wave velocity (Vs) profiles. Vs range from 80 – 2000 m/s within the unconsolidated sediment overburden. Our results indicate that resonator topography can have a significant impact on peak shape and amplitude. In relatively simple 2-layer cases, peak frequencies decrease and broaden over dipping resonators and even disappear over very steep resonator slopes, indicating that twoand three-dimensional subsurface resonator topography is highly influential on peak shape. Additionally, we present examples where sharp increases in shear wave velocity within the sediment column form strong resonating horizons, producing a high amplitude peak which does not necessarily correlate with the bedrock surface. Our results suggest that resonator topography and velocity structure need to be well understood by a practitioner before interpreting geological conditions