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TitleHVSR measurements in complex sedimentary environment and highly structured resonator topography - comparisons with seismic reflection profiles and geophysical borehole logs
AuthorDietiker, B; Pugin, A J -M; Crow, H L; Mallozzi, S; Brewer, K D; Cartwright, T J; Hunter, J A
SourceSAGEEP 2018 - Symposium on the application of geophysics to environmental and engineering problems, Nashville TN, online SAGEEP 2018 technical program; p. 1-7
Alt SeriesNatural Resources Canada, Contribution Series 20180033
PublisherEnvironmental and Engineering Geophysical Society
MeetingSAGEEP 2018 - Symposium on the application of geophysics to environmental and engineering problems; Nashville, TN; US; March 25-29, 2018
Mediaon-line; digital
File formatpdf
Subjectsgeophysics; stratigraphy; surficial geology/geomorphology; seismic methods; seismic waves; seismic interpretations; seismic reflection surveys; seismic profiles; amplitude spectra; stratigraphic analyses; glacial deposits; overburden thickness; bedrock topography; horizontal-to-vertical spectral ratio (HVSR) measurements; resonator topography; geological mapping techniques
Illustrationsseismic profiles; graphs; spectra; cross-sections, stratigraphic; tables
ProgramAssessing Earthquake Geohazards, Public Safety Geoscience
LinksOnline - En direct (PDF, 1095 KB)
AbstractOver 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 two- and 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 from HVSR data.
Summary(Plain Language Summary, not published)
Over the last two decades, microtremor measurements have gained popularity for assessments of site response to earthquake shaking. In simple subsurface structures, where a 2-layer-model is a good approximation, bedrock depth can be calculated from the measured fundamental frequency. However, in a complex glacial environment, sediment types often change from very soft clays to hard tills; shear wave velocities range from 80 - 2000 m/s. We observe that these changes lead to strong contrasts, and the measured fundamental frequency indicates a layer well above bedrock. Hence, bedrock depth could be significantly under-estimated. In simple 2-layer cases, the peak of the fundamental frequency is smaller over inclining layers and even disappears over a very steep slope. This indicates that two- and three-dimensional layer topography strongly influences the measured frequencies. Our results suggest that topography and velocity structure of the sediments need to be well understood before geological conditions can be interpreted from microtremor measurements.