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Abstract One of the main sources of seismic noise below 0.05 Hz is the atmospheric pressure variation, especially when surface pressure variations are large. When surface broadband seismic stations are equipped with pressure sensors, there is high coherence between pressure and seismic signals at low frequencies. The amount of ground deformation under surface pressure variations reflects the characteristics of near‐surface elastic structure and allows us to estimate near‐surface shear‐modulus structure using an inversion method. In the inversion method, we have the surface observable η(f) = Sz/Sp, where f is a frequency between 0.01 and 0.05 Hz, and Sz and Sp are the power spectral densities of vertical seismic data and of surface pressure data. We derive depth sensitivity kernels for η(f) with which we invert for elastic moduli of the shallow structure. Between 0.01 and 0.05 Hz, sensitivity kernels typically have peaks at depths within the uppermost 100 m. Based on vertically heterogeneous 1‐D structures, we estimate Vs30 at 744 USArray Transportable Array stations. Vs30 is the time‐averaged shear‐wave velocity from the surface to the 30‐m depth. We compare our results with various surficial geology maps. Although Vs30 has high horizontal variability over a short distance on the scale of hundreds of meters, we find correlations between Vs30 and large‐scale geological structures, such as mapped units and surficial materials. We find good agreement between estimated Vs30 and mapped Quaternary sediment depths, where stations with thicker underlying sediment tend to have slower Vs30.
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Using a Dense Seismic Array to Determine Structure and Site Effects of the Two Towers Earthflow in Northern California
Abstract We deployed a network of 68 three-component geophones on the slow-moving Two Towers earthflow in northern California. We compute horizontal-to-vertical spectral ratios (HVSRs) from the ambient seismic field. The HVSRs have two prominent peaks, one near 1.23 Hz and another between 4 and 8 Hz at most stations. The 1.23 Hz resonance is a property of the background noise field and may be due to a velocity contrast at a few hundred meters depth. We interpret the higher frequency peaks as being related to slide deposits and invert the spectral ratios for shallow velocity structure using in situ thickness measurements as a priori constraints on the inversion. The thickness of the shallowest, low-velocity layer is systematically larger than landslide thicknesses inferred from inclinometer data acquired since 2013. Given constraints from field observations and boreholes, the inversion may reflect the thickness of deposits of an older slide that is larger in spatial extent and depth than the currently active slide. Because the HVSR peaks measured at Two Towers are caused by shallow slide deposits and represent frequencies that will experience amplification during earthquakes, the depth of the actively sliding mass may be less relevant for assessing potential slide volume and associated hazard than the thicknesses determined by our inversions. More generally, our results underscore the utility of combining both geotechnical measurements and subsurface imaging for landslide characterization and hazard assessment.
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- Award ID(s):
- 1848302
- PAR ID:
- 10183745
- Date Published:
- Journal Name:
- Seismological Research Letters
- Volume:
- 91
- Issue:
- 2A
- ISSN:
- 0895-0695
- Page Range / eLocation ID:
- 913 to 920
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0220190206
