Effects of Meteorological Variations and Sensor Burial Depths on HVSR Analysis

Kang, Su Young; Kim, Kwanghee; Kim, Doyoung; Jeon, Byeong-Uk; Lee, Jinwook

doi:10.5467/jkess.2020.41.6.658

Cited by 4 publications

(2 citation statements)

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“…The duration of noise measurement varies from a couple of hours to 6 months. The all-in-one equipment is buried ~0.3 m below the surface to ensure a good coupling and to avoid undesirable noises due to meteorological variations including wind and precipitation (Foti et al, 2018;Kang et al, 2020b). The observation times at an individual site varied from a couple of hours to 6 months.…”

Section: Methods and Data Processingmentioning

confidence: 99%

Soil liquefaction potential assessment using ambient noise: A case study in Pohang, Korea

Kang

Kim²,

Gihm³

et al. 2022

Front. Earth Sci.

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An MW 5.5 earthquake occurred in Pohang, Korea, on 15 November 2017 and caused large damage. In and around the epicentral area the earthquake also remained numerous sand boils, which is a surface representation of soil liquefaction at shallow depth during the earthquake. Soil liquefaction is one of the most dangerous consequences of an earthquake. Here we show that the spatial distribution of the ground vulnerability index (Kg), which we estimate via ambient noise analysis, correlates very well with the distribution of sand boils. Our Kg model based on the dense microtremor surveys at differing geological conditions and urbanizations indicates that only 28.4% of the study area is vulnerable to ground shaking (Kg > 20), with 91.1% of the observed sand boils occurring in these vulnerable areas. We also observed that Kg values estimated in the study correlate well with both VS30 values and geologic units in the study area. This case report confirmed that the Kg can be an affordable supplement to traditional, but expensive and time-consuming, geotechnical/geophysical techniques for the initial screening and regional evaluations. Such a Kg map can assist stakeholders in earthquake-prone regions in identifying areas more susceptible to liquefaction and bring a recommendation to consider seismic hazard mitigation.

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Section: Methods and Data Processingmentioning

confidence: 99%

Soil liquefaction potential assessment using ambient noise: A case study in Pohang, Korea

Kang

Kim²,

Gihm³

et al. 2022

Front. Earth Sci.

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“…HVSR curves with single or multiple peaks indicate the presence of one or more impedance contrasts beneath the observation sites. Resonance frequencies with lower H/V amplitudes indicate minor impedance contrasts, while sites with no discernible HVSR peak suggest very weak impedance contrasts beneath the site [30,31]. Formations of younger age (Quaternary age) exhibit higher amplification factor values compared to formations of later age (Tertiary age) [7].…”

Section: Characteristics Of the Amplification Factor (Ao) In The Opak...mentioning

confidence: 99%

Microzonation of Seismic Parameters in Geological Formation Units Along the Opak River Using Microtremor Measurements

Budi Wibowo

2023

GEOMATE

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The 2006 Yogyakarta earthquake, with a magnitude of Mw 6.3, caused significant damage primarily on the west side of the Opak River. The Opak River area encompasses formations that exhibit distinct responses to seismic wave propagation. Formations consisting of unconsolidated and young sedimentary materials tend to experience wave amplification during seismic events. This study aims to investigate the seismic wave characteristics within each formation unit using microtremor measurements. The seismic wave parameters analyzed include dominant frequency, amplification factor, shear wave velocity, weathered layer thickness, seismic vulnerability index, and lithology based on N-SPT data. The study utilized 190 microtremor data sets and 7 N-SPT data sets. The microtremor data was processed using the Horizontal to Vertical Spectral Ratio Nakamura method, while the shear wave velocity data was processed using the Imai and Tonouchi approach. The findings revealed higher amplification factors in areas dominated by unconsolidated sedimentary materials and young formations. Quaternary formations are dominated by unconsolidated sedimentary material composed of sand, silt, clay, and breccia. Formations with a tertiary age are composed of more complex lithologies such as breccia-tuff, dacite tuff, andesitic tuff, volcanic breccias, lavas, siltstone, sandstone, and conglomerate. These conditions make Quaternary formations have seismic parameter characteristics such as lower dominant frequency, higher amplification factor, thicker sediment layer thickness, and higher seismic vulnerability index compared to Tertiary formations. Quaternary formations correlate with damage to buildings after the Yogyakarta earthquake, with an average shear wave velocity character of 279-293.67 m/s, and are in a zone with a vulnerability index >20. Tertiary formations demonstrated higher seismic resistance compared to Quaternary formations, indicating their relative stability.

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Mapping bedrock topography and detecting blind faults using the fundamental resonance of microtremor: a case study of the Pohang Basin, southeastern Korea

Kang

2024

Geophysical Journal International

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SUMMARY The Pohang Basin sustained the most extensive seismic damage in the history of instrumental recording in Korea due to the 2017 Mw 5.5 earthquake. The pattern of damage shows marked differences from a radial distribution, suggesting important contributions by local site effects. Our understanding of these site effects and their role in generating seismic damage within the study area remains incomplete, which indicates the need for a thorough exploration of subsurface information, including the thickness of soil to bedrock and basin geometry, in the Pohang Basin. We measured the depth to bedrock in the Pohang Basin using dense ambient noise measurements conducted at 698 sites. We propose a model of basin geometry based on depths and dominant frequencies derived from the horizontal-to-vertical spectral ratio (HVSR) of microtremor at 698 sites. Most microseismic measurements exhibit one or more clear HVSR peak(s), implying one or more strong impedance contrast(s), which are presumed to represent the interface between the basement and overlying basin-fill sediments at each measurement site. The ambient seismic noise induces resonance at frequencies as low as 0.32 Hz. The relationship between resonance frequency and bedrock depth was derived using data from 27 boreholes to convert the dominant frequencies measured at stations adjacent to the boreholes into corresponding depths to the strong impedance contrast. The relationship was then applied to the dominant frequencies to estimate the depth to bedrock over the whole study area. Maps of resonance frequency and the corresponding depth to bedrock for the study area show that the greatest depths to bedrock are in the coastal area. The maps also reveal lower fundamental frequencies in the area west of the Gokgang Fault. The results indicate a more complex basin structure than previously proposed based on a limited number of direct borehole observations and surface geology. The maps and associated profiles across different parts of the study area show pronounced changes in bedrock depth near inferred blind faults proposed in previous studies, suggesting that maps of bedrock depth based on the HVSR method can be used to infer previously unknown features, including concealed or blind faults that are not observed at the surface.

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Effects of Meteorological Variations and Sensor Burial Depths on HVSR Analysis

Cited by 4 publications

References 0 publications

Soil liquefaction potential assessment using ambient noise: A case study in Pohang, Korea

Soil liquefaction potential assessment using ambient noise: A case study in Pohang, Korea

Microzonation of Seismic Parameters in Geological Formation Units Along the Opak River Using Microtremor Measurements

Mapping bedrock topography and detecting blind faults using the fundamental resonance of microtremor: a case study of the Pohang Basin, southeastern Korea

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