• KSCE Journal of Civil and Environmental Engineering Research
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• v.42 no.1
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• pp.23-33
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• 2022

The horizontal-to-vertical spectral ratio (HVSR) of ambient noise is widely used to identify the resonant frequency of a site. The frequency at the largest HVSR is regarded as the resonant frequency. The source of ambient noise is impossible to identify and control. Therefore, obtaining reliable HVSR of ambient noise requires sufficient measurement time and absence of near-field vibration. In this study, we investigated the minimum stabilization time required for a portable seismometer and the effect of the distance between the seismometer and artificial vibration on HVSR estimation. In the case of a soil site, the HVSR was stabilized after 5 minutes after sensor installation. In the case of a rock site, stabilization required more than an hour. Human-footsteps within 10 m of the seismometer strongly influenced the HVSR for the soil site. These results provide a field guideline when measuring ambient noise for HVSR.

• The Journal of Engineering Geology
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• v.1 no.1
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• pp.109-120
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• 1991

Applications of microtremor to geological engineering are widely reviewed and observed microtremors are processed to show evidences of close relationship between the predominant periods of microtremor and underground structure. The ground vibrates continuously at all times and the elastic vibration is called microtremor (0.5-20Hz) or microseisms (0.01-0.1Hz) according to their frequency range. The vibration is believed to have propagative nature like those of the dispersive surface waves or multireflected shear waves. Microtremors were recorded at selected thress places of which subsoil structures are well distinguished in the Kyongsang Sedimentary Basin. It is found that the underground structures estimated from microtremor analysis coincide well with the known structures. The microtremor analysis of the long period range can be an inexpensive and effective tool in geological engineering for the evaluation of the underground structure, site-specific reponse spectrum, and seismic microzonations.

• Journal of the Korean earth science society
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• v.31 no.6
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• pp.541-554
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• 2010

Fifteen min-microtremor data sets were collected at 136 sites from a coastal area of Kunsan and 117 sites from an inland area of Jeonju located in SW Korea, and were analyzed for the HVSR (Horizontal to Vertical Spectral Ratio) of the sites. The microtremor spectra of the coastal area have stronger energy in the lower frequency range from 1-6 Hz than those of the coastal area. This result can be attributed to the effect of the waves and tides in the Keum river and the Yellow sea. Twenty four hours of measurement of the microtremor indicated that the microtremor spectrum correlates with the human activities, but the microtremor HVSR peak was observed consistently at the characteristic frequency for the site. The HVSR peaks were grouped into 4 types -"single peak", "double peak", "broad peak" or "no peak"- based on their shapes. More than 90% of the data sets exhibit peak frequencies ($F_0$) which can be easily identified. The distribution of $F_0$ reveals a close relationship with the topography and local geology of the areas, exhibiting high F0s in the hillside areas and low $F_0s$ in the reclaimed land area. While the amplitudes of microtremor HVSR peak frequencies are less than 4 in the downstream of the inland area, those of the recently reclaimed land in the coastal area are extremely high (more than 10). The results of this study indicate that detailed HVSR studies are essential for the earthquake hazard reduction of reclaimed lands.

• Journal of the Korean earth science society
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• v.23 no.4
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• pp.380-392
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• 2002

The purpose of the present study is to evaluate the usage of microtremor in estimation of subsurface structure and ground response to strong ground motion. To accomplish the purpose, the current status of microtremor study are reviewed and microtremors recorded at several stations are analysed. First of all, the stability of microtremor is examined through the analysis of microtremors recorded for 80 seconds per hour during the time from 10 p.m. to 6 a.m. for eight hours at night time. It is found that the shape of microtremor spectra of low frequency below 10Hz is approximately invariable with time and the spectra contain informations about subsurface structure. The subsurface structures estimated from the predominant frequency determined from the recorded microtremors are compared with the known ones from geophysical surveys at several stations in Kyungju. The comparison of structures shows rough agreements at most stations. Horizontal to vertical spectral ratio(HVSR) technique for microtremor has been proposed as an indirect method to determine ground response to strong ground motion. The HVSR for microtremors recorded in Kyungju is calculated and compared with theoretical transfer function calculated from the known structures. The comparison shows rough coincidence of the peak frequency of spectra between them.

• The Journal of Engineering Geology
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• v.16 no.4 s.50
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• pp.381-392
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• 2006

The passive surface wave survey using microtremor is conducted in areas of crystalline rock basements to obtain average shear-wave velocity structures to 30 m deep (Vs30), on which the earthquake-resistant design standard is based. Test data were recorded at two sites with triangular and L-shaped arrays for 4 seconds with an sampling interval of 2 ms. The microtremor recorded at a site were analysed using the spatial autocorrelation method to obtain phase-velocity spectra and effects of major factors such as size and shape of away and number of record and receiver were examined. At the other site, shear-wave velocities were derived from VSP and microtremor data separately. The results from these two methods agree to each other reasonably well, indicating that the microtremor method can be an effective geophysical tool to measure Vs30.

• The Journal of Engineering Geology
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• v.31 no.2
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• pp.211-222
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• 2021

The magnitude 5.4 earthquake occurred in 2017 near 7.5 km north of Buk-gu, Pohang. In order to find out the characteristics of microtremor in Buk-gu, Pohang, Gyeongsangbuk-do, where earthquake damage occurred significantly, a total of 39 points were acquired to analyze the horizontal to vertical spectral ratio (HVSR). Microtremor vary from region to region, so the resonant frequency of the region is obtained by examining the microtremor. For Thickness analysis, we analyze the structure and properties of shear wave velocity (Vs) up to the underlying rock to compare resonance frequencies to match the horizontal to vertical spectral ratio (HVSR) analysis technique against nearby boring data. Using F₀ = V_s/4H with a resonance frequency of alluvium is 1.3 ± 0.07 Hz and a resonance frequency of Yeonil group is 0.69 ± 0.22 Hz, the alluvium thickness was found to be 26~30 m and the Yeonil group thickness was 170~250 m.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05d
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• pp.373-379
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• 1996

모든 물체는 물체를 구성하는 재료의 성질과 구조의 형식에 따라 각각의 고유주기를 갖고 있으며, 구조물을 구성하는 재료는 시간의 경과와 더불어 경년열화 과정을 거치므로 구조물의 고유주기는 조금씩 길어지는 경향을 갖는다. 그러므로 구조물의 건설 당시에 동적특성을 파악해 놓았다면 구조물에 문제가 발생했을 시의 동적특성을 측정하여 구조물의 손상 여부를 판단하는 중요한 열쇠가 된다. 본 연구는 자연적으로 발생하는 진동 또는 인간이 사회생활을 영위하면서 항상 발생하고 있는 지반의 미소한 외력인 상시미동을 이용하여 구조물의 동적특성을 측정하고 구조해석을 통한 해석결과와 비교 분석하여 구조물의 진동특성을 파악하는 편리한 도구로써 상시미동이라는 진동측정방법의 실용성을 확인하였으며, 고유진동수의 변화추이는 구조물의 건전성을 평가하는 새로운 지표가 될 수 있음을 제안하였다.

• 한국지구물리탐사학회:학술대회논문집
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• 2010.10a
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• pp.113-116
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• 2010

• Geophysics and Geophysical Exploration
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• v.13 no.2
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• pp.144-152
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• 2010

To study the thickness variation of volcanic rocks of Jeju Island, microtremor data were recorded at eight and four sites using short-period and broadband seismometers, respectively, for 30 ~ 60 minutes with a 100 Hz sampling rate. During the daytime, these records show increased cultural noise at frequencies above 1.8 Hz. Natural noise occurs in the frequency range of 0.4 to 0.8 Hz in both daytime and nighttime data. Predominant frequencies determined by the H/V spectral-ratio method are in the range of 0.2 ~ 0.7 Hz. These frequencies decrease gradually as the central part of the Mt. Halla is approached. This may indicate that the basement is warped downward beneath the center of the island, which is consistent with previous gravimetric and magnetic models. Assuming an average shear-wave velocity of 1,800 m/s for the overburden basalts, the depths to basement are estimated to be between 640 and 2,140 m.

• Journal of the Korean Geotechnical Society
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• v.38 no.8
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• pp.17-27
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• 2022

To characterize the dynamic properties of Gimhae Plains sediments, we calculated natural frequencies using microtremor horizontal-to-vertical spectral ratios and derived shear wave velocity profiles by inversion of Rayleigh-wave dispersion curves obtained by the high frequency-wavenumber and modified spatial autocorrelation methods. Our results suggest that in this region, strong amplification of ground motion is expected in the vibration frequency (f ≥ 1 Hz). Additionally, obtained velocity profiles show that shear wave velocities are ~200 and 400 m/s for the shallow marine and old fluvial sediments, respectively. Bedrock is possibly encountered at depths of 60-100 m at most sites. We developed a simplified shear wave velocity model of shallow sediments based on the obtained profiles. Our results suggest that a large area in the Gimhae Plains could be categorized as an S6 site based on the Korean seismic design code (KDS 17 10 00).