Microtremor HVSR analysis of heterogeneous shallow sedimentary structures at Pohang, South Korea

Abstract

We apply the single-station microtremor horizontal-to-vertical spectral ratio (HVSR) method to image the geometry of Pohang Basin, South Korea, which experienced the greatest earthquake damage in Korea during a series of anthropogenic earthquakes between November 2017 and February 2018. We collected and analysed the ambient seismic noise at 124 temporary stations. The resonance frequencies, which vary significantly across the area (0.35–19.86 Hz), were inverted to constrain the depth of the major impedance contrast, which is interpreted to be the sediment–bedrock interface beneath each station. The sedimentary layer thickness is generally thin in the north and thickens to the southern and central parts of the basin, where the depth to bedrock is up to 340 m. We compare the HVSR results with six borehole observations in the area, whose depth to bedrock ranges from 189 to 359 m. The sediment thicknesses obtained via the direct borehole measurements and HVSR method are comparable with each other. The resultant three-dimensional shape of the sedimentary basin provides crucial information for the microzonation of the Pohang area for seismic risk mitigation. It also provides a realistic initial velocity model for three-dimensional tomographic inversions to elucidate the detailed subsurface structure of the region.

Ground vulnerability derived from the horizontal‐to‐vertical spectral ratio: Comparison with the damage distribution caused by the 2017 ML 5.4 Pohang earthquake, Korea

“…The HVSR method is a well-established and widely applied technique used to study subsurface basin geometries, site effects and microzonations (e.g., Bignardi et al, 2016;Hassani and Atkinson, 2016;Lee et al, 2017;Kang et al, 2020). It is a passive seismic method that uses ambient noise measured by a three-component seismometer.…”

“…We assumed that the microtremor data during the chosen time period were subject to lower levels of cultural or artificial noise than at other times. All three-component seismometers in the array were buried approximately 0.5 m below the ground surface to reduce the contributions from surface effects such as wind (Kang et al, 2020).…”

“…In the central-southern parts of the study area, where Tertiary sediments or Quaternary alluvial layers are present at the surface, the lowest resonance frequencies of <0.5 Hz are clustered. Kang et al (2020) studied the structure of the Pohang Basin using resonance frequencies with the mean shear-wave velocity (480 m/s) proposed by the Geological Society of Korea (2019). They reported that the thickness of the sedimentary sequence in the basin is consistent with previously reported depths to bedrock measured using borehole observations in the area.…”

“…5). The highest amplification factors of up to 8.3 Figure 4 Distribution of the resonance frequency (F 0 ) calculated using the HVSR method (modified from Kang et al, 2020).…”

Estimasi ketebalan lapisan sedimen permukaan menggunakan pengukuran mikrotremor di Pemalang, Jawa Tengah

“…Sedangkan metode HVSR merupakan metode analisis data yang membandingkan spektrum mikrotremor vertikal dan horizontal [11]. Hasil analisis HVSR berupa kurva HVSR dimana tinggi puncak kurva disebut faktor amplifikasi dan frekuensi pada puncak kurva merupakan nilai frekuensi dominan lapisan sedimen permukaan di lokasi pengukuran [12], [13]. Frekuensi dominan merupakan frekuensi resonansi lapisan sedimen permukaan saat mencapai nilai amplifikasi maksimum.…”