Assessment of seismic vulnerability using the horizontal-to-vertical spectral ratio (HVSR) method in Haenam, Korea
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Geosciences Journal
GJ
Assessment of seismic vulnerability using the horizontalto-vertical spectral ratio (HVSR) method in Haenam, Korea Su Young Kang1, Kwang-Hee Kim2*, and Byungmin Kim3 1
Institute of Geologic Hazard & Industrial Resources, Pusan National University, Busan 46241, Republic of Korea Department of Geological Science, Pusan National University, Busan 46241, Republic of Korea 3 School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea 2
ABSTRACT: The town of Haenam in southwest Korea lies partially on reclaimed coastal land and experienced an unprecedented earthquake swarm during April and May 2020. Construction of a new town in the area means that there is demand to evaluate the seismic hazard caused by site-specific amplification of seismic ground motions by artificial unconsolidated cover. We used a microtremor horizontal-to-vertical spectral ratio (HVSR) method to identify resonance frequencies, image depths to bedrock, and assess seismic ground vulnerability across the epicentral area of the recent earthquake swarm. Microtremor measurements were taken at 144 sites across an 18.5 km × 12.5 km study area. The HVSR analysis shows resonance frequencies as low as 1.1 Hz. Using mean shear-wave velocities obtained by Multi-Channel Analyses of Surface Waves (MASW), we find that resonance frequencies correspond to depths to bedrock of between 3 m and 56 m. There are clear relationships between the distribution of resonance frequencies, depths to bedrock, surface geology, and areas of development in recent land reclamation projects. Higher resonance frequencies characterize areas of thin sedimentary cover or hard rock sites, while relatively low resonance frequencies characterize reclaimed land or greater depths to bedrock. We quantify the seismic hazard potential across the study area using the seismic vulnerability of ground index (Kg), and observe a clear association between zones of high Kg and areas of reclaimed land. This study demonstrates the value of the HVSR technique in characterizing the subsurface and assessing seismic vulnerability in areas of reclaimed land, where it can be used as a viable and low-cost alternative to large-scale active seismic refraction or MASW measurements. The results can also be used to inform and guide urban planning projects. Key words: Haenam earthquake swarm, horizontal-to-vertical spectral ratio method, resonance frequency, ground vulnerability index, urbanization Manuscript received August 14, 2020; Manuscript accepted September 7, 2020
1. INTRODUCTION On 3 May 2020, an ML (local magnitude) 3.1 earthquake occurred in Haenam in southwestern Korea (Figs. 1 and 2a). The earthquake, which had a maximum Modified Mercalli Intensity (MMI) of III, was the first felt by residents in the area *Corresponding author: Kwang-Hee Kim Department of Geological Science, Pusan National University, Busan 46241, Republic of Korea Tel: +82-51-510-2250, Fax: +82-51-5187-6389, E-mail: [email protected] Electr
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