seismic resp。nse fr。m micr。trem。r 。f ch。gye basin, K。rea

Earth History of Asia- II October 31 - November 3, Niigata, Japan
Seismic response from microtremor of Chogye basin, Korea
LEE, Heekyoung, KIM, Roungyi, JANG, Bora, JEON, Hyejin, JO, Ahyeon,
HAN, Hyojeong, YANG, Sujin, KANG, Tae-Seob
Department of Earth and Environmental Sciences, Pukyong National University, Busan 608-737, Korea
[email protected]
When seismic waves corresponding to the resonance frequencies reach a certain
area, the ground motion can be rapidly amplified. This is especially true if the nature
of the soil is composed of sedimentary layers, not the bedrock in the area, as in the
Chogye basin. Ground motion amplification can be observed more easily when seismic
waves propagate through a basin with slow velocity. Chogye basin, which is
surrounded by country rock, has a closed-basm form (Choi et al., 2001). In such a
basin, incident seismic energy can form multiply reflected waves, thus causing energy
concentration to occur at this closed-basin area. Therefore, measuring resonance
frequencies of closed-basins is very important to estimate the earthquake response for
seismic hazard evaluation. Two field investigations were conducted for this research at
the Chogye basin, which is located in Chogye-myeon and Jeokjung-myeon,
Hapcheon-gun, Gyeongsangnam-do, Republic of Korea. For the first on-site inspection,
we set 23 observation points, which were divided into 8 transfer routes, and recorded
background noise by moving two to three times. The 2nd on-site inspection was also
done in the same way as before; however, this time, we included the measurement of
microtremors from the basm area to the surrounding mountains. Based on the
observations of microtremors acquired from two field inspections, we calculated the
Horizontal to Vertical Spectral Ratio (HVSR) using on the 3 component microtremor
data. Using the HVSR ratio, we were able to derive the resonance frequencies for
every observation point. Through this process, we derived the link between an
observed elevation and resonance frequencies. We were able to identify the thickness
of the sediments is inversely proportional to the resonance frequencies. We noticed
that the thicker sedimentary layers are more likely to be very sensitive to long period
waves. In contrast, shallow sedimentary layers respond well to short period waves.
This implies that sediment thickness is an important parameter that can be used to
predict the ground response when seismic waves arrive.
Acknowledgment: This study is based upon a project completed in the undergraduate
research class by Jang, B., Jeon, H., Jo, A., Han, H., and Yang, S.
Choi, G. S., Lee, S. W. and Lee, Y. A., 2001, Circular topography and basin of the
Chogye and Jeokjung, Hapcheon, regions: Is this result of meteorite impact?
Abstract Volume of the 2001 Joint Conference of Korean Geological Societies, 35.