## 5 Results and Discussion
### Horizontal and Vertical Response Spectra of the Jeju Region  
Individual, mean, and mean plus 1\(\sigma\) response spectra of 8 stations and all the events for horizontal and vertical components in Jeju are shown in Figure 8a,b. Since Jeju island is approximately 74 km long and 32 km wide in roughly east-west and north-south directions, respectively (Figure 1), 8 stations can be considered to be fairly well representative of characteristics of response spectra for the Jeju island (9.25 km long and 4.0 km wide per each station). The mean plus 1\(\sigma\) horizontal response spectra (Figure 8a) increases gradually in the low frequency band, reached a maximum of \(\sim\)5.7 at 2.6 Hz, and gradually decreases and converges to \(\sim\)1.6 at high frequencies. Many regulatory design criteria, including Reg. Guide 1.60, 1.165, 1.208 (US NRC, 1973, 1997, 2007), and ASME ([PERSON] and [PERSON], 2017) adopt the mean plus 1\(\sigma\) values to ensure the conservatism of the seismic design response spectrum [10; 17; 20; 21]. In recent years, many studies have investigated the response spectra of inland Korea, except Jeju Island, using ground motions Fukuoka events ([PERSON] and [PERSON], 2016), Pohang events series ([PERSON] et al., 2018), and the Gyoungju events ([PERSON] et al., 2019) [14; 15; 16]. These previous studies (conducted 2014 onward) state that the horizontal response spectra commonly exhibit peak values ranging from 4.5 to 5.0 at frequencies of 9 Hz or higher. In contrast, the maximum horizontal response in the Jeju area is \(\sim\)5.6, which is a fairly higher (12-24 %) than that of the inland area, whereas the frequency band with very high responses ranges between 2.0 and 6.0 Hz, which is significantly lower than that of the inland areas. Figure 8: Mean and mean +1 \(\sigma\), (**a**) horizontal and (**b**) vertical response spectra of all the events and 8 stations. The higher responses including maximum amplitude formed in the lower frequency band could be attributed to the local geological environment of Jeju island, which was formed by the eruption of a surface volcano with lower shear velocity in contrast to the inland granite-based bedrock having higher shear wave velocity ([PERSON] and [PERSON], 2012) [7]. This could also be attributed to the existence of unconsolidated Pleistocene marine sediments distributed widely in the Jeju area, which were identified by penetrating well data ([PERSON] et al., 2000) having a remarkably low shear wave velocity at ~200 m under the surface [32]. The layer was also identified by the inversion of the HVSR and dispersion curves ([PERSON] and [PERSON], 2012) and the deep resistivity sounding having relatively higher conductivity (<100 \(\Omega\) m) compared to that of the over- and underlying layers ([PERSON] et al., 2007) [7,8]. Owing to the larger contrast of elastic impedance (shear velocity multiplied by density) among adjacent layers, the higher responses, including the maximum amplitude, can be developed in a frequency band much lower than that of the inland areas. Considering the general relationship between the resonant frequency and the thickness and shear wave velocity of the layers, the resonant frequency of the site decreases as the velocity of the layers with an assumed thickness decreases ([PERSON] et al., 2006, [PERSON] et al., 2007) [33,34]. Furthermore, the amplitude of the maximum response in Jeju being 12-24% higher than that of land can be attributed to the greater elastic impedance contrast between the strata owing to the existence of low-velocity layers distributed widely in Jeju. In the near future, it would be necessary to thoroughly investigate the influence of low shear wave velocity layers, surface volcano eruption, and marine sediments in depth on the seismic response spectra with frequency bands and amplitudes. Considering a simple formula, let To = 0.1 N (equation 0306.5.6) be the resonance frequency of the buildings suggested by KBC 2016, where N indicates the number of floors.