## 4 Characteristics of the Horizontal Response Spectra in Jeju Island  
Figure 4 shows the horizontal response spectra of 7 stations (Table 2), including GOS, HALB, JJB, JJU1, MRD, and SGP1, for the 15 Kumamoto earthquake sequence (ML: 4.8-7.3). It can be seen that as the frequency increases in the low-frequency band, the response gradually increases. The response reaches a peak value in the mid-frequency band and finally decreases in the high-frequency band, as suggested by various seismic design standards (Reg Guide 1.60, 1.165, and 1.208 (US NRC 1973, 1997, 2007) and ASME ([PERSON] and [PERSON], 2017) [10; 17; 20; 21]. Particularly, the responses of GOS, HALB, MRD, and SSP exhibited a characteristic section where the slope was remarkably gentle in the frequency range of 0.3-0.5 Hz. Figure 4: \(Cont.\)  
Figure 4: Normalized horizontal response spectra at **(a)** GOS, **(b)** HALB, **(c)** JJB, **(d)** JJU1, **(e)** MRD, **(f)** SGP1, and **(g)** SSP using Kumamoto earthquakes series. It is necessary to consider ground motion data as much as possible to determine a response spectrum that was more representative of the Jeju region. Therefore, we included four additional stations, that is, CJD (northwest area), JJU2 (central), and SGP2 (northeast), and UDO (northeast island) (Table 2). JJU2 and SGP2 replaced JJU1 and SGP1 and the newly installed UDO after the Kumamoto earthquake. In particular, although CJD was installed before the Kumamoto earthquake, no data are available on that particular event. Therefore, to investigate the response spectrum of the CJD, JJU2, SGP2, and UDO stations, we considered 9 Pohang earthquake sequences (ML: 3.5-5.4), which occurred in 2017 on the inland Korean Peninsula. Figure 5 shows individual horizontal response spectra of CJD, JJU2, SGP2, and UDO. CJD exhibited a very large variability in response; the amplitude of response spectra decreased with a decrease in the earthquake magnitude. Similarly, JJU2 was characterized by very high variability. SGP2 also showed a large response at ML 5.4, whereas some showed responses that were abnormally very far from most responses in the high frequency bands. Although UDO showed fairly large variability in the high frequency band, it was comparatively lower than that of the other stations in the entire frequency band. Therefore, all three stations except UDO were excluded owing to their relatively large variability compared to the previous seven stations. As a result, the responses of the remaining 8 stations were investigated to represent the Jeju area. Figure 5: Normalized horizontal response spectra at (**a**) CJD, (**b**) JJU2, (**c**) SGP2, and (**d**) UDO using Pohang earthquakes series. The Pohang earthquake series (Table 1) exhibited an ML of 5.4, with three earthquakes in the ML 4 level (4.6, 4.3, and 4.1) and five in the ML 3 level. In contrast, seismic ground motions from the Kumamoto earthquake (table x) can be classified into three groups: Four with ML greater than 6.0 (7.3, 6.4, 6.3, and 6.0), six with ML greater than 5.3 (5.8, 5.8, 5.7, 5.5, 5.4, 5.3), and five with ML greater than 4.8 (5.0, 5.0, 4.9, 4.8, 4.8). It can be seen that a very large difference exists in the total numbers of events (15 in Kumamoto and. 9 in the Pohang series) and the number of events with ML greater than 5.0 (12 in Kumamoto and 1 in the Pohang series). On comparing the seismic energy release, the largest earthquake (ML 7.3) in the Kumamoto series was found to have approximately 708 (\(10^{(1.5\times 1.9)}\)) times the seismic energy (ML 5.4) of the Pohang sequence, indicating a large asymmetry of the distributions of the earthquake scale. Since the above four stations are permanent and included in national networks, devices such as sensors and recorders and the site effects of the stations need to qualify a certain level of national quality assurance.