BROADBAND SOURCE MODEL AND STRONG MOTIONS OF THE 2018 HOKKAIDO EASTERN IBURI EARTHQUAKE

<p> Large ground motions with seismic intensity of 7 were observed at Atsuma-cho Shikanuma (BBC) during the 2018 Hokkaido eastern Iburi earthquake (M_J6.7) although the hypocentral depth of 37 km was very deep as crustal earthquakes. Strong motions at KiK-net Oiwake (IBUH01) were also equivalent to seismic intensity of 7. We examined the causes of large ground motions to improve strong motion predictions for feature crustal earthquakes.</p><p>  Firstly, we separated the source, path and site effects on strong motions by the spectral inversion using the main shock and aftershock records. The estimated Q for the path is modelled by 44f^0.84 using frequency f. The estimated f_max for the main shock is 6 Hz. Both Q and f_max are average as crustal earthquakes. Empirical amplification factors for weak motions in frequencies 5-10 Hz are largest at IBUH01 among 33 stations. The amplification factor at IBUH01 during the main shock becomes smaller in frequencies 5-10 Hz but becomes larger at 2 Hz due to nonlinearity of shallow soils. The amplification at 2 Hz contributes to large peak at 2 Hz of spectra and seismic intensity of equivalent 7. The empirical amplification factors in frequencies 0.5-1 Hz are largest at three stations of KiK-net Atsuma (IBUH03), K-NET Ukawa (HKD126) and BBC. These stations are in a basin-like shape region where the thickness from the layer with S-wave velocity of 1.7 km/s to the layer with S-wave velocity of 0.35 km/s is thick (around 2.5 km). The predominant frequency of 0.8 Hz for weak motions shifts to 0.6 Hz at BBC and 0.3 Hz at IBUH03 during the main shock due to nonlinear site effects. At these stations the peak levels of the amplification factors become larger than those for weak motions. The large amplifications contribute to the large peak at 0.6 Hz and seismic intensity of 7 at BBC. Since the spectra at IBUH03 and IBUH01 in the boreholes have no clear peaks at 0.3 Hz and 2 Hz, respectively, these peaks are mainly generated by site responses.</p><p>  Secondary, we estimated the broadband source model composed of three strong motion generation areas by the empirical Green’s function method. We used records at 9 KiK-net stations in boreholes and 4 K-NET stations where effects of nonlinearity are small. It is found that the strong motion generation area SMGA2 with the stress drop of 74 MPa located at depths 21-25 km in the southern region of the fault is the main cause of large ground motions. BBC is the nearest station to SMGA2 and IBUH03 is also near SMGA2. Both stations are in the forward side from SMGA2. Records in borehole at IBUH03 are mostly reproduced by strong motions generated from SMGA2. SMGA2 starts to rupture about 7.7 s after the rupture of the small strong motion generation area SMGA1 located at the hypocenter. The other strong motion generation area SMGA3 with the stress drop of 52 MPa located at the same depths to SMGA2 in the northern region slightly contributes to the later part of S-wave potions especially at stations in backward side from SMGA2. Although the depths of SMGA2 and SMGA3 are about twice of the average as crustal earthquakes, the stress drops are 4-6 times of the average. The short period spectral level is more than 4 times of the average of crustal earthquakes and is the same level to intraslab earthquakes. These facts reveal that the large short period spectral level due to SMGAs with large stress drops located at 12-16 km shallower region from the hypocenter caused large ground motions during the main shock.</p>