Preliminary analysis of nonlinear site response at the S-net seafloor sites during three Mw 7 class earthquakes

In this paper, we investigated the characteristics of nonlinear site response (NLSR) at 23 S-net seafloor sites using strong-motion records obtained during three Mw 7 class earthquakes that occurred directly beneath the network. During the earthquakes, horizontal peak accelerations as large values as 1,400 and 1700 cm/s2 were recorded at the land (KiK-net) and S-net stations, respectively. The S-net is a large-scale inline-type seafloor observation network for earthquake and tsunami in the Japan Trench area. Characterization of NLSR is important because, in most common cases, it can cause a reduction of higher frequency components and a shift of predominant site frequency to lower one. Obtaining high-quality strong-motion records at seafloor sites is extremely difficult and expensive. Some of the records from the three earthquakes used in this study were contaminated by the rotations of the sensor houses, resulting in the ramps and offsets after the arrival of strong S-wave phases. We used a time window of 10 s starting from the S-wave onset, that avoided the ramps and offsets mostly. Using the horizontal-to-vertical spectral ratio (HVSR) technique, we found that the selected S-net sites might have experienced substantial degrees of NLSR during the three earthquakes with peak accelerations greater than about 60 cm/s2. To investigate that the obtained features of NLSR were realistic or not at the S-net sites, we examined the NLSR at nine KiK-net sites on land where high-quality strong-motion records were obtained. We found that the KiK-net sites experienced various degrees of NLSR during the three earthquakes, and the obtained characteristics of NLSR at the KiK-net and S-net sites were comparable. We found that the NLSR affected the ground motions at frequencies mainly higher than 1 Hz at both Kik-net and S-net sites. Despite these similarities, by analyzing the spectral ratios between two horizontal component records, we suspected that the induced rotations contributed to some extent in exaggerating the degree of NLSR at the S-net sites, primarily when the components perpendicular to the cable axes were used. We concluded that consideration of induced rotational effects is necessary to understand the NLSR at the S-net sites better

Assessment of nonlinear site response at ocean bottom seismograph sites based on S-wave horizontal-to-vertical spectral ratios: a study at the Sagami Bay area K-NET sites in Japan

Additional file 4. Relationship between DNL values and PGAs at the OBS and land sites

Evidence of strong long-period ground motions of engineering importance for Nankai Trough plate boundary earthquakes: comparison of ground motions of two moderate-magnitude earthquakes

Abstract We analyzed strong-motion and broadband recordings of two moderate-magnitude earthquakes that occurred in the Nankai Trough. The first event was the 2004 Mw 6.5 southeast off-Kii peninsula earthquake, an aftershock event inside the Philippine Sea Plate near the Nankai Trough axis. The second event was the 2016 Mw 5.8 southeast off-Mie Prefecture earthquake, an independent event in the rupture area of the 1944 Mw ~ 8 Tonankai earthquake. The centroid depths were 11 and 14 km for the 2004 and 2016 events, respectively. Despite a large difference in the moment magnitude between the two events, the JMA magnitude (Mj) was 6.5 for both the events. We found that the short-period ground motions (e.g., response spectra at periods  20 s) for the 2016 event scaled generally well with the moment magnitude of the event. In contrast, the ground motions from the 2016 event were comparable to those for the larger-moment-magnitude 2004 event at equal distances at periods of about 2–20 s in wide areas and the observed acceleration response spectra at those periods were noticeably underestimated for the 2016 event by the ground motion prediction equation (GMPE) that employs Mw. An examination of the existing subsurface velocity model suggested that the difference in the relative location of the two events with respect to the thick accretionary prism of low seismic velocity most probably caused the comparable amplitude of the seismic waves at those periods. As a result, we posit that the values of Mj are equal for the two events because Mj is estimated using the displacement amplitude of ground motions at periods smaller than about 6 s. On the other hand, GMPE employing Mj generally described the observed data well. The results suggested that the plate boundary earthquakes in the Nankai Trough may excite strong long-period ground motions of engineering importance, and these ground motions appear to be better explained by Mj than by Mw in GMPEs for moderate-magnitude earthquakes in the Nankai Trough subduction zone