Effect of the surface geology on strong ground motions due to the 2016 Central Tottori Earthquake, Japan
© The Author(s) 2017
Received: 28 February 2017
Accepted: 31 July 2017
Published: 15 August 2017
The authors conducted microtremor observations in the damaged area and evaluated the predominant period of the ground surface (Noguchi and Kagawa 2014) based on horizontal to vertical (H/V) spectral ratios. Figure 1 shows the distribution of the predominant period and locations of the observation sites mentioned in this study. First, the spectral properties of the observed strong ground motions are considered, referring to the predominant period distribution map in the area with respect to the effect of surface geology.
Another key issue observed during the earthquake is the nonlinear site response of soft ground sites, especially the Togo Site, which is located in a reclaimed area close to a pond shore; an anomalous accelerogram was obtained from there, showing pulsating swings and cyclic mobility behavior (e.g., Iai et al. 1995). The peak periods of the main shock, longer compared with those of small events, suggest nonlinear site response at this site. The same phenomenon was observed at the Hojo Site at which a JMA intensity of 6 lower was observed. The peak periods from strong ground motions at the two sites did not agree well with predominant periods obtained from preliminary microtremor observations. The second objective of this study is to evaluate the effect of the surface geology on strong ground motions at soft sedimentary sites.
Effect of surface geology on observed main shock records
Site information and characteristic values of the main shock
Kurayoshi City Office
6 lower (5.8)
Ryuto, Yurihama Town
6 lower (5.8)
Hisadome, Yurihama Town
5 upper (5.2)
Hashita, Hokuei Town
6 lower (5.8)
Yurashuku, Hokuei Town
5 upper (5.4)
5 upper (5.4)
The damage around the Kurayoshi Site, such as falling ridge roof tiles, crashing clay walls, and tumbling tombstones, suggests strong effects of the ground motion with dominant short periods. However, complete collapse of buildings was not observed in the area because the ground motions of ~1–2 s affecting building collapse (Kawase 1998) are not large enough (Fig. 2a). Japanese wooden houses generally have a natural period of ~0.3–0.5 s; they suffered resonance between ground motions and housing vibrations and therefore were slightly damaged. The damage extended the natural period of the house and further resonance promoted further damage if the ground motions had enough power in the new natural period range. The first resonance in Kurayoshi City occurred during the period of ~0.4 s; however, further resonance did not occur because the ground motions did not have enough power in the longer period range. Therefore, much damage was observed in the Kurayoshi area; however, the damage was not severe. In the area between the Hojo and Daiei sites, several aged and low earthquake-resistant houses completely collapsed. As shown in the response spectrum in Fig. 2c, the ground motion in the period range lower than 0.5 s that is close to the natural period of wooden houses was not large at the Hojo Site; however, the higher peak of ~1.5 s is suggested to affect the resonance of old houses. In general, robust wooden houses were not damaged in this area because no first resonance occurred between the ground motion and natural period of the houses.
The authors installed a strong ground motion observation site at Takatsuji, Yurihama Town, in October 2015 (Fig. 1) because seismic swarms with a maximum JMA magnitude of 4.3 had occurred in this area. Fortunately, strong motion records of the main shock and many small events were obtained at this site. A brief explanation of the site is given in Table 1; the data observed at this site were provided by Tottori University (CRMSE 2016).
As demonstrated above, strong effect of surface geology on ground motion is suggested in the area based on the peak period during the main shock of the 2016 Central Tottori Earthquake. It is important to prepare a predominant period distribution map for seismic zonation; however, the predominant periods estimated from microtremor observations do not agree with those based on strong ground motion at soft sediment sites.
Suspected nonlinear behavior at the Togo and Hojo sites
Anomalous ground motion was observed in the NS component at the Togo Site (Fig. 2b). The site is located on reclaimed land close to a pond shore (Fig. 1). The original accelerogram in the figure shows pulsating swings. The amplitudes of pulses to the north are larger than those to the south. In addition, the pulses indicate rather cyclic mobility behavior, which usually is observed for liquefied ground during earthquakes (e.g., Iai et al. 1995). Despite the shape of the accelerogram, liquefactions and resulting sand boils were not identified at and around the site. However, ground subsidence of ~20 cm was observed close to the sensor and its hood in a tree garden near the building of the town office (CRMSE 2016). Land subsidence was widely observed in and around the office. The JMA checked the setting of the accelerometer at the site shortly after the main shock because large ground motion with JMA seismic intensity of 6 lower was observed at the site. However, no issues with respect to sensor leveling or anchoring were reported.
The same analysis was conducted at the Hojo Site (Fig. 2c). The results are shown in Fig. 4. The predominant period of 1.5 s in Fig. 2c is not clear in Fig. 4b; the longest peak estimated from the strong motion H/V spectral ratio is ~2.2 s. The peaks observed for the small event (dashed lines) shift to longer periods during the main shock. The changes are estimated as 0.5–0.7 s and 1.2–2.2 s (Fig. 4), respectively. The estimated rigidity reduction is ~0.3, suggesting a larger strain level than that at the Togo Site. No liquefaction or ground deformation was observed around the site.
For both sites, the peak period during stage 3 (aftershock) agrees well with that of stage 1 (foreshock). It is suggested that the high strain level that causes the nonlinear site response recovered within 30 min after the main shock. Hata et al. (2017) reported the recovery of shear wave velocities in Mashiki Town due to the main shock of the 2016 Kumamoto Earthquake. Based on their observations, the high strain level recovered 1000 h after the main shock. This is much longer than our result of 30 min, which might be due to the local soil profile suffering from ground motions.
The nonlinear site response suggests that the estimation of the predominant period distribution from microtremor observations is not enough to induce microzonation under strong ground motions. The predominant period might be longer, particularly at soft sediment sites. Such effects were observed during the 2016 Kumamoto Earthquake (Goto et al. 2017). Nonlinear site responses should be considered with respect to seismic microzonation in the future.
Summary and conclusions
A JMA seismic intensity of 6 lower was observed at three sites: Kurayoshi, Hojo, and Togo. The intensity values were 5.8 at all sites; however, the PGAs varied between 0.3 and 1.4 G.
The PGA values were affected by distinguished periods of observed ground motions; they were basically controlled by predominant periods of the ground surface estimated from previous microtremor observations.
It is true for the hard site Kurayoshi; however, the predominant periods of the main shock were longer than those of microtremors at the soft sites Hojo and Togo.
Nonlinear site responses were suggested for soft sites. The distinguished periods of the main shock record shifted to longer periods than those of the weak motion data. A stiffness decrease due to the nonlinear site effect was estimated for each site.
Predominant periods estimated from microtremor observations are not sufficient for disaster mitigation of soft sites. The consideration of nonlinear site responses might be the key for future seismic microzonation studies.
Based on records of the strong ground motion due to the earthquake, the ground motion characteristics from previous surveys were reconfirmed for the target area. In the future, we would like to carefully examine the effect of the surface geology using aftershock records at temporal sites around permanent observatories that were installed shortly after the main shock and the dense microtremor observation data obtained at strong ground motion observation sites. In addition, the nonlinear site response in the target area needs to be estimated.
Data and resources
Strong motion data obtained at the Kurayoshi City Office Site (K-NET TTR005) are available from the National Research Institute for Earth Science and Disaster Resilience (NIED) at http://www.kyoshin.bosai.go.jp (last accessed February 2017). The main shock data of the Takatsuji Site can be obtained from the Center for Regional Management and Safety Engineering (CRMSE), Faculty of Engineering, Tottori University, at http://www.eng.tottori-u.ac.jp/anzen/takatsuji.csv (last accessed February 2017). The main shock data for other sites maintained by the Tottori Prefecture are provided by the Japan Meteorological Agency (JMA) at http://www.data.jma.go.jp/svd/eqev/data/kyoshin/jishin/1610211407_tottoriken-chubu/index.html (last accessed February 2017, in Japanese).
TK, TN, SYo, and SYa conducted the microtremor observations and TN integrated the results. SYa maintained the strong motions site at Takatsuji. SYo carried out the basic analysis of strong ground motion records. TK drafted the manuscript after discussions among all authors. All authors read and approved the final manuscript.
The authors thank the Crisis Management Bureau of the Tottori Prefecture for providing small event data obtained at the Togo Site.
The authors declare that they have no competing interests.
This study was partially supported by a Grant-in-Aid for Scientific Research, KAKENHI, from the Japan Society for the Promotion of Science (Grant No. 15K01250) and Observation and Research Program for the Prediction of Earthquakes and Volcanic Eruptions, Subdivision on Geodesy and Geophysics, Council for Science and Technology of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT).
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