Local site effects in Kumamoto City revealed by the 2016 Kumamoto earthquake
© The Author(s) 2017
Received: 1 August 2016
Accepted: 28 February 2017
Published: 7 March 2017
Strong ground motions were observed in Kumamoto Prefecture, Japan, during the 2016 Kumamoto earthquake sequence, which occurred on April 14, 2016, at 21:26 JST (Mj 6.4) and April 16, 2016, at 1:25 JST (Mj 7.3). In Mashiki, where the maximum intensity value on the Japanese intensity scale (i.e., 7) was recorded, buildings were heavily damaged by the earthquake; therefore, a great deal of preliminary research has been carried out to investigate the strong ground motions in this area (e.g., Kawase et al. 2016; Yamanaka et al. 2016). However, in Kumamoto City, which is located approximately 15 km from the Futagawa–Hinagu fault zone (e.g., Shimizu et al. 2016), a Japanese seismic intensity of >6 was recorded, with slight earthquake damage to buildings. Moreover, in Kumamoto City, large site amplifications are expected because of the soft soil sediments comprising the Kumamoto Plain.
The Kumamoto Plain extends across the northeastern part of Kumamoto Prefecture, Japan, and consists of a diluvial plateau on the western slope of Mt. Aso (Hoshizumi et al. 2004) and an alluvial plain formed by the Shirakawa and Midorikawa rivers (Ishizaka et al. 1995). In particular, the geology of the central to northern parts of Kumamoto City comprises alluvium deposited by the Shirakawa River, andesite widely distributed in and around Mt. Kinbo, and pyroclastic flow deposits from Mt. Aso (Hoshizumi et al. 2004). Regarding soil conditions, the Japan Seismic Hazard Information Station (J-SHIS) (Headquarters for Earthquake Research Promotion, Cabinet Office, Government of Japan, 2005) indicates V S 30 (average shear wave velocity down to 30 m depth) values ranging from 155 to 405 m/s on the Kumamoto Plain, and a value of 510 m/s on Mt. Kinbo, which suggests that earthquake ground motions are greatly and intricately amplified in Kumamoto City, because of the soft and complex soil sediments comprising the Kumamoto Plain.
In this study, we investigate the local site effects in an area extending from the central to the northern parts of Kumamoto City, based mainly on seismic data obtained from temporary seismic observations conducted immediately after the 2016 Kumamoto earthquake foreshock.
List of temporary seismic stations and one permanent seismic station with site information from surficial geological data and seismic data
Wald and Allen (2007)
Pyroclastic flow deposits
Early pleistocene products (Ko-kinbo volcanic rocks)
Epicentral distance (km)
Maximum amplitude 2016/04/18 20:41 (JST) Mj5.8
Site amplification factors using weak motion data
2016/04/16 (JST) Mj7.3
2016/04/18 20:41 (JST) Mj5.8
Horizontal acceleration (cm/s2)
Horizontal velocity (cm/s)
Fundamental frequency (Hz)
Amplification over a range of frequencies (s)
In addition, a velocity seismometer (VSE-11F/12F, Tokyo Sokushin Co. Ltd.) was previously installed at station KU.KMP1, which is located on Mt. Kinbo where andesite is widely distributed (Hoshizumi et al. 2004), by the Institute of Seismology and Volcanology, Faculty of Science, Kyushu University. We adopted the seismic data recorded at KU.KMP1 as a reference.
Site amplifications in Kumamoto City
We estimated site amplification factors for the KR stations (KR01–KR06), using KU.KMP1 as a reference. We analyzed weak ground motions from the earthquake events that occurred in and around the Aso region (Fig. 1). To estimate the site amplification factors, firstly, we manually picked the S-wave components of the accelerations and filtered velocities and calculated the Fourier spectra for a time window of 5.12 s, with a Parzen window of 0.4 Hz. Secondly, the Fourier spectra for the sediment-based KR stations were divided by those of the rock-based KU.KMP1, and these spectra were multiplied by the epicentral distances to account for seismic attenuation (Table 1). Finally, the derived spectral ratios were averaged as the arithmetic means of both their NS and EW components.
Nonlinearity during the main shock
Of the sediment-based sites, seismic data from the 2016 Kumamoto earthquake main shock (Mj 7.3) were recorded at KR02 and KR04, as well as at the rock-based KU.KMP1 (Figs. 3, 4). Therefore, we estimated site amplification factors for the strong ground motions at KR02 and KR04 using the same procedure and reference site as those adopted in the analysis of the weak ground motions.
In contrast to the categorized accelerations, the site amplifications for the 2016 Kumamoto earthquake at KR02 and KR04 are significantly larger within the frequency range of 1–3 Hz. Moreover, the site amplification factors decrease notably in the high-frequency range of 10–20 Hz. These observations (e.g., Field et al. 1998) indicate that the seismic response of subsurface soils in Kumamoto City during the 2016 Kumamoto earthquake main shock was nonlinear. In contrast, the site amplifications for the categorized accelerations tend to be similar, which indicate that the subsurface soils behaved linearly during ground shaking at accelerations of less than 150 cm/s2.
As mentioned above, the strong ground motions in the frequency range of 1–3 Hz observed at sites KR02 and KR04 in Kumamoto City during the 2016 Kumamoto earthquake main shock (Mj 7.3) were influenced by nonlinear behavior of the subsurface soil, which shortens the peak frequency and weakens the amplitude at high frequencies. However, the Fourier spectra for KR02 and KR04 differ significantly within the frequency range of 0.7–3 Hz (Fig. 5), despite their close proximity of 600 m (Fig. 1). To understand the lateral heterogeneity in subsurface structure between KR02 and KR04, we performed single-station microtremor measurements with an interval distance of approximately 100 m along the survey line between KR02 and KR04 on May 22 and 23, 2016.
For these single-station microtremor measurements, we used the same instruments (accelerometer: JEP-6A3; data logger: LS-8800) as for the temporary seismic observations. We calculated the horizontal-to-vertical (H/V) spectral ratios of the microtremors using the geometric means of the NS and EW components and selected the signal 15 min in length with a high signal-to-noise ratio by removing artificial noise from the original data.
An important issue for earthquake engineering in this area is to understand the differences between the Fourier spectra for sites KR02 and KR04 (Fig. 5), which are located in close proximity of 600 m; however, we could not determine the reason for these differences from the H/V spectral ratio of microtremor results. The nonlinear behavior characteristics identified at KR02 and KR04, caused by local site conditions in the near surface, may be a major cause of these differences in strong ground motions. However, as pointed out by Kaklamanos et al. (2015), compounding factors such as basin waves, path effects, soil heterogeneity, and nonvertical incidence can greatly reduce the accuracy of 1D site response assumptions. The north–south survey line in this study is located close to Mt. Kinbo; therefore, basin waves refracted by the edge of the basin as well as topographic effects associated with Mt. Kinbo may influence the site responses and compound the nonlinear behaviors.
Seismic data from the 2016 Kumamoto earthquake (Mj 7.3) were successfully recorded at sites KR02 and KR04 located along a north–south survey line in Kumamoto City; these data show that the seismic waves recorded at these stations had large amplitudes in the frequency range of 1–3 Hz. Site amplifications for the weak ground motions were calculated with respect to a permanent station located at Mt. Kinbo as a reference, and the derived amplifications are relatively variable along the 6-km survey line; however, site amplification factors in the frequency range of 0.5–3 Hz are not large enough to explain the amplitudes produced by the main shock. Moreover, site amplifications for the 2016 Kumamoto earthquake at KR02 and KR04 are large within the frequency range of 1–3 Hz, despite the dominance of those for the weak ground motions at a frequency of 5 Hz. It is inferred that the large site amplification factors in the frequency range of 1–3 Hz were responsible for the nonlinear behavior of subsurface soil in Kumamoto City during the 2016 Kumamoto earthquake.
To understand the differences in the Fourier spectra between the closely located (600-m interval) stations KR02 and KR04, we also performed single-station microtremor measurements with an interval distance of approximately 100 m between these stations. We confirmed that the peak frequencies of the H/V spectral ratios have similar characteristics to those of the site amplification factors along the survey line. However, the H/V spectral ratio results could not explain the differences between the Fourier spectra at KR02 and KR04. In future research, to further investigate the differences in strong ground motions between these two sites during the 2016 Kumamoto earthquake, we will perform array microtremor observations and examine the nonlinear behavior characteristics by drillings at both sites.
ST and MK analyzed the seismic data. ST drafted the manuscript. All the authors participated in the observations of strong ground motions and revised the manuscript.
We thank the Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, for providing us with seismic data from the 2016 Kumamoto earthquake. We thank Mr. K. Wada and Dr. K. Sakai of the Railway Technical Research Institute for help in conducting the temporary seismic observations. We also thank two anonymous reviewers and the editor for their comments and suggestions, which significantly contributed to improving the quality of this paper.
The authors declare that they have no competing financial interests.
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