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
© The Author(s) 2017
Received: 3 October 2016
Accepted: 2 February 2017
Published: 8 February 2017
KeywordsOcean bottom seismograph Nonlinear site response Horizontal-to-vertical spectral ratio Sagami Bay
Soft soil sites undergo nonlinear site responses during strong shakings. A peak ground acceleration (PGA) of 100–200 cm/s2 has generally been cited as a threshold motion that causes a nonlinear site response (e.g., Beresnev and Wen 1996). The classical approach compares the spectral ratios of observed recordings at a site with respect to those at a reference rock site to identify nonlinearity. A major limitation of this approach, however, is the availability of a reference rock site that is close enough to the soil site to cancel the source and propagation path effects (e.g., Field et al. 1997). In contrast, the deployment of vertical array strong-motion observations in different parts of the world provides a unique opportunity to understand linear as well as nonlinear site responses during strong shakings (e.g., Satoh et al. 1995; Wen et al. 1995; Bonilla et al. 2002; Tsuda et al. 2006). Using the surface and borehole site pairs of KiK-net stations, Wu et al. (2010) pointed out nonlinear site responses even for shakings as low as about 20–30 cm/s2. The nonlinear site response is identified by the reduction in amplitude of high-frequency components and the shift of predominant frequencies to lower ones during strong motions in comparison with those during weak motions; these effects are due to the increase in damping and degradation of the shear rigidity of soils during strong shakings (e.g., Beresnev and Wen 1996).
Wen et al. (2006) proposed a single station method using the horizontal-to-vertical spectral ratios of S-waves (S-H/V) to identify the nonlinear site effects. In this method, the mean S-H/V spectral ratios for weak motions at a site are considered to be the reference spectral ratios and are compared with the S-H/V spectral ratios for strong motions at the same site. Using the method of Wen et al. (2006), Noguchi and Sasatani (2008, 2011) identified the nonlinear site effects at KiK-net sites during strong shakings; it was verified that the S-H/V spectral ratios of surface recordings have features similar to those depicted by the surface and borehole station pairs. The S-H/V method is especially useful for sites where the station pairs or vertical array measurements are unavailable.
Deployments of large-scale ocean bottom networks that comprise seismometers and pressure gauges (e.g., DONET in the Nankai Trough area, DONET 2016; S-net in the Japan Trench area, Kanazawa 2013) are expected to contribute to earthquake and tsunami early warnings by prompt detection of earthquakes at subduction zones. The amplification effects of soft sediments at the ocean bottom seismograph (OBS) sites on the overestimation of displacement–amplitude-based magnitudes have already been discussed (Hayashimoto and Hoshiba 2013; Nakamura et al. 2014). Hayashimoto et al. (2014) analyzed the S-H/V spectral ratios at three off Kushiro OBS sites and showed that recordings having a PGA of about 100 cm/s2 or greater display the nonlinear site response. These results indicated that the reliable prediction of strong motions using the OBS sites as front stations is impossible without taking the nonlinear site effects into account. In this regard, it is important to identify nonlinear site response characteristics such as threshold PGAs, shift of predominant frequencies, and reduction in high-frequency amplifications at the OBS sites to effectively utilize the strong-motion recordings at the OBS sites for real-time applications.
Data and methodology
We used 935 three-component ocean bottom accelerograms from 315 earthquakes. The accelerograms have the following properties: (1) horizontal vector PGA > 5 cm/s2, (2) both P- and S-wave onsets are included in the recordings, and (3) signal-to-noise ratio is greater than 3 for each frequency component. In this paper, we define the PGA as the horizontal vector PGA obtained from two horizontal component recordings. The epicenters of the earthquakes used in this study are shown in Fig. 1. We used recordings from earthquakes having Japan Meteorological Agency magnitudes (Mj) between Mj = 3 and Mj = 7. Most of the recordings having PGAs > 20 cm/s2 are from earthquakes between Mj = 4.0 and Mj = 6.6. An example of recordings with pre-event noise, S-wave time windows, and their Fourier amplitude spectra is shown in Additional file 1. The epicentral distance was arbitrarily restricted to 200 km for recordings having PGA < 20 cm/s2 to reduce the data processing time because a large number of recordings are available for smaller motions and to 300 km for recordings having PGA ≥ 20 cm/s2 to increase the number of such strong-motion recordings. The focal depths of the earthquakes were shallower than 150 km.
Wu et al. (2010) noted a time window of 6 s as a balance between the stability of the computed spectra and the temporal changes for recordings from earthquakes with relatively smaller magnitudes. In this study, we used a time window of 10 s starting from 1 s before the S-wave onset considering the stability of the computed spectra as temporal changes were not investigated. We picked the S-wave onset manually, and half-cosine tapering was applied for 1 s at both ends of the time window. The time window was extended to 40.96 s by padding zeroes to compute the Fourier spectral amplitudes. The horizontal spectral amplitudes were derived as the vector sum of two horizontal components, and both horizontal and vertical spectral amplitudes were smoothed by applying a Parzen window of 0.4 Hz. Then, S-H/V spectral ratios were obtained.
Results and discussion
A comparison of the S-H/V spectral ratios at the KNG203 site for four recordings having PGAs 150, 175, 202, and 284 cm/s2 is shown in Fig. 3b. S-H/V spectral ratios of the weak motion at the site show peak frequencies between 5 and 10 Hz. Unlike the KNG201 site, a clear decrease in high-frequency spectral ratios between 7 and 20 Hz can be seen for all of the PGAs mentioned above, and the clear shift of peak frequencies to lower ones can be identified for the two largest PGAs. The computed DNL values are greater than 7.5 and agree with the visual comparisons from which substantial nonlinearity can be stipulated. It is noted that the largest PGA in the analyzed data is 467 cm/s2 recorded at the KNG205 site; the DNL value is 7.3; and a clear shift of peak frequency and reduction in high-frequency spectral ratios can be identified (see Additional file 3). Our analysis indicated that the spectral ratios at frequencies lower than 2 Hz are not affected by the nonlinear site response in the analyzed data ranges.
As we applied the same processing technique as Noguchi and Sasatani (2011), it is interesting to see how the DNL values obtained at the OBS sites and those obtained at the land sites by Noguchi and Sasatani (2011) are different with respect to the values of PGAs. The results from the present study and those from Table 1 of Noguchi and Sasatani (2011) are compared in Additional file 4. We found that the DNL values at the land sites and four OBS sites, namely KNG201, KNG202, KNG205, and KNG206, change almost similarly with the comparable PGAs. However, at two OBS sites, KNG203 and KNG204, the DNL values for PGAs of about 200 cm/s2 are about twice the DNL values for the corresponding PGAs at the land sites. These results are not surprising given the possibility of very soft site conditions at the OBS sites, where the nonlinear site response may begin at much lower levels of shakings. Furthermore, strong attenuation of high-frequency motions due to the soft layers at the OBS sites is possible.
We analyzed S-H/V spectral ratios for identifying the nonlinear site responses at six OBS sites of K-NET located in the Sagami Bay area of Japan. S-H/V spectral ratios from weak motions having PGA < 20 cm/s2 were used as reference spectra for each OBS site. We found that the weak-motion S-H/V spectral ratios differ from site to site. The difference suggests that the local geology is not uniform beneath the recording stations. The degree of nonlinearity was computed by using the method proposed by Noguchi and Sasatani (2008, 2011) for recordings having PGA ≥ 20 cm/s2. Our results showed that the S-H/V spectral ratios for strong-motion recordings having horizontal PGAs greater than 50–150 cm/s2, depending on the site, display signatures of a nonlinear site response at the OBS sites. That is, the shift of peak frequencies to lower ones and the decrease in high-frequency spectral ratios are well identified above the threshold PGAs, which are site dependent. We found that the degree of nonlinearity was remarkably larger at some of the OBS sites due to the smaller threshold motions to cause a nonlinear site response at the OBS sites. The lower threshold PGAs at some of the OBS sites might indicate that pervasive nonlinear site effects occur at the OBS sites during offshore earthquakes of large magnitude and greatly diminish the high-frequency components of strong motions and cause a considerable shift of peak frequencies to lower ones. These results suggest the need for careful use of the recorded strong motions at the OBS sites for applications such as real-time ground motion predictions as front detections.
degree of nonlinearity
dense oceanfloor network system for earthquakes and tsunamis
Japan Meteorological Agency
Kiban Kyoshin network
ocean bottom seismographs
peak ground acceleration
S-wave horizontal to vertical
seafloor observation network for earthquake and tsunami along the Japan Trench
YPD analyzed the data, interpreted the results, and drafted the manuscript. SA took part in the interpretation of the results and design of the study. TK, WS, and TK took part in the design of the study. All authors read and approved the final manuscript.
We would like to thank the Japan Meteorological Agency for providing us with hypocenter information of the earthquakes used in this study. We are grateful to two anonymous reviewers for their constructive and helpful comments that improved the quality of the manuscript. We also are grateful to Yasuo Ogawa, Editor-in-Chief, Masato Furuya, Editor, and Kuo-Liang Wen, Lead Guest Editor, at Earth, Planet and Space for facilitating the review of this manuscript. We also would like to thank Wessel and Smith (1998) for providing us with Generic Mapping Tools, which were used to make some figures in this manuscript.
The authors declare that they have no competing interests.
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- Aoi S (2004) Strong-motion seismograph network operated by NIED: K-NET and KiK-net. J Jpn Assoc Earthq Eng 4(3):65–74Google Scholar
- Beresnev IA, Wen KL (1996) Nonlinear soil response—a reality? Bull Seismol Soc Am 86:1964–1978Google Scholar
- Bonilla FL, Steidl JH, Gariel JC, Archuleta RJ (2002) Borehole response studies at the Garner Valley downhole array, Southern California. Bull Seismol Soc Am 92:3165–3179View ArticleGoogle Scholar
- Dhakal YP, Suzuki W, Kimura T, Kunugi T, Aoi S (2016) Analysis of S-wave H/V spectral ratios at the ocean bottom strong motion sites for soil nonlinearity. In: 5th IASPEI/IAEE international symposium: effects of surface geology on seismic motion, Aug 15–17, 2016, Taipei, Taiwan, I102C
- DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) 2016. https://www.jamstec.go.jp/donet/e/. Accessed 9 Sep 2016
- Eguchi T, Fujinawa Y, Fujita E, Iwasaki SI, Watabe I, Fujiwara H (1998) A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan. Mar Geophys Res 20:73–94View ArticleGoogle Scholar
- Field EH, Johnson PA, Beresnev IA, Zeng Y (1997) Nonlinear ground-motion amplification by sediments during the 1994 Northridge earthquake. Nature 390:599–602View ArticleGoogle Scholar
- Hayashimoto N, Hoshiba M (2013) Examination of travel time correction and magnitude correction of Tonankai ocean bottom seismographs for earthquake early warning. Quart J Seismol 76:69–81 (in Japanese with English abstract) Google Scholar
- Hayashimoto N, Nakamura T, Hoshiba M (2014) The characteristics of unusual OBS data exposed to strong shaking and the influence of applying these data to EEW processing: examples of Off-Kushiro OBS, JAMSTEC. AGU Fall Meeting, S33C-4543
- Kanazawa T (2013) Japan trench earthquake and tsunami monitoring network of cable-linked 150 ocean bottom observatories and its impact to earth disaster science. Underwater Technology Symposium (UT), 2013 IEEE International, 1–5. doi:10.1109/UT.2013.6519911
- Kawase H, Sanchez-Sesma FJ, Matsushima S (2011) The optimal use of horizontal-to-vertical spectral ratios of earthquake motions for velocity inversions based on diffuse-field theory for plane waves. Bull Seismol Soc Am 101:2001–2014View ArticleGoogle Scholar
- Nakamura T, Nakano M, Hayashimoto N, Takahashi T, Takenaka H, Okamoto T, Araki E, Kaneda Y (2014) Anomalously large seismic amplifications in the seafloor area off the Kii peninsula. Mar Geophys Res 35:255–270View ArticleGoogle Scholar
- Noguchi S, Sasatani T (2008) Quantification of degree of nonlinear site response. In: 14th world conference on earthquake engineering, Beijing, paper ID: 03-03-0049
- Noguchi S, Sasatani T (2011) Nonlinear soil response and its effects on strong ground motions during the 2003 Miyagi-Oki intraslab earthquake. Zisin 63:165–187 (in Japanese with English abstract) View ArticleGoogle Scholar
- Satoh T, Sato T, Kawase H (1995) Nonlinear behavior of soil sediments identified by using borehole records observed at the Ashigara Valley, Japan. Bull Seismol Soc Am 85:1821–1834Google Scholar
- Tsuda K, Steidl J, Archuleta R, Assimaki D (2006) Site-response estimation for the 2003 Miyagi-Oki earthquake sequence considering nonlinear site response. Bull Seismol Soc Am 96:1474–1482View ArticleGoogle Scholar
- Wen KL, Beresnev IA, Yeh YT (1995) Investigation of nonlinear site amplification at two downhole strong ground motion arrays in Taiwan. Earthq Eng Struct Dyn 24(313):324Google Scholar
- Wen KL, Chang TM, Lin CM, Chiang HJ (2006) Identification of nonlinear site response using the H/V spectral ratio method. Terr Atmos Ocean Sci 17(3):533–546Google Scholar
- Wessel P, Smith WHF (1998) New improved version of generic mapping tools released. EOS Trans AGU 79:579View ArticleGoogle Scholar
- Wu C, Peng Z, Ben-Zion Y (2010) Refined thresholds for non-linear ground motion and temporal changes of site response associated with medium-size earthquakes. Geophys J Int 182(3):1567–1576View ArticleGoogle Scholar