Assessment of nonlinear site response at ocean bottom seismograph sites based on Swave horizontaltovertical spectral ratios: a study at the Sagami Bay area KNET sites in Japan
 Yadab P. Dhakal^{1}Email author,
 Shin Aoi^{1},
 Takashi Kunugi^{1},
 Wataru Suzuki^{1} and
 Takeshi Kimura^{1}
DOI: 10.1186/s4062301706155
© The Author(s) 2017
Received: 3 October 2016
Accepted: 2 February 2017
Published: 8 February 2017
Abstract
Keywords
Ocean bottom seismograph Nonlinear site response Horizontaltovertical spectral ratio Sagami BayBackground
Soft soil sites undergo nonlinear site responses during strong shakings. A peak ground acceleration (PGA) of 100–200 cm/s^{2} 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 strongmotion 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 KiKnet stations, Wu et al. (2010) pointed out nonlinear site responses even for shakings as low as about 20–30 cm/s^{2}. The nonlinear site response is identified by the reduction in amplitude of highfrequency 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 horizontaltovertical spectral ratios of Swaves (SH/V) to identify the nonlinear site effects. In this method, the mean SH/V spectral ratios for weak motions at a site are considered to be the reference spectral ratios and are compared with the SH/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 KiKnet sites during strong shakings; it was verified that the SH/V spectral ratios of surface recordings have features similar to those depicted by the surface and borehole station pairs. The SH/V method is especially useful for sites where the station pairs or vertical array measurements are unavailable.
Deployments of largescale ocean bottom networks that comprise seismometers and pressure gauges (e.g., DONET in the Nankai Trough area, DONET 2016; Snet 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–amplitudebased magnitudes have already been discussed (Hayashimoto and Hoshiba 2013; Nakamura et al. 2014). Hayashimoto et al. (2014) analyzed the SH/V spectral ratios at three off Kushiro OBS sites and showed that recordings having a PGA of about 100 cm/s^{2} 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 highfrequency amplifications at the OBS sites to effectively utilize the strongmotion recordings at the OBS sites for realtime applications.
Data and methodology
We used 935 threecomponent ocean bottom accelerograms from 315 earthquakes. The accelerograms have the following properties: (1) horizontal vector PGA > 5 cm/s^{2}, (2) both P and Swave onsets are included in the recordings, and (3) signaltonoise 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/s^{2} are from earthquakes between Mj = 4.0 and Mj = 6.6. An example of recordings with preevent noise, Swave 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/s^{2} 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/s^{2} to increase the number of such strongmotion 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 Swave onset considering the stability of the computed spectra as temporal changes were not investigated. We picked the Swave onset manually, and halfcosine 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, SH/V spectral ratios were obtained.
Results and discussion
A comparison of the SH/V spectral ratios at the KNG203 site for four recordings having PGAs 150, 175, 202, and 284 cm/s^{2} is shown in Fig. 3b. SH/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 highfrequency 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/s^{2} recorded at the KNG205 site; the DNL value is 7.3; and a clear shift of peak frequency and reduction in highfrequency 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/s^{2} 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 highfrequency motions due to the soft layers at the OBS sites is possible.
Conclusions
We analyzed SH/V spectral ratios for identifying the nonlinear site responses at six OBS sites of KNET located in the Sagami Bay area of Japan. SH/V spectral ratios from weak motions having PGA < 20 cm/s^{2} were used as reference spectra for each OBS site. We found that the weakmotion SH/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/s^{2}. Our results showed that the SH/V spectral ratios for strongmotion recordings having horizontal PGAs greater than 50–150 cm/s^{2}, 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 highfrequency 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 highfrequency 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 realtime ground motion predictions as front detections.
Abbreviations
 DNL:

degree of nonlinearity
 DONET:

dense oceanfloor network system for earthquakes and tsunamis
 JMA:

Japan Meteorological Agency
 KiKnet:

Kiban Kyoshin network
 KNET:

Kyoshin network
 OBS:

ocean bottom seismographs
 PGA:

peak ground acceleration
 SH/V:

Swave horizontal to vertical
 Snet:

seafloor observation network for earthquake and tsunami along the Japan Trench
Declarations
Authors’ contributions
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.
Acknowledgements
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, EditorinChief, Masato Furuya, Editor, and KuoLiang 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.
Competing interests
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
References
 Aoi S (2004) Strongmotion seismograph network operated by NIED: KNET and KiKnet. 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 Swave 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 realtime observation network of oceanbottomseismometers 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 groundmotion 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 OffKushiro OBS, JAMSTEC. AGU Fall Meeting, S33C4543
 Kanazawa T (2013) Japan trench earthquake and tsunami monitoring network of cablelinked 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, SanchezSesma FJ, Matsushima S (2011) The optimal use of horizontaltovertical spectral ratios of earthquake motions for velocity inversions based on diffusefield 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: 03030049
 Noguchi S, Sasatani T (2011) Nonlinear soil response and its effects on strong ground motions during the 2003 MiyagiOki 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) Siteresponse estimation for the 2003 MiyagiOki 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, BenZion Y (2010) Refined thresholds for nonlinear ground motion and temporal changes of site response associated with mediumsize earthquakes. Geophys J Int 182(3):1567–1576View ArticleGoogle Scholar