KiK-net Network Research Articles

Regional Correlations of VS30 and Velocities Averaged Over Depths Less Than and Greater Than 30 Meters

Using velocity profiles from sites in Japan, California, Turkey, and Europe, we find that the time-averaged shear-wave velocity to 30 m (VS30), used as a proxy for site amplification in recent ground-motion prediction equations (GMPEs) and building codes, is strongly correlated with average velocities to depths less than 30 m (VSz, with z being the averaging depth). The correlations for sites in Japan (corresponding to the KiK-net network) show that VS30 is systematically larger for ag ivenVSz than for profiles from the other regions. The difference largely results from the placement of the KiK-net station locations on rock and rocklike sites, whereas stations in the other regions are generally placed in urban areas underlain by sediments. Using the KiK-net velocity profiles, we provide equations relating VS30 to VSz for z ranging from 5 to 29 m in 1-m increments. These equations (and those for California velocity profiles given in Boore, 2004b) can be used to estimate VS30 from VSz for sites in which velocity profiles do not extend to 30 m. The scatter of the residuals decreases with depth, but, even for an averaging depth of 5 m, a variation in logVS30 of � 1 standard deviation maps into less than a 20% uncertainty in ground motions given by recent GMPEs at short periods. The sensitivity of the ground motions to VS30 uncertainty is considerably larger at long periods (but is less than a factor of 1.2 for averaging depths greater than about 20 m). We also find that VS30 is correlated with VSz for z as great as 400 m for sites of the KiK-net network, providing some justification for using VS30 as a site-response variable for predicting ground motions at periods for which the wavelengths far exceed 30 m. Online Material: Estimates of VS30 at K-NET stations.

Analysis of the Origins of (Kappa) to Compute Hard Rock to Rock Adjustment Factors for GMPEs

The observable decay with frequency ( f ) of Fourier amplitude spectra for ground-motion recordings is controlled by a parameter κ : exp(- πκf ). We analyze data from the KiK-net network, which is composed of stations with paired ground-motion sensors, one at the surface and one in a borehole. This study estimates κ for all KiK-net stations, using earthquakes recorded between 1998 and 2006. An inversion scheme is used to separate site, source, and path contributions to κ . We correlate the site component of κ ( κ ), estimated at the surface and the depth, with different S -wave velocity measures and with the fundamental resonant frequency of the site. The results show that the best correlations involve shallow soil S -wave velocity measures. The superficial layers of the soil predominantly influence κ , but a remaining component with a deep origin is also observed. The source component of κ is small and presents a clear regional dependence, while no correlations with magnitude or depth of the earthquake are observed. Data from the Next Generation of Attenuation (NGA) database are also used to estimate κ (surface site component) at the stations with a measured V S 30 value. The V S 30- κ couples estimated from both KiK-net and NGA data are compared with results from the literature, and a new correlation is established. Most of the ground-motion prediction equations (GMPEs) for stable continental regions are derived for very hard rock sites ( V S 30>2000 m/s). In order to use these equations for standard rock sites ( V S 30 around 800 m/s), conversion factors are required. Using the new correlation V S 30- κ and the host-to-target adjustment method of Campbell (2003), we compute amplification factors from very hard rock to rock sites. We show that in these conversions, the effects of both V S 30 and κ have to be taken into account.

Erratum to: Site effect assessment using KiK-net data: Part 1. A simple correction procedure for surface/downhole spectral ratios

When data is available, the estimation of site effects is usually performed using the “standard spectral ratio” (SSR) technique with respect to an outcropping, reference rock site. This study uses the Japanese KiK-net network, which has more than 600 pairs of surface-downhole stations allowing the computation of empirical borehole transfer functions, consisting of mean spectral ratios of surface over downhole recordings. The borehole transfer function deviates from the SSR in two respects: the reference is located at depth, and the downhole velocity varies from one site to another. These differences bias the estimation of the transfer function with reference to a standard outcrop rock site. The goal of this paper is to develop a simple and robust methodology to correct for such bias. The proposed correction procedure consists of two steps: a depth correction designed to account, in a simplified and physically acceptable way, for the existence at depth of destructive interferences and the absence of free-surface effects in the high-frequency range; and an impedance correction designed to normalize the shear wave velocity at depth. The depth correction involves a simple, frequency-dependent curve to be adapted for each site as a function of the first destructive interference frequency at depth. The impedance normalization combines the use of “generic” rock velocity profiles and a quarter-wavelength approach, resulting in a smooth frequency-dependent amplitude correction. The proposed methodology is applied on a large subset of KiK-net data in view of analysing the correlation between site amplification factors and site parameters in a companion paper.

Temporal changes of site response during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

The recent Mw 9.0 off the Pacific coast of Tohoku Earthquake and its aftershocks generated widespread strong shakings as large as ~3000 Gal along the east coast of Japan. Here we systematically analyze temporal changes of material properties and nonlinear site response in the shallow crust associated with the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KiK-Net. We compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of peak ground acceleration (PGA). Our preliminary results show clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites with PGA from 600 to 1300 Gal. In the site MYGH04 where two distinct groups of strong ground motions were recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. We investigate the percentage drop of peak frequency and peak spectral ratio during the Tohoku main shock at different PGA levels, and find that at most sites they are correlated.

Analysis of Single-Station Standard Deviation Using the KiK-net Data

Estimates of single-station standard deviation can be used as a lower bound to probabilistic seismic hazard analyses that remove the ergodic assumption on site response. This paper presents estimates of single-station standard deviation using data from the KiK-net network. The KiK-net network has a dense array of stations that recorded a large number of earthquakes over the period of study, both at the surface and at colocated borehole instruments. The large number of records implies that there are a large number of stations with recordings from multiple events; hence, site terms and single-station standard deviations can be properly estimated. Borehole instruments permit a breakdown of residuals, considering the effect of amplification in the shallow surface layers. Random-effects regression was first used to develop a ground-motion prediction equation (GMPE) using both the surface and borehole data. The GMPE was constrained such that event terms were the same at the surface and borehole. Residuals were then computed and the within-event (intraevent) residuals were separated into a repeatable site-term and a remaining residual, for both the ground motion itself and for the empirical amplification factor between surface and borehole. Results show that single-station standard deviations are considerably lower than standard deviations using the ergodic assumption, and these standard deviations are further reduced if only a small bracket of station-to-event azimuths is considered for each station such that path variability is minimized. Moreover, analyses of residuals indicate that most of the differences between ergodic standard deviations of surface and borehole data are the results of a poor parametrization of shallow site effects. However, the contribution of site-to-site variability in the empirical amplification factor is only limited. Finally, a comparison with results from other studies at different tectonic regions indicates that the values of single-station standard deviations are strikingly similar for all studies.

Spectral Analysis of K-NET and KiK-net Data in Japan, Part I: Database Compilation and Peculiarities

Abstract The wealth of accelerometric recordings collected by the K-NET and KiK-net networks in Japan since 1996 provides a unique opportunity to improve our understanding of many important seismological research questions. Subsets of these data have been used for many case studies, most of them, however, not focusing specifically on the best practices for data selection and giving relatively little attention to the properties and peculiarities directly observable from the data. Yet for many applications, these steps are an important prerequisite for successful and reliable analysis. For this reason, we devote this article to the extraction of a large data set of surface and borehole recordings from the K-NET and KiK-net databases with strong emphasis on data quality and reliability. The final data set available for subsequent work consists of 78,840 records from 2201 earthquakes covering the Japan Meteorological Agency (JMA) magnitude range 2.7–8, observed at 1681 sites throughout Japan. We explain how this data set has been compiled, including automatic phase picking and relocation of events. We also present an overview of the general features of the data set, providing important information for subsequent analysis. Strong amplification effects at high frequencies are immediately visible on the surface recordings. Furthermore, there is a clear presence of downgoing waves in the borehole records, as deconvolution of borehole/surface recording pairs indicates.

Spectral Analysis of K-NET and KiK-net Data in Japan, Part II: On Attenuation Characteristics, Source Spectra, and Site Response of Borehole and Surface Stations

Abstract In this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the K-NET and KiK-net networks in Japan in order to derive attenuation characteristics, source spectra, and site response. For this purpose, we use a total of more than 67,000 S -wave records from 2178 earthquakes ( M JMA 2.7–8) obtained at 1555 stations at the Earth’s surface and more than 29,000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m. Attenuation characteristics are investigated in five separate regions, showing that crustal Q depicts lower values in central compared to southern Japan, and a significant frequency dependence is observed in every region. The source spectra follow the ω 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones. While strong amplification effects dominate the site contributions for the surface sensors, those for the borehole sensors are characterized by smaller variability. Nevertheless, consistent with observations from deconvolution of borehole/surface recording pairs, downgoing wave effects are visible in the site contributions for many borehole stations. Finally, the site amplification functions obtained at the surface are compared with surface-to-borehole (S/B) and horizontal-to-vertical (H/V) spectral ratios, showing that the S/B ratios generally provide better estimates of the horizontal amplification than the H/V ratios due to amplification of the vertical component of ground motion.

Refined thresholds for non-linear ground motion and temporal changes of site response associated with medium-size earthquakes

SUMMARY We systematically analyse non-linear effects and temporal changes of site response associated with medium-size earthquakes, using seismic data recorded by the Japanese Strong Motion Network KIK-Net. We apply a sliding-window spectral ratio technique to surface and borehole strong motion records at six sites, and stack results associated with different earthquakes that produce similar peak ground acceleration (PGA). In some cases we observe a weak coseismic drop in the peak frequency when the PGA is as small as ∼20–30 Gal, and near instantaneous recovery after the passage of the direct S waves. The percentage of drop in the peak frequency starts to increase with increasing PGA values. We also observe a coseismic drop in the peak spectral ratio for two sites. When the PGA is larger than ∼60 Gal to more than 100 Gal, we observe considerably stronger drops of the peak frequencies followed by logarithmic recovery with time. The observed weak reductions of peak frequencies with near instantaneous recovery likely reflect non-linear response with essentially fixed level of damage, while the larger drops followed by logarithmic recovery reflect the generation (and then recovery) of additional rock damage. The results indicate clearly that non-linear site response may occur during medium-size earthquakes, and that the PGA threshold for in situ non-linear behaviour is lower than the previously thought value of ∼100–200 Gal.

On the use of JMA intensity in earthquake early warning systems

The estimation of strength of shaking at a site from the initial P-wave portion of ground motion is the key problems for shortening the alert time of the earthquake Early Warning (EEW). The most of the techniques proposed for the purpose utilize (a) ground motion modelsbasedonthe estimated magnitudeandhypocentraldistance, or(b)theinterim proxies, such as initial vertical displacement Pd. We suggest the instrumental Japan Meteo- rological Agency (JMA) intensity (JMAI) as a characteristic for fast estimation of damage potential in the EEW systems. We investigated the scaling relations between JMAI measured using the whole earthquake recordings (overall intensity) and using particular time intervals of various duration (2.0-8.0s) starting from the P-wave arrival (preliminary intensity). The dataset included 3,660 records (K-NET and the KiK-net networks) from 55 events (MW 4.1-7.4) occurred in 1999-2008 in Japan. We showed that the time interval of 4-5s from the P-wave arrival can be used for reliable estimations of the overall intensity with the average standard error of about 0.5 JMA units. The uncertainty in the prediction may be reduced by consideration of local site conditions or by development of the station-specific models.

Site Amplification and Attenuation via Downhole Array Seismogram Inversion: A Comparative Study of the 2003 Miyagi-Oki Aftershock Sequence

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M_w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of near-surface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V_s), attenuation (Q_s), and density (ρ) profiles at the sites of interest. Based on the inversion results, V_s versus Q_s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surface-to-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average Vs of the upper 30 m (V_(s30)) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B–C boundary site conditions (V_(s30) = 760 m/sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations.

IMPROVEMENT OF THE SHAPING WINDOW IN THE STOCHASTIC METHOD AND REPRODUCTION OF ACCELERATION RECORDINGS OF THE HYOGO-KEN NANBU EARTHQUAKE

The stochastic method is widely used to simulate high-frequency earthquake motions. However, it still has a crucial defect that simulated high-frequency motions are underestimated in amplitude and duration. This is mainly caused by neglecting the effects of multiply reflected and transmitted waves and scattered waves. We remove this defect from the method in a simple practical way. We firstly introduce into the method the hypocentral-distance dependent shaping window which is inferred from borehole data provided by KiK-net network. We secondly introduce the radiation coefficients transiting along with propagation from the theoretical respective value for SH and SV waves to the isotropic value common to the two wave types. The usefulness of the improved method is demonstrated by reproducing three-component acceleration time histories of the 1995 Hyogo-ken Nanbu earthquake in amplitude and duration as well as spectra at all the sites in the Osaka Basin

Nonlinear Behavior of Soils Revealed from the Records of the 2000 Tottori, Japan, Earthquake at Stations of the Digital Strong-Motion Network Kik-Net

Acceleration records of the Tottori earthquake (6 October 2000), pro- vided by stations of the Digital Strong-Motion Network Kik-Net, show clear evidence of the nonlinearity of soil response at sites located in near-fault zones. In this study, records of the mainshock of the Tottori earthquake are analyzed, and stresses and strains, induced by the strong motion in the upper 100 or 200 m of soil, are recon- structed at sites located within 80 km from the fault plane. For reconstructing stresses and strains, the method is applied, which we developed and used previously for studying the response of soils during the 1995 Kobe earthquake. Nonlinear time- dependent stress-strain relations in the soil layers are estimated based on vertical- array records. A good agreement between the observed and simulated accelerograms of the Tottori earthquake testifies to the validity of the obtained vertical distributions of stresses and strains in the soil layers. We also evaluated variations of the shear moduli of the soil layers, caused by the strong motion, at stations located at different distances from the fault plane. Changes in the rheological properties of the upper soil layers were found at the stations closest to the fault-plane. A similarity in stress- strain relations, describing the behavior of similar soils during the 1995 Kobe earth- quake and the 2000 Tottori earthquake, was obtained, indicating the possibility of precasting soil behavior in future earthquakes at sites where profiling data are available.