Preliminary calibration of the

numerical large-scale scenario of the

Niigata-Ken Chuetsu-Oki earthquake

Filippo GATTI, Fernando LOPEZ-CABALLERO,Roberto PAOLUCCI, Didier CLOUTEAU

Introduction

This study presents a first calibration of a large-scale seismological model of the July, 16th 2007 MW6.6 Niigata-Ken Chuetsu-Oki earthquake (NCOEQ). The analysis is intended to clarify some aspects of the recorded site-effects in the epicentral area (within the japanese Niigata prefecture) and at the Japanese nuclear site ofKashiwazaki-Kariwa (KKNPP), held by the Tokyo Electric Power Company (TEPCO). A map of the KKNPPsite is shown in Figure 1a.

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Figure 1: (a) Map of the KKNPP with the accelerometric stations installed that recorded the NCOEQ. (b)Map of the Niigata area surrounding the NCOEQ epicentre. The slip model proposed by Shiba et al. [10] isshown along with the KKNPP site. Several recording stations belonging to the japanese KNET and KikNetaccelerometric network are shown.

The main objectives of this study are (1) the choice of a suitable geological model and (2) the quantitative yetpreliminary assessment of the parameters required for the kinematic modelling of the source mechanism (e.g.the fault plane location and dimensions originated, the slip pattern, rupture velocity, rise time).Due to the relative small source-to-site distance and shallow hypocenter depth, the mentioned seismic scenariois extremely complex to be characterized, although very appealing due to the consistent seismic record databaseavailable. From this point of view, different assumptions of the SINAPS@ project work-packages may be tested.For instance, several source-inversions were performed on the NCOEQ [2, 4, 7, 10, 3]. Most of the proposedslip models are reliable up to 0.5-1 Hz and poorly constrained by near-source data. In this study, the slipcontour proposed by Shiba et al. [10] is considered (the fault plane is depicted in Figure 1b), duly calibratedupon KKNPP records. Moreover, a physics-based 3D numerical scenario is herein built-up, to overcome someof the major simplifications commonly made in the estimation of strong-ground motion time-histories (e.g. thedisregarded topography effect, the 1D geological profile).In the following, a preliminary calibration of the mentioned scenario is presented. In this sense, the work ispresented in three steps: the semi-analytical validation of (1) a simplified 1D geological model and of (2) aninverted source model; (3) the numerical simulation of the July, 16th 2007 MW4.4 aftershock wave-propagationin the Niigata region, by means of a Spectral Element Method (SEM) based numerical code.

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Validation of seismological model

A numerical large-scale earthquake scenario requires the standalone identification of the source and path effectson the radiated wave-field. This identification is performed by applying complex wave-form inversion analyseson mainshock and aftershock data. In this section, the geological and source models proposed in the literatureare validated against the available set of recordings. Specifically, the analyses were performed by means of theWave-Number Integration (WNI) method, proposed and numerically implemented by Hisada [5, 6]. The WNIis a semi-analytical method that simulates the complete 3D wave propagation field radiated from an extendedkinematic seismic source in an extended half-space. This approach is based on the computation of static anddynamic Green’s functions of displacements and stresses for a viscoelastic horizontally layered half-space (notopography considered).

Simplified geological model of the epicentral area Several simplified 1D geological models are availableof the Niigata region. VP and VS profiles are depicted in Figure 2a and 2b respectively. Their validation wasperformed by runnning a WNI analysis of the July 16 2007 MW4.4 aftershock (aftershocks are considered aspoint-wise seismic sources).

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Figure 2: Several VP (a) VS (b) 1D geological models of the Niigata basin from literature.

The analyses carried out clarified that the two most reliable 1D geological profiles were the one proposed byAochi et al. [1] and the one proposed by Shinohara et al. [11] (tagged as Aochi2013 and Shin.2008 respectivelyin Figure 2a, 2b). This is proved by the good fit provided by the time-histories shown in Figure 3a,3b (recordsfilter at between 0.05 and 0.5 Hz).

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Figure 3: Comparisons between recorded (blue) and synthetic (red) velocity wave-forms at NIG018 (KNETstation) for the 16th July 2007 MW4.4. Both the time-histories were filtered within a 0.05-0.5 Hz frequencyband. (a) Synthetics were obtained by using the geology profile Aochi2013 [1]. (b) Synthetics were obtained byusing the geology profile Shin.2008 [11].

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Seismic source characterization Average co-seismic slip values barely estimate the ground motion com-plexity, even in far-field conditions, because the area of strong motion generation usually coincides with slipheterogeneities, i.e. asperities area. As depicted in Figure 1b, it composes of three major asperities. To testlatter slip model, the same approach presented in the previous section was exploited although the fault plane issubdivided into rectangular sub-faults at constant slip and rake angle. Each contribution is then convolved toobtain the wave-forms at each receiver’s location. The analysis is limited to 0.5 Hz, since the Green’s functionsare computed by means of a truncated wave-number series.

Results at KKNPP are shown in Figures 4a, 4b. The synthetics are compared to the recordings at G.L.-250mwithin the nuclear site (KSH-SG4, in Figure 1a). The velocity time-hitories were obtained by using the WNI,with the same geological profiles considered above.

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Figure 4: Comparisons between recorded (blue) and synthetic (red) velocity wave-forms at KSHSG4 (TEPCOstation) for the NCOEQ. Both the time-histories were filtered within a 0.05-0.5 Hz frequency band. (a) Syn-thetics were obtained by using the geology profile Aochi2013 [1]. (b) Synthetics were obtained by using thegeology profile Shin.2008 [11].

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Figure 5: (a) Mesh of the Niigata region including surface topography. (b) Snapshot of the displacement fieldnumerically reproduced by Spectral Element Method.

Large-scale SEM wave-propagation of NCOEQ aftershock

The final yet preliminary phase of this study concerns the forward numerical wave-propagation of the July 162007 MW4.4 aftershock. The simulation was performed by means of a SEM-based numerical code implemented

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on massively parallel computer architecture. Figure 5a shows a detail of the meshed domain. Figure 5b depictsthe propagated displacement contour.The numerical model is featured by the topography of the Niigata area, along with the 1D geological profilecalibrated in the previous section. The seismic source is represented by a point-wise double-couple calibratedon the Hi-Net moment tensor solution [8, 9] of the July 16 2007 MW4.4 aftershock. Time-histories at severallocations (K-NET, KiK-net and KKNPP stations) were compared to test the main difference between semi-analytical and SEM analyses, addressing the effect of topography and the source time-function. Moreover,different degree of mesh refinement were tested to increase the frequency range of reliability of the numericalmodel.

Acknowledgement

This work, within the SINAPS@ project, benefited from French state funding managed by the National Re-search Agency under program RNSR Future Investments bearing reference No. ANR-11-RSNR-0022-04. Theresearch reported in this paper has been supported in part by the SEISM Paris Saclay Research Institute.Time histories and velocity profiles used in this study were collected from the KiK-net website: http://www.kik.bosai.go.jp/kik/ (last accessed November 2011). The authors are very grateful to the National Research Insti-tute for Earth Science and Disaster Prevention (NIED) for providing such high- quality earthquake recordings.

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