STRONG MOTION SIMULATION OF THE 2004 NIIGATA-KEN CHUETSU EARTHQUAKE BY USING A MULTI-ASPERITY MODEL AND A Vs30 MAP

Nelson PULIDO1, Masashi MATSUOKA1, Hisakazu SAKAI2 and Kouichi HASEGAWA1

1 National Research Institute for Earth Science and Disaster Prevention 2 Ritsumeikan University

SUMMARY: In this study we analyzed the contribution of fault rupture process and site effects to the extremely large recorded near-fault ground motions and widespread road damage during the 2004/10/23, M6.8 Niigata-ken Chuetsu earthquake. For that purpose we calculate seismic-bedrock broadband ground motions every 250m within the near-fault region, and incorporate site amplifications to simulated PGV values, from a 7.5-arc second (250m) Vs30 map in the Niigata region. We derive the expressions relating average S-wave velocity of the shallow soil to PGV amplification values with respect to a seismic bedrock condition. These relationships are useful to predict PGV when a detailed velocity 3D structure is not available. Our simulated ground motions are in good agreement with observed strong motion data and damage distribution to roads during the Chuetsu earthquake. Predicted PGV values at a dense grid cell provide a useful information at localities with no strong motion records.

1 INTRODUCTION The 2004/10/23, M6.8 Niigata-ken Chuetsu earthquake is the largest damaging earthquake in Japan since the 1995 Kobe earthquake. The earthquake recorded accelerations above 1.7g and velocities of 130 kines. The Chuetsu earthquake produced an extensive damage to roads around the epicentral region. In order to allow a comparison between damage to roads and ground motion indexes it is necessary to calculate ground motion at a finer spacing than the one provided by observed ground motions (K-NET and KiK-net) and JMA observed instrumental intensities. For that purpose we calculate seismic-bedrock broadband ground motions every 250m within the near-fault region, and incorporate site amplifications to simulated PGV values, from a 7.5-arc second (250m) Vs30 map in the Niigata region. Forward seismic-bedrock ground motions (by a hybrid method that combines wave propagation within a 1-D crustal model at low frequencies, with a semi-stochastic approach at high frequencies), are calculated from a broadband multi-asperity source model obtained by optimizing the fitting to observed Fourier spectra and acceleration envelopes of near-fault ground motions (Pulido and Kubo [1]). 2 SITE AMPLIFICATIONS WITHIN NIIGATA PREFECTURE Inversion of spectral ratios We derive the expressions relating the average S-wave velocity of the upper 5, 10, 20 and 30m to PGV amplification factors with respect to a seismic bedrock site (Vs=2600 km/s), by using site amplification functions obtained from a two-step spectral inversion technique; we estimated site amplifications at 31 strong motion stations in Figure 1 (23 K-NET and 8 KiK-net), using records from 50 aftershocks (M3 to 5) of the Mid-Niigata earthquake. We first estimated relative spectral ratios at every site with respect to the surface recordings of a reference borehole site (FKSH07 KiK-net site), and perform the spectral inversion to obtain relative amplification functions, including borehole bottom site responses. Then we use the theoretical 1-D response of the reference borehole site to obtain corrected amplification functions with respect to an S-wave velocity of 2600 m/s, which is the velocity value at the bottom of the reference borehole site. The reason in selecting FKSH07 as a reference site is because its underground (GL-203m) S-wave velocity is located near a seismic bedrock condition (2600 m/s). To check the inverted site effects values we calculate the ratios of surface to bottom site effects and compare them with the observed borehole response using all the aftershocks database at the KiK-net sites (Figure 2). We can observe a very good agreement between the ratios from inverted site effects (black lines) and observed boreholes response (gray lines) (Figure 2). Our inversion assumes an spherical attenuation (1/R geometrical spreading) and a frequency dependent Q. We obtained a Q value of 63 f 1.14 from inversion. This value is in agreement with other Q values obtained for the region (Jin and Aki [2]). PGV amplification to seismic bedrock and Vs30 Using the reference station corrected site effects we de-convolved the site amplification functions from all the aftershocks records and calculated the ratio of observed to site-corrected PGV values. In this way we obtain PGV amplifications with respect to a seismic bedrock (Vs=2600 m/s). These PGV amplifications include the contribution from all frequencies as we used frequency dependent site response functions to obtain them. Finally we calculate the time averaged S-wave velocities for different soil column thickness (5 to 30m) from log information at each site, and derive the best fitting log-linear expressions relating PGV amplification and average S-wave velocities. We obtained that the PGV amplification with respect to the seismic bedrock is better correlated with the average S-wave velocity of the upper 10m of soil (Vs5 and Vs10) than with Vs30, which is a widely used index to estimate PGV amplifications with respect to an engineering bedrock site (Vs=600m/s) (Figure 3). This result implies that most of the amplification due to site effects is experienced in the very shallow layers, as can be clearly observed in sites like K-Net Ojiya. The PGV amplification with respect to a seismic bedrock (Vs=2600m/s) is obtained as follows:

log PGVamp = 1.83 – 0.53 log Vs30 (1) In Figure 3 (Vs30) we can observe that the PGV amplification with respect to a seismic bedrock obtained in this study, is twice the PGV amplification with respect to an engineering bedrock (Vs30=600m/s) (Midorikawa and Matsuoka [3]). This factor of 2 represents the contribution of the deep velocity structure to PGV values. In a later section we will use information of a 250m Vs30 map of the Niigata region to predict the ground motion at a finer grid than observed data (KiK-net, K-NET and JMA stations

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Figure 1. KiK-net and K-NET stations used to estimate site amplifications.Color scale corresponds to a 7.5 arc-second (250m) map of the upper 30maverage S-wave velocity within the Niigata region.

Figure 3. PGV amplification with respect to a seismicbedrock condition (Vs= 2600m/s) for different averageS-wave velocity soil column thickness. Correlation values(R) and best-fitting equations are shown inside each panel.

Figure 2. Ratios of surface to bottom inverted site effects for KiK-Net borehole stations in Niigata prefecture region (black lines). Observed surfaceto bottom ratios are shown in gray lines for all aftershocks database. Yellow lines are observed averages ratios. Red line depicts the theoreticaloutcrop response for the reference station (for spectral inversion). Borehole depths are shown within each box (in meters).

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including municipalities). Despite Vs10 gives a better correlation to PGV amplification than Vs30, there is no information available on a Vs10 map for Japan, and therefore we will use the Vs30 map available for the Niigata region (Wakamatsu and Matsuoka [4]). 3 SOURCE MODEL ESTIMATION We estimated an optimum multi-asperity model of the Niigata-ken Chuetsu earthquake by maximizing the fitting to observed acceleration Fourier spectra and velocity envelopes of near-fault ground motions. For this purpose we use a genetic algorithm scheme combined with a broadband strong motion simulation procedure (Pulido and Kubo [1]). For the inversion, we selected 10 stations (3 KiK-net, 7 K-NET) within 50 km from the fault, as shown by filled triangles and squares in Figure 5. Before performing the inversion, we de-convolved all observed waveforms to a seismic-bedrock ground condition (Vs=2600), by using the site effects functions obtained in the previous section. We used a slip model obtained from inversion of near-fault strong motion and teleseismic waveforms (Yagi [5]), as an initial source model (Figure 4). Our optimum source model consists of four asperities the first one centered at the hypocenter (Figure 4). In Table 1 we show the asperity parameters obtained from the inversion. We obtained that asperity 2 has a very large stress drop (271 bar), which makes a significant contribution to the extremely large PGA value of 1.7g, recorded at the Tokamachi K-NET station (NIG021). 4 STRONG MOTION SIMULATION WITHIN NIIGATA PREFECTURE In order to compare damage to civil infrastructures during the 2004 Niigata-ken Chuetsu earthquake, with ground motion indexes, we estimated the ground motion every 250m. Forward strong motions are first calculated at a seismic-bedrock for a 1km grid mesh within the Niigata region, from an optimum multi-asperity model combined with a broadband strong motion simulation procedure (Pulido and Kubo [1]). Then we linearly interpolate the seismic bedrock ground motions to a 250m mesh corresponding to the grid cells of a Vs30 map available for the Niigata region (Wakamatsu and Matsuoka [4]). Finally we apply the PGV amplification factors with respect to the seismic bedrock obtained from equation 1 to every interpolated seismic bedrock PGV, and get the ground motion at the surface (Figures 5 and 6).

Parameter Value

Stress drop asperity 1 100 bar

Stress drop asperity 2 271 bar

Stress drop asperity 3 187 bar

Stress drop asperity 4 61 bar

f max 5 Hz

Q(f ) for Niigata region 63 f 1.14

Figure 5. Simulated PGV values of the 2004 Niigata-ken Chuetsu earthquake (colorscale). Black contour lines represent the observed JMA instrumental intensity duringthe Chuetsu earthquake. The figure inset shows the comparison between simulated andobserved PGV values at Kik-net and K-Net sites within the Niigata region. Filledsymbols represent stations used for asperity parameters estimation.

Figure 6. Simulated PGV values of the 2004 Niigata-kenChuetsu earthquake (color scale) for a region around theruptured fault (black box). Open circles show damagepoints to roads during the Chuetsu earthquake.

Figure 4. Multi-asperity model of the 2004 Niigata-ken Chuetsu earthquake.Slip model was obtained from inversion of near-fault strong motions andteleseismic data (Yagi 2004).

Table 1. Source Model and attenuation parameters from inversions

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In Figure 5 we show the simulated PGV values for the Niigata region. For comparison we include the observed JMA instrumental intensity values (black contour lines in Figure 5) obtained from JMA intensity recorders located at every municipality (approximately one instrument per municipality). Municipalities boundaries within the Niigata prefecture are depicted by white lines in Figure 5. From the overall picture of the Niigata region we can see that our simulated PGV’s follow a similar trend compared to JMA recorded intensities. However, looking at a fine scale we can observe that our simulation is able to provide a detailed picture of peak ground motions at a municipality level, were there are only one or two JMA records available (Figure 5 and Figure 6). In order to evaluate the accuracy of our simulations forward ground motions were calculated at all the K-NET and KiK-net sites within the Niigata prefecture, incorporating PGV amplifications from equation 1, and using actual Vs30 values from log information data at every site. We obtained a good agreement to observed strong ground motions as displayed at the inset in Figure 5. The simulated PGV values for the 10 sites used for estimation of asperity parameters are within 40% deviation from observed values, as shown by filled circles in Figure 5 inset. Our simulated PGV values show a complex pattern around the hypocenter (blue stars in Figures 5 and 6). We can observe that ground motion is relatively small just above the hypocenter, compared with the adjacent areas around the hypocenter (Figure 5 and 6). This feature may be explained by the radiation pattern of a reverse type earthquake like the Chuetsu earthquake, because for this type of mechanism a minimum nodal plane of SH-waves corresponds approximately to a region above the hypocenter. We obtained large values of PGV at the western and southern regions of Ojiya city as well as the entire Kawaguchi and Yamakoshimura towns, where simulated PGV’s are above 80 kines for large areas (Figure 6). We obtained that asperity 1 have a large contribution to the ground motion at western Ojiya-city and northern Kawaguchi town. Large ground motions at Yamakoshimura town are generated by asperity 3, and ground motions at southern Ojiya and Kawaguchi are mainly generated by asperity 2 (Figure 6). 5 STRONG MOTION AND DAMAGE TO ROADS In Figure 6 we show the simulated PGV for the epicentral region of the 2004 Chuetsu earthquake. Damage points to major local and prefecture roads (Sakai et. al. [6]) are shown by open circles and black lines in Figure 6. We can observe that PGV is above 40 kines for the most of the region (Figure 6). In southern Ojiya, northern Kawaguchi and Yamakoshimura regions, where simulated PGV is above 100 kines, a large concentration of road damage was observed (Sakai et al. [6]). On the other hand simulated PGV values at Kashiwazaki-city, 30 km North-West of the mainshock epicenter, are relatively larger than the surrounding areas. This is also in good agreement with the large road damage concentration at Kashiwazaki-city (Figure 6). On the other hand, despite having large values of PGV, northern Ojiya did not sustained significant road damage. This is maybe related to the fact that the urban area of Northern Ojiya city is located within a region without pronounced topographic changes, contrasting with the heavily damaged roads in the Yamakoshimura mountain region. 6 DISCUSSIONS AND CONCLUSIONS -We derived expressions relating average S-wave velocity of soil to PGV amplification with respect to a seismic bedrock condition for the Niigata region, by using results from a spectral ratios inversion of aftershocks recordings from the Chuetsu earthquake. These equations were derived for the Niigata region and therefore they are not necessarily applicable for other regions. However they represent a good tool for approximately estimating PGV values when information of a detailed 3D velocity structure (above the seismic bedrock and below the engineering bedrock) is not available. Future studies may address the calculation of similar equations for other regions with different geological conditions. -We obtained that the PGV amplification with respect to the seismic bedrock is better correlated with the average S-wave velocity of the upper 10m of soil (Vs5 and Vs10) than with Vs30, which is a widely used index to estimate PGV amplifications with respect to an engineering bedrock site. The reason for this is probably the larger impedance between the upper 10m of soil and the seismic bedrock, compared with that of the upper 30m. -We simulated ground motion at a 250m grid in the Niigata region by incorporating site effects and using a multi-asperity model derived from near-fault ground motions. Our agreement to observed KiK-net, K-NET and JMA intensity records within the Niigata region is good. -We also obtained a good correlation between observed damage to roads and simulated PGV values. We are able to reproduce large ground motions in regions with a high concentration of road damage, and with few available ground motion recordings, highlighting the importance of ground motion prediction.

ACKNOWLEDGMENTS K-NET and KiK-net strong motion data used in this study was provided by NIED.

REFERENCES [1] Pulido N., and T. Kubo. Near-Fault Strong Motion Complexity of the 2000 Tottori Earthquake (Japan) from a Broadband Source

Asperity Model, Tectonophysics, 390, 177-192, 2004. [2] Jin, A. and K. Aki. High-resolution maps of Coda Q in Japan and their interpretation by the brittle-ductile hypothesis, Earth

Planets Space, 57, 403-409, 2005. [3] Midorikawa, S., M. Matsuoka, and K. Sakugawa. Site effects on strong-motion records observed during the 1987 Chiba-ken

Toho-oki , Japan earthquake, The 9th Japan Earthquake Engineering Symposium, 3, 85-90, 1994. [4] 若松加寿江，松岡昌志，坂倉弘晃：新潟地域の地形・地盤分類 250m メッシュマップの構築とその適用例，第 28 回地

震工学研究発表会報告集，CD-ROM, ID180, 4p.,2005. [5] Yagi, Y., http://iisee.kenken.go.jp/staff/yagi/eq/20041023/source.pdf, 2004. [6] 酒井久和，長谷川浩一，ネルソン・プリード，佐藤忠信: 新潟県中越地震における強震動と道路被害の関係, 構造工

学論文集, 52A, No.1, 301-308, 2006.

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