(Aug. 1, 2007; last update on Aug. 17, 2007)

Source Process of the 2007 Niigata-ken Chuetsu-oki Earthquake
Derived from Near-fault Strong Motion Data
National Research Institute for Earth Science and Disaster Prevention
The 2007 Niigata-ken Chuetsu-oki earthquake occurred on July 16th, 2007, 10:13 JST( 37.557N, 138.608E, 17 km depth; Japan Meteorological Agency). We performed a multi-time
window linear waveform inversion analysis (Hartzell and Heaton, 1983) to estimate therupture process from the near fault strong motion data of K-NET, KiK-net, and others.
Fault Model and Parameterization
Open circles in Fig. 1 are aftershock distribution within 24 hours of the mainshock
located by manual picking of Hi-net data. Fault mechanisms are estimated by the distributionof the polarity for the P-wave first motion by Hi-net (left) and by the moment tensor analysisof F-net data (right).
The fault plane for the mainshock has not been clearly determined yet from theaftershock distribution, so that we performed two waveform inversions for north-west dipping
fault (Model A) and south-east dipping fault (Model B). Their strike, dip and rake (as thecenter of the rake angle variation constraint) are set to those of the moment tensor solutions
by F-net. The former is N215E, 49, 80 degrees, and the latter is N49E, 42, 101 degrees forstrike, dip, rake angles, respectively. The hypocenter determined by hypoDD using Hi-net
data (Yukutake et al., 2007) was 37.5397N, 138.6091E, and 7.417 km depth. For waveform
inversion, we assumed a rupture starting point at the same epicenter and 8 km depth.
We assumed a fault plane model of 30 km length by 24 km width to cover aftershockdistribution within 24 hours after the mainshock. In the multi-time window linear waveform
inversion procedure, the moment-release distribution is discretized in both space and time.
Discretization in space is done by dividing the model fault planes into 180 subfaults, each 2
km× 2 km. We assumed six smoothed ramp functions with 1.0 sec duration separated by 0.5
sec to represent slip velocity time function on each subfault.
Theoretical Green's functions were calculated by the discrete wavenumber method(Bouchon, 1981) and the R/T matrix method (Kennett, 1983) with the different stratified
medium for each station based on the velocity structure including the information form thereflection survey and borehole logging data. Convolution of moving dislocation was introduced
to represent the rupture propagation in an each subfault (Sekiguchi et al., 2002).
Waveform Data
We used strong motion data of 14 stations shown in Fig. 1 from K-NET, KiK-net

(downhole), F-net, JMA (JMACB6), and Niigata prefecture (L65039). The observed
acceleration records were integrated into velocity except of F-net velocity data recorded byVSE-355G2, and bandpass filtered between 0.1 and 1.0 Hz. We inverted 14 s of the S-wave
portion from 1 s before the S-wave arrivals.
Waveform Inversion
We solved least-squared equation to obtain slip amount of each time window on eachsubfault to minimize squared residual of the waveform fitting between observed and synthetic
waveforms. To reduce instability or excessive complexity, smoothing constraint is introduced
to reduce differences among slips close in space and in time. Non-negative constraints(Lawson and Hanson, 1974) to limit the rake-angle variation are also adopted. The rake
angles were allowed to vary within plus-minus 45 degree centered at 101 degree fornorth-west dipping fault and 80 degree for south-east dipping fault, which are rake angles ofthe moment tensor solutions by F-net. We performed many inversions changing the rupture
velocity of the first time windows and the smoothing strength. Appropriate values for thesmoothing strength and the rupture velocity were selected based on ABIC (Akaike's BayesianInformation Criterion; Akaike, 1980) and the residual of the waveform fitting, respectively.
ResultFig. 2 show slip distributions and Fig. 3 displays comparison between observed andsynthetic velocity waveforms.
Regarding Model A (north-west dipping fault plane), estimation of the total seismic
moment was 1.42 x 1019 Nm which corresponds to Mw = 6.7. We obtained large slip in thesouth-west deeper part of the rupture starting point, which is close to Kashiwazaki-city. The
second or third velocity pulses of observed velocity waveforms seem to be composed of slipfrom the asperity.
Regarding Model B (south-east dipping fault plane), estimation of the total seismic
moment was 1.62 x 1019 Nm which corresponds to Mw = 6.7. We obtained large slip in thesouthwest shallower part of the rupture starting point, which is also close to
Kashiwazaki-city.
In both models, we found small slip near the rupture starting point, and largest slip at
about ten kilometer in the south-west of the rupture starting point with the maximum slip of
2.3 and 2.5 m for Models A and B, respectively. The difference of the residual between
observed and synthetic waveforms for both models is not significant, therefore it is difficult toconclude which fault plane is appropriate to explain. The estimated large-slip regions in theinverted source models with the Models A and B are located near the cross point of the two
fault plane models, which should have similar radiation pattern. This situation may be one of

the reasons why judgment of the fault plane orientation is such difficult. We need careful
examinations not only strong motion data but also geodetic data to further explore the fault
orientation and the source process of this earthquake.
Notes: This result is preliminary and will make a change.
(*Aoi, S.(aoi@bosai.go.jp), **H. Sekiguchi, *N. Morikawa, *T. Kunugi, and ***M. Shirasaka)* National Research Institute for Earth Science and Disaster Prevention** National Institute of Advanced Industrial Science and Technology*** Japan Meteorological Agency
Acknowledgement: We used seismic intensity data of Niigata prefecture.

http://www.k-net.bosai.go.jp/k-net/topics/chuetsuoki20070716/inversion/
References
Akaike, H. (1980), Likelihood and the Bayes procedure, in Baysesian statics, edited by J. M.
Bernardo, M. H. DeGroot and A. F. M. Smith, University Press, Valencia, Spain.
Bouchon, M. (1981), A simple method to calculate Green's function for elastic layered media, Bull.
Seism. Soc. Am., 71, 959-971.
F-net, www.fnet.bosai.go.jp/freesia/index.html
Hartzell, S. H. and T. H. Heaton (1983), Inversion of strong ground motion and teleseismic
waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake,Bull. Seism. Soc. Am., 73, 1553-1583.
Hi-net, www.hinet.bosai.go.jp/ (in Japanese).
Kennett, B. L. and N. J. Kerry (1979), Seismic waves in a stratified half space, Geophys. J. R. astr.
Soc., 57, 557-583.
Lawson, C. L., and R. J. Hanson (1974). Solving Least Squares Problems, Prentice-Hall, Inc., New
Jersey, 340 pp.
Sekiguchi, H., K. Irikura, and T. Iwata. (2002). Source inversion for estimating continuous slip
distribution on the fault, --- Introduction of Green's functions convolved with a correction
function to give moving dislocation effects in subfaults ---, Geophys. J. Int., 150, 377-391, 2002.
Yukutake, Y., T. Takeda, K. Obara (2007), Spatial distribution of aftershocks in the region ofNiigata Tyuetsu-oki Earthquake in 2007, by waveform correlation analysis, fall meeting ofSeism. Soc. Japan (in Japanese).
Wessel, P., and W. H. F. Smith (1995). New version of the Generic Mapping Tools released, EOS
Trans. AGU 76, 329.

Fig. 1: Distribution of the observation stations used for the inversion analysis. Two rectangles
show the projection of the assumed fault planes. Red star indicates the rupture
starting point. Open circles are aftershock distribution within 24 hours after themainshock located by manual picking of Hi-net data. Fault mechanisms are estimatedby the distribution of the polarity for the P-wave first motion by Hi-net (left) and by
the moment tensor analysis of F-net data (right).

Fig. 2A: Estimated total slip distribution (north-west dipping fault; Model A). Star indicatesthe rupture starting point. Arrows show the amplitudes and directions of slip.

Fig. 3A: Comparison between observed (black) and synthetic (red) velocity waveforms(north-west dipping fault; Model A). The maximum values of each component aregiven to the upper-left of each trace in m/s.

Fig. 2B: Estimated total slip distribution (south-east dipping fault; Model B) Star indicatesthe rupture starting point. Arrows show the amplitudes and directions of slip.

Fig. 3B: Comparison between observed (black) and synthetic (red) velocity waveforms(south-east dipping fault; Model B). The maximum values of each component are given
to the upper-left of each trace in m/s.

Fig. 4: The projection of the inverted slip distribution. Left panel is Model A(north-west dipping fault), and right panel is Model B (south-east dipping fault). Bluecircles are aftershock distributionlocations estimated by hypoDD using Hi-net data
(Yukutake et al., 2007). The large slip area falls on to the area with relatively sparseaftershock distribution.