Rupture process of the 2011 off the Pacific coast of Tohoku Earthquake (M9.0) obtained from strong-motion data

Introduction

We derive the rupture process of the 2011 off the Pacific coast of Tohoku Earthquake (M9.0) at 14:46 on March 11 (JST) using the strong-motion data.

Data

Strong motion data recorded at 36 stations (10 K-NET stations and 26 KiK-net stations) shown in Figure 1 are used in the inversion analysis. The velocity waveforms are derived by integration of the original accelerations, band-pass filtering between 0.01 and 0.125 Hz, and resampling down to 1 Hz. The data for inversion are windowed from 10 s before S-wave arrival for a duration between 225 s and 280 s, which varies depending on stations.

Fault model and discretization of the rupture process

We assume the 510 km × 210 km rectangular fault model that has a strike of 195 degrees and a dip of 13 degrees. The rupture starting point is set at 38.10N, 142.85E, and a depth of 24 km, referring to the hypocenter information by Hi-net and JMA.
The rupture process is spatially and temporally discretized following the multiple-time-window linear waveform inversion scheme (Olson and Apsel, 1982; Hartzell and Heaton, 1983). For the spatial discretization, the fault plane is divided into 17 subfaults along the strike and 7 subfaults along dip directions, with a size of 30 km × 30 km each. For the temporal discretization, the moment rate function of each subfault is represented by 25 smoothed-ramp functions (time windows) progressively delayed by 3.0 s and having a duration of 6.0 s each. The first-time-window starting time is defined as the time prescribed by a circular rupture propagation with the constant speed of Vftw. Thus, the rupture process and the strong-motion waveforms are linearly related via Green's functions.
The Green's functions between each subfault and each station are calculated using the discrete wavenumber method (Bouchon, 1981) and the reflection/transmission matrix method (Kennett and Kerry, 1979) assuming a 1-D layered velocity structure model. The underground structure model is obtained for each station from the 3-D structure model (Fujiwara et al., 2009). Logging data are also referred to for the KiK-net stations. The rupture propagation effect inside each subfault is included in the Green's function by the convolution of the moving dislocation effect, following Sekiguchi et al. (2002).

Waveform inversion

Moment of each time window at each subfault is derived by minimizing the difference between the observed and the synthetic waveforms using the least-squares method. To stabilize the inversion, the slip angle is allowed to vary within ±45 degrees centered at 90 degrees using the non-negative least-squares scheme (Lawson and Hanson, 1974). In addition, we impose the spatiotemporal smoothing constraint on slips (Sekiguchi et al., 2000).

Results

Figure 2 shows the total slip distribution on the fault. Figure 3 shows the comparison between the observed and the synthetic waveforms. Figure 4 shows the rupture progression. Vftw, the maximum slip, and the seismic moment are 3.2 km/s, 48 m, and 4.42×10²² Nm (M_W 9.0), respectively.

This English page was created on March 30, 2020 based on the Japanese report released on August 12, 2011.
The peer-reviewed article was published as Suzuki et al. (2011, GRL).

Figure 1:Station distribution and fault model on the map. Red and blue triangles denote K-NET and KiK-net stations, respectively. A star denotes the rupture starting point. Gray circles denote the epicenters of earthquakes which occurred within one day after the mainshock.

Figure 2: Total slip distribution on the fault. The vectors denote the direction and amount of the slip of the hanging wall side. A star denotes the rupture starting point.

Figure 3: Comparison between the observed and the synthetic waveforms. The maximum values are shown on the upper right of each waveform.

Figure 4: Rupture progression in terms of slip amount for each 10 s time window.

[References]

Bouchon, M. (1981), A simple method to calculate Green's function for elastic layered media, Bull. Seismol. Soc. Am., 71, 959-971.
Kennett, B. L. N. and N. J. Kerry (1979), Seismic waves in a stratified half space, Geophys. J. R. Astr. Soc., 57, 557-583.
Fujiwara, H. et al. (2009), A Study on Subsurface Structure Model for Deep Sedimentary layers of Japan for Strong-motion Evaluation, Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No.337 (in Japanese).
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. Seismol. Soc. Am., 73, 1553-1583.
Lawson, C. L., and R. J. Hanson (1974), Solving Least Squares Problems, Prentice-Hall, Inc., New Jersey, 340 pp.
Olson, A. H. and R. J. Apsel (1982), Finite faults and inverse theory with applications to the 1979 Imperial Valley earthquake, Bull. Seismol. Soc. Am., 72, 1969-2001.
Sekiguchi, H., K. Irikura, and T. Iwata (2000), Fault geometry at the rupture termination of the 1995 Hyogo-ken Nanbu earthquake, Bull. Seismol. Soc. Am., 90, 117-133.
Sekiguchi, H., K. Irikura, and T. Iwata (2002), Source inversion for estimating the continuous slip distribution on a 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.