Evidence of large scale repeating slip during the 2011 Tohoku-Oki earthquake

(Finite-fault source rupture process inversion, version 3)

Shiann-Jong Lee, Bor-Shouh Huang, Masataka Ando, Hung-Chie Chiu and Jeen-Hwa Wang

Institute of Earth Sciences, Academia Sinica

Last update: 2011/10/15  |  version 1   version 2

*This paper had been published in GRL. For more details:  http://www.agu.org/journals/gl/gl1119/2011GL049580/

Movie S1: This movie is the rupture process of Tohoku-Oki earthquake. Left side shows the slip at each moment, and right side is the accumulated slip.

■ Introduction
The repetition of slip during rupture process of earthquake is a debate issue which had never been confirmed clearly in the past big events due to the lack of dense near-field observations and limited resolution in time of source model. The 2011 M9.0 Tohoku-Oki earthquake generated a wealth seismic records which provided us an unprecedented opportunity to study the rupture evolution of giant earthquake at a high spatio-temporal resolution. Here we use teleseismic, local strong motion and near-field coseismic geodetic data to investigate the source rupture process of this event based on the parallel inversion technique. The results reveal a broad slip zone with remarkable large scale repeating slip during the earthquake. The inverted source model shows several time periods of energy release with three main peaks. These energy bursts and temporal rupture snapshots suggest repetition of a large scale slip on the biggest asperity. This rupture behavior resulted in >50 m slips on the slip zone and prolonged the entire rupture process for a long duration of ~160 seconds. The proposed source model is in a good agreement with the aftershock distribution and can interpret the characteristics of local strong motions. Further investigations of repeating slip during this event are crucial which will deeply transform earthquake science from dynamic point of view.

■ Inversion results

Joint source inversion results show that the rupture occurred on a large triangular shaped slip zone with an area of 400x200 km² and the average slip was about 18 m (see Fig. 1). The slip concentrated in several areas, with one extremely large asperity and two secondly large asperities. These asperities were not only defined from the slip amount but also mainly from the slip characteristics and their developments in temporal rupture process which is discussed in the next section. The largest asperity developed around the hypocenter with a maximum slip over 50 m. This asperity covered a broad area of about 200x200 km² with a reverse motion fan spreading originated from the hypocenter. Comparisons between inverted synthetic and observation of the three data sets are shown in Figure 2.

Figure 1: Map view of spatial slip distribution of the 2011 Tohoku-Oki earthquake. The slip values are shown in the color scale indicated at the bottom. The vectors indicate slip directions. The red star is the hypocenter reported by the JMA. The hypocenter of March 9, 2011 M7.2 foreshock reported by USGS is presented by open blue star. Aftershocks occurred within 2 months after the mainshock are shown by solid circles. The beach balls show the USGS W-phase focal mechanisms.

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Figure 2: (a) Comparison between teleseismic body wave and synthetic waveforms. Black and red lines are observed and synthetic waveforms respectively. All the waveforms are vertical component in displacement type, band-pass filtered between 0.005 and 0.2 Hz. The maximum observed ground displacement is shown at each station. Forward synthetic waveforms (purple dotted lines) for stations not been used in the inversion (open blue triangle) are also shown for comparison. (b) Comparison between observed GPS and synthetic coseismic displacements. The numbers show the deformation amount of seafloor geodetic observations in meter. (c) Comparison between local K-NET observation and synthetic waveforms. Black and red lines are observed and synthetic waveforms respectively. All the waveforms are band-pass filtered between 5 and 100 second in velocity type.

■ Rupture process & Source time function

The slip snapshots and accumulated slip are shown in Figure 3. From the temporal variation in slip, the Tohoku-Oki earthquake appears to be caused primarily in the form of a large scale repeating slip related to the development of the largest asperity. By comparing the moment rate function (see Fig. 4) with the rupture snapshots, three time periods of energy release can be identified. We infer that the first energy release time (T1) is related to the rupture nucleation near the hypocenter. In the second energy release time (T2), Asperity IA grew at the shallow part just above the hypocenter. Meanwhile, the rupture front also extended to the deeper part of the subduction zone. The fault plane was quiet at ~100 seconds, and then the area above hypocenter was initiated to slip again between 100 and 140 seconds (T3). These two repeating slip events resulted in a long duration time and extremely large movement (>50 m) on Asperity I. The overall duration of the Tohoku-Oki earthquake is about 160 seconds and the total seismic moment is 0.367×10³⁰ dyne-cm, which is equivalent to an earthquake of Mw 9.0.
　

Figure 3: The snapshots of the rupture process: (a) the slip at each moment, and (b) accumulated slip. The numbers in (a) identify the appearance of repeating slips (marks 1 to 3) and the development of southern rupture (mark 4).

Figure 4: The moment rate function. Three time periods of moment release, i.e. T1, T2 and T3, are separated by dotted lines. The percentage of the moment release from the entire earthquake and their equivalent moment magnitude are also shown.

■ Discussion

It is expected that repeating slip would dominate the characteristics of local seismograms. To confirm this point, we check the acceleration seismograms recorded by K-NET as shown in Figure 5. By tracking the travel times of seismograms, two predominant waveform groups (mark 1, 2) are observed in the northern stations (above 37.5° N). The time difference between these two wavefronts is about 40 seconds. Another two waveform groups (mark 3, 4) can also be found from the northern and southern stations, but their wavefronts are not as obvious as the first two. Comparison between synthetic and observed data shows a good agreement in waveforms. This result would confirm the phenomenon of repeating slip.

We infer that the first waveform group is related to the nucleation near hypocenter that the initial slip occurred on the biggest asperity (T1 in moment rate function, see Fig. 4), and the second group is caused by the repeating slip related to the development of Asperity I (T2). Although the time difference between seismic energy release of Asperity IA in T2 and that of Asperity IB in T3 is ~50 seconds, their contributions to waveforms are not indivisible due to long rupture duration times (see mark 2, 3). Furthermore, the rupture area of Asperity IA extended to the western part of the fault plane close to Honshu. This caused stronger ground shaking compare to Asperity IB that ruptured farther from Honshu. The wavefront of the fourth group (mark 4) is not obvious, we still can trace its source which came from the development of Asperity II in the southern slip zone.

Figure 5: Local strong motion records: (a) The distribution of K-NET stations in used, (b) the raw E-W component acceleration waveforms, and (c) comparison between observed (black lines) and synthetic (red lines) E-W component velocity waveforms, which are band-pass filtered between 0.01 and 0.2 Hz. The travel time curves of four waveform groups are identified with the color shading and numbers.

■ Conclusions

Based on the results of high resolution joint source inversion analysis, repetition of a large scale slip in area near the hypocenter has been observed during the Tohoku-Oki earthquake. The repeating slip caused an anomalously 50 m slip on the largest asperity which had a dimension of about 200x200 km² developed from shallow to deeper subduction zone. The temporal rupture processes show that the slip nucleated slowly near the hypocenter at the beginning, and then propagated to the shallow part causing the second slip on the biggest asperity. A remarkable slip developed in the deeper subduction zone then enforced the previous ruptured area to slip again. Finally, the rupture front extended to the south and north along the Japan Trench. A relatively large stress drop of 7 MPa is obtained from the inversion result. It is worth noting that this high stress drop was derived from at least twice repeating ruptures in the main asperity. How can a repeating slip occur from a dynamic point of view? Did this rupture behavior cause the shallow dynamic overshoot? Further investigations of dynamic rupture process of the Tohoku-Oki earthquake based on high resolution kinematic source model will be crucial to answer these questions.

■ Forward 3D wave propagation simulation

A 3D spectral-element method forward simulation is performed by using the source model determined from this study (source model version 3). For more information, see real-time computational seismology special event report  (http://www.earth.sinica.edu.tw/~sjlee/rcs/index.htm).

■ Acknowledgements

We thank Dr. Kuo-Fong Ma(NCU), Dr. Takashi Furumura(ERI), Dr. Takuto Maeda(ERI) and Dr. Laetitia Mozziconacci(IES) for fruitful suggestions and discussions. The terrestrial coseismic deformation data are taken from GPS displacement data (version 0.3) provided by the ARIA team at JPL and Caltech. All Original GEONET RINEX data provided to Caltech by the Geospatial Information Authority (GSI) of Japan. The seafloor geodetic observations were provided and processed by Japan Coast Guard.

Citation: Lee, S.-J., B.-S. Huang, M. Ando, H.-C. Chiu, and J.-H. Wang (2011), Evidence of large scale repeating slip during the 2011 Tohoku-Oki earthquake, Geophys. Res. Lett., 38, L19306, doi:10.1029/2011GL049580. (Full text)

Correspondence: Shiann-Jong Lee (sjlee@earth.sinica.edu.tw)