Rupture process of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) as imaged with back-projection of teleseismic P-waves
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We use the back-projection method, with data recorded on the dense USArray network, to estimate the rupture propagation for the Mw 9.0 earthquake that occurred offshore of the Tohoku region, Japan. The results show a variable rupture propagation ranging from about 1.0 to 3.0 km/s for the high-frequency radiation. The rupture propagates over about 450 km in approximately 150 s. Based on the rupture speed and direction, the high-frequency source process can be divided into two parts. The first part has a relatively slow rupture speed of 1.0 to 1.5 km/s and propagates northwestward. In the second part, the rupture progresses southwestward starting with a slow speed of about 1.5 km/s and accelerating to about 3.0 km/s. We see three large pulses at 30 s, 80 s and 130 s. The first two, including the largest second pulse, were located 50 to 70 km northwest of the epicenter. The third occurred about 250 km southwest of the epicenter. The variability of rupture velocity may be associated with significant changes of physical properties along the fault plane. Areas of low/high rupture speed are associated with large/small energy releases on the fault plane. These variations may reflect the strength properties along the fault. Also, locations of the high-frequency radiation derived from the back-projection analysis are significantly different from the areas of very large slip for this earthquake.
Key wordsBack-projection the 2011 Tohoku Earthquake high frequency energy rupture velocity
The great 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) on March 11, 2011 at 05:46 (UTC) occurred offshore of the east coast of Honshu, Japan and caused a devastating tsunami with considerable loss of life and property. The sequence started with a M 7.2 earthquake east of Miyagi Prefecture on March 9 at 02:45 (UTC). The mainshock followed two days later with the hypocenter about 40 km southwest of the previous event. There have been many aftershocks, including three events greater than M 7. In order to investigate the rupture propagation and energy release of this earthquake, we used a back-projection technique with the USArray data. As shown by Ishii et al. (2005), the back-projection can image the rupture extent, duration and speed of large earthquakes. Past studies have estimated the rupture velocities of the Denali and Kokoxili earthquakes (Walker and Shearer, 2009) and 2008 Wenchuan, China earthquake (Xu et al., 2009; Mori and Smyth, 2009; Zhang and Ge, 2010). For large earthquakes, rupture velocity is an important parameter that reflects the fault properties and rupture complexity. For example, tsunami earthquakes usually have slow rupture velocities due to rupture complexity and/or low rigidity material, whereas some onshore crustal earthquakes can have very fast supershear speeds that might be attributed to the accumulated stress and smoothness of the fault planes.
2. Data and Method
The top portion of Fig. 2 shows the time series of the (squared) stack amplitudes for each time window, as calculated by Eq. (1). The unfiltered data (gray line) shows one large pulse which is located northwest of the epicenter. The 1 Hz high-pass data (black line) and similar 0.5 Hz high-pass data (yellow line), show three large subevents, with the two larger ones occurring in the general region of the epicenter. The longer period results may be representative of the continuous slip which seems to be dominated by one area of large slip. However, even these unfiltered data probably do not contain sufficient long-period information to correctly map out the total slip distribution. The filter ranges were chosen to look at a range of frequencies with constraints of the data and method. The higher frequencies around 3 Hz are much less coherent so the rupture cannot be seen clearly, and the lower frequencies around 0.1 Hz lose spatial resolution for imaging the details of the rupture.
Our results show a significantly variable speed for the rupture propagation of this huge earthquake. The rupture velocity for the first part of the earthquake source is relatively slow at 1.0 to 1.5 km/s and for the second part the values accelerate from about 1.5 km/s to about 3.0 km/s. The rupture speed can be linked with physical properties on the fault surface, and may show the relative amount of non-radiated energy, such as fracture energy (Fossum and Freund, 1975; Kanamori and Brodsky, 2004). Higher fracture energy implies processes with more dissipated (non-radiated) energy and may be an indication of lower dynamic friction (Kanamori and Brodsky, 2004), For this earthquake, the areas of slow rupture speed seem to be associated with the areas of large slip near the hypocenter. In contrast, the areas of higher rupture velocity and less slip can correspond to lower fracture energy, indicating more brittle failure on a high friction fault. Thus, the spatial distribution of rupture speed determined in this study (Fig. 3) may be interpreted in terms of differences in dynamic friction on the fault. Although there are many complicating factors that control the fracture energy and rupture velocity (Bizzari, 2010) that need to be clarified to support this interpretation.
This work is partly supported by NSFC grant 41004020. The USArray Transportable Array data were obtained from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center. All the figures were plotted with the Generic Mapping Tools (GMT). We appreciate the constructive comments from Ryou Honda and an anonymous reviewer.
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