Spatio-temporal distribution of seismic moment release near the source area of the 2011 Tohoku-Oki earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 29 December 2011
Accepted: 9 April 2012
Published: 28 January 2013
To understand the generation mechanism of the 2011 Tohoku-Oki earthquake, we analyzed the spatial and temporal distribution of the cumulative seismic moment (ΣM0) for all earthquakes along the Japan Trench listed in the global centroid moment tensor catalog between January 1976 and November 2011. Three areas with distinct characteristics of ΣM0 are identified: (1) in the 2011 Tohoku-Oki source area, the ΣM0 released in the down-dip portion (≥30 km) was greater than that in the up-dip portion (<30 km) before the mainshock; (2) the Σ M0 of the up-dip portion in the region where slow slip activity prevails is greater than that of the down-dip portion throughout the study period; (3) in the surroundings of the source area, where interplate coupling is relatively low and the largest foreshock occurred, values of ΣM0 for the down-dip and up-dip portions are both intermediate. Our results show that a megathrust rupture could be generated by large accumulations of strain energy at the subduction interface, resulting from the differential strain energy released in the down-dip and up-dip portions during the interseismic interval. We propose that the variation pattern of ΣM0 may offer spatial constraints for seismic hazard assessment in the future.
The Japan Trench, where the Pacific plate subducts beneath the Okhotsk plate at an average rate of approximately 8 to 8.5 cm/year (DeMets et al., 2010), has experienced numerous interplate earthquakes with a moment magnitude (MW) of 7 to 8 (Sagiya, 2004; Ishibe and Shimazaki, 2009; Ozawa et al., 2011). The 2011 Tohoku-Oki earthquake, a MW 9 megathrust earthquake, occurred in this area. The earthquake generated an enormous tsunami that killed more than ten thousand people (Hirose et al., 2011; Maeda et al., 2011).
During the past several decades, various geophysical methods and datasets have been used to understand the mechanisms behind these devastating earthquakes, including seismic and geodetic ground motion datasets (Mazzotti et al., 2000; Kawasaki et al., 2001; Koyama et al., 2004; Hashimoto et al., 2009; Igarashi, 2010; Katsumata, 2011; Uchida and Matsuzawa, 2011), historical tsunami deposits (Minoura et al., 2001; Satake et al., 2008), aftershock distributions (Katsumata et al., 1995), and seafloor bathymetry (Umino et al., 1990; Tanioka et al., 1997; Tsuji et al., 2011). The most traditional method is to evaluate the coupling on the subduction thrust faults by calculating the ratio of the observed to the expected moment release rate (Ruff and Kanamori, 1980; Peterson and Seno, 1984; Pacheco et al., 1993; Ohnaka, 2004; Oishi and Sato, 2007). However, some studies have obtained weak interplate coupling (20–40%) (Peterson and Seno, 1984; Pacheco et al., 1993; Matsumoto et al., 2008), whereas others have shown strong coupling along the Japan Trench (Nishimura et al., 2004; Suwa et al., 2006; Hashimoto et al., 2009). This contradiction is probably the result of two major limitations in the estimation of the moment release rate. One is that the available earthquake history is often too short compared with the long and variable recurrence time of large earthquakes. Another is that summed moment release rates are sensitive to the assumed width of seismically-active zones (Sandiford, 2008). Several improvements have been proposed to remedy the above limitations (McCaffrey, 1997; Frohlich, 2007; Frohlich and Wetzel, 2007); however, the computation procedure still seems a little complicated.
Here, we would like to use a simple approach without any assumptions by examining the cumulative scalar seismic moment release (hereafter, ΣM0) in and around the 2011 Tohoku-Oki earthquake rupture area. Specifically, we quantify the energy release of the earthquakes by summing their seismic moment release (M0) within a given time or space window and we investigate its spatio-temporal variation along the Japan subduction system. The magnitude of the ΣM0 can be viewed as a first-order approximation to the regional changes of shear strain in response to earthquakes (Sykes and Jaumé, 1990). In our calculation, we used earthquakes listed in the GCMT catalog, including the interplate and intraplate earthquakes. We recognize that the principal contribution to ΣM0 comes from the large earthquakes in the subduction seismogenic zone (Pacheco and Sykes, 1992). Hence, we consider our result to be representative for the study of the seismogenic behavior of the subduction area. Our purpose has been simply to obtain the evolution pattern of ΣM0 prior to the occurrence of large earthquakes, for providing an additional tool for seismic hazard assessment, even if the duration of our current seismic record is too short compared with a complete cycle of large earthquakes.
2. Data Processing
2.1 Seismic data acquisition
2.2 Up-dip portion and down-dip portion
As shown in Fig. 1, most rupture zones of the major historic earthquakes are distributed along the plate boundary between the forearc and the oceanic crust, at depths shallower than 30 km. The behavior of earthquakes seems to vary with depth. In addition, seismic tomographic images show that the depth of the forearc Moho along the Japan Trench is approximately 30 km (Zhao et al., 1994, 2011; Satoh et al., 2001; Miura et al., 2005; Shinohara et al., 2005; Huang et al., 2011), which suggests the possible location of the brittle-ductile transition zone. We therefore separate the earthquakes into up-dip (<30 km, near trench) and down-dip (≥30 km) parts in the following analysis. Furthermore, the lateral variation of the depth of the forearc Moho (e.g., Nakajama and Hasegawa, 2006; Uchida et al., 2010) and the hypocenter location uncertainty might affect the classification of earthquakes and change our result. To take these factors into consideration, we will perform our calculation by taking different boundary depths for the determination of the up-dip and down-dip events. The results of using 20- and 40-km boundary depths will be presented in Section 3, and we will demonstrate that there are no significant differences between the results with 20, 30 and 40 km as boundary depth.
2.3 Along-strike segmentation of the 2011 Tohoku earthquake
The focal mechanism of aftershocks of the 2011 Tohoku-Oki earthquake varies from the north to the south (Fig. 2(b)), which may relate to the tectonic heterogeneity and the source effect of the mainshock. We therefore divide the study area into five segments along the Japan Trench based on the faulting types of the aftershocks (Fig. 2).
The most striking feature of a comparison of the distribution of post- and pre-seismicity is that only one earthquake occurred in Segment A after the mainshock (Fig. 2(b)). Due to its great distance from the rupture area, this event may not belong to the 2011 Tohoku-Oki earthquake sequence. However, it is worthwhile noting that the 2003 MW8.3 Tokachi-Oki earthquake has ruptured this area (earthquake 6 in Fig. 2(a)). The significant contribution of this event to the calculation of ΣM0 will be discussed in the next section.
Most aftershocks in Segment B have a similar focal mechanism, showing NE-SW thrusting. Segments C, D and E show a complex pattern of focal mechanisms, including both extensional and compressive earthquakes (Fig. 2). The focal mechanisms of the down-dip earthquakes indicate a NE-SW thrust-faulting system, whereas the earthquakes in the up-dip portion indicate an extensional-faulting component, with a T-axis oriented in a roughly E-W direction (Fig. 2(b)) (Kato et al., 2011). The absence of near-trench aftershocks is observed in Segment C. The source area of the 2011 Tohoku-Oki earthquake is located in Segment D, and numerous normal-faulting events have occurred in the outer-rise area. Few normal-faulting outer-rise events are observed in Segment E, and the near-trench area is characterized as the thrust-faulting events.
3. Spatial and Temporal Variation of ΣM0
3.1 General features of ΣM0
3.2 Spatial variation of ΣM0
To examine the effect of the boundary depth and show the robustness of our results, we repeat the calculation of ΣM0 by changing the boundary depth to 20 and 40 km, respectively (Figs. 4(c)–(d) and 4(e)–(f)). As shown in Fig. 4, the value of ΣM0 in each grid changed with a different boundary depth. However, the spatial patterns of ΣM0 still seem similar: a relatively-large seismic moment was released in the down-dip portion around the rupture zone of the 2011 Tohoku-Oki earthquake before the mainshock.
3.3 Temporal variation of ΣM0
For the down-dip portion of the subduction zone (Fig. 5(b)), a one order difference in ΣM0 exists between the two segments which have the largest ΣM0 value— Segments A (~ 5.5 * 1019) and D (~ 4.5 * 1020 N-m)— throughout the period from 1976 to 2003. Thus, the ΣM0 of Segment A becomes close to the value of Segment D due to the 2003 Hokkaido earthquake. Consequently, Segments A and D have the largest ΣM0 among all the segments throughout the period of 1976 to 2011. Meanwhile, their ΣM0 in the up-dip portion were not always dominant (Fig. 5(a)). Similarly, although the ΣM0 of Segment B was the highest in the period 1990 to 2003 in the up-dip part, its ΣM0 in the up-dip portion is relatively small.
To summarize, our observations show three distinct patterns of the ΣM0 distribution with time and space: (1) in Segments A and D, where the 2003 Tokachi-Oki and the 2011 Tokoku-Oki earthquakes occurred, the strain energy in the down-dip area has been released more than in the up-dip area prior to the mainshock; (2) in Segment B, where the 1989 (MW 7.4), 1992 (MW 6.9) and 1994 (MW 7.7) earthquakes occurred, strain energy in the up-dip area has been released more than that in the down-dip area throughout our study time; (3) in Segments C and E, where the largest fore-shocks of the 2011 Tohoku-Oki earthquake (MW7.4) and the 1982 earthquake (MW 7.0) occurred, the values of ΣM0 in both the down-dip and up-dip areas are intermediate.
4. Conceptual Mode of Megathrust Event
Many source inversion and geodetic modeling results have shown that the primary seismic moment contributions of the 2011 Tohoku mainshock were from the up-dip portion, possibly extending to the trench (Ammon et al., 2011; Iinuma et al., 2011; Lay et al., 2011b; Miyazaki et al., 2011) and that the source area was relatively strongly coupled (Mazzotti et al., 2000; Hashimoto et al., 2009; Loveless and Meade, 2010; Uchida and Matsuzawa, 2011). We thus propose a first scenario to explain the occurrence of the 2011 Tohoku-Oki, and the 2003 Tokachi-Oki, earthquakes. If the up-dip region was completely locked, the energy released in the down-dip area may induce a large component of vertical upward slip and the strain energy could build up in the intersection area of the up- and down-dip portions during the interseismic interval (Category 1, Fig. 7(a)). Once the friction on the subduction interface exceeds a critical value, strike-slip occurs resulting in the occurrence of a large earthquake (Category 1, Fig. 7(b)).
A second scenario is suggested based on our observations in Segment B (Category 1, Fig. 7(c)). By analyzing permanent GPS data, it has been suggested that the area located at the down-dip limit of the seismogenic zone off northern Tohoku and south of Hokkaido, extending from 39°N to 42°N (Segment B), has been decoupled (Mazzotti et al., 2000). Segment B coincides with the lower limit of the locked portion, which was ruptured during the MW 7.6 San-riku earthquake in December 1994 (Nakayama and Takeo, 1997) (Fig. 1). Thus, a large amount of energy was released by the 1994 mainshock and slow slips followed subsequently in the rupture area (Kawasaki et al., 2001), as expressed by the substantial seismic moment release in the up-dip portion shown in category 2 (Fig. 5(a)). Meanwhile, a significant part of the elastic energy is dissipated silently through the postseismic afterslip generated in the down-dip area (Heki et al., 1997; Mazzotti et al., 2000; Uchida et al., 2006). An excess of seismic slip in the down-dip area has also been suggested by geodetic data analysis (Hashimoto et al., 2009). As a result, the strain energy has been released aseismically in the down-dip portion of Segment B, which is reflected in the low ΣM0 value in Figs. 4(a) and 5(b). In addition, the large seismic moment released by the previous large earthquakes and the subsequent slow slip in Segment B acted as a barrier to prevent the extension of both the 2011 Tohoku mainshock rupture and its aftershock activity (Kosuga and Watanabe, 2011). This mechanism may explain why there have been only a few postseismic seismic moment increases in the northern part of this area (Fig. 2).
A third scenario corresponding to the Segments C and E (Fig. 7(d)), which are the northern and southern extended content of the 2011 Tohoku-Oki rupture area, respectively, has a relatively low coupling (Uchida and Matsuzawa, 2011) and small to intermediate values in ΣM0 (Fig 5). Moreover, these two zones are characterized by moderate earthquakes (M ~ 7) that repeat approximately every ~20 yr (Loveless and Meade, 2010), which may indicate a plate interface behavior only capable of producing moderate earthquakes. Additionally, two M > 7.0 aftershocks of the 2011 Tohoku earthquake occurred approximately 20-30 min later at both the northern and southern edges of the rupture area, inducing an increase of up-dip seismic moment release in Segments C and E (Fig. 5(a)). These two segments may have been loaded by the mainshock (Miyazaki et al., 2011).
Here, it should be emphasized that although we cannot specifically estimate the timing and threshold of megathrust fault ruptures, our conceptual model provides plausible rupture scenarios in a spatial resolution for the occurrence of the 2011 Tohoku-Oki earthquake. More megathrust events in the worldwide subduction zones should be examined for a consistent pattern, as observed in this study. Furthermore, techniques which are more quantitative need to be applied in further studies to determine, more objectively and accurately, the difference between the spatial and temporal variations of up-and down-dip earthquakes before, and after, a large earthquake.
In summary, we integrate our results with the schematic cross-sections proposed by Lay et al. (2011a) regarding the possible stress variation and failure processes along the Japan Trench (Fig. 7). Three distinct seismic moment release processes (Categories 1 to 3) along five rupture segments were classified. Our results show that the great earthquakes, such as the 2011 MW 9.0 Tohoku earthquake and the 2003 MW 8.3 Hokkaido earthquake, could be produced in the first category. In this scenario, the energy was released in the down-dip portion but not in the up-dip portion, resulting in a large energy accumulation at their boundary prior to the mainshock. Our observations also show that the summation of GCMT catalog moments could be a potential tool for seismic hazard evaluation.
We thank the editor, Dr. Masanao Shinohara, and two anonymous reviewers for their helpful comments. Figures were prepared with the Generic Mapping Tool (GMT) software (Wessel and Smith, 1998). This study was under the financial support of the National Science Council of Taiwan (grant NSC-100-2119-M-008-028).
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