A scenario for the generation process of the 2011 Tohoku earthquake based on dynamic rupture simulation: Role of stress concentration and thermal fluid pressurization
© 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: 27 December 2011
Accepted: 24 May 2012
Published: 28 January 2013
We perform dynamic rupture simulations to improve the understanding of the generation process of the 2011 Tohoku earthquake. We assume a dynamic weakening mechanism (dynamic thermal pressurization of pore fluid, hereinafter called TP) on the fault plane to represent nonlinear weakening friction, and take into account the shear stress changes before the Tohoku earthquake, due to the four M 7-class earthquakes that occurred during 2003–2011. To constrain the dynamic rupture simulation, the moment release rate obtained by seismic slip inversions is referred. The simulation result implies the following about the 2011 Tohoku earthquake: (1) The rupture around the hypocenter was enhanced by the stress accumulation due to the preceding M 7-class earthquakes. (2) The enhanced rupture triggered the TP mechanism in the near-trench area causing large slip, which promoted propagation of the rupture over a wide region including the source areas of the M 7-class earthquakes and surrounding conditionally-stable areas. (3) Without sufficient stress accumulation, the moment release of the Tohoku earthquake ended as an M 8-class earthquake. (4) TP in the near-trench area should be effective but moderate. The occurrence time of the next megaquake would be strongly affected by the nonlinear effects of TP and the stress conditions. Thus, our model may contradict the concept of the (quasi-)cyclic occurrence of M 9 earthquakes.
A giant earthquake, the 2011 off the Pacific Coast of To-hoku earthquake (hereafter called the Tohoku earthquake), occurred at the subduction plate boundary east of northern Honshu on March 11, 2011. Its magnitude, Mw 9.0, was far larger than the previous M 7-class earthquakes in the region (Yamanaka and Kikuchi, 2004; Uchida and Matsuzawa, 2011). Such a giant event had not previously been reported, with the exception of geological studies of tsunami deposits (e.g., Minoura et al., 2001).
Several GPS inversion studies using an on-land GPS network (e.g. Ito et al., 2000; Nishimura et al., 2004; Suwa et al., 2006; Hashimoto et al., 2009) proposed that the source region of the 2011 Tohoku earthquake had been locked (a clearly slower slip rate on the plate interface than the plate convergence rate) in aggregate, at least prior to 2003, when an M 7-class earthquake occurred in this region. Studies of small repeating earthquakes (Uchida et al., 2006; Uchida and Matsuzawa, 2011) also implied a locked fault. Moreover, Yamanaka and Kikuchi (2004) mentioned that the M 7-class earthquakes in this region did not fully release the accumulated slip due to the plate convergence. Mitsui and Iio (2011) suggested that locked conditions had continued until the occurrence of the Tohoku earthquake, except in the areas of the M 7-class earthquakes and their after-slip. However, such scenarios are only applicable in a region deeper than the hypocenter of the 2011 Tohoku earthquake and conditions in the shallower part have not been well resolved because of the power lack of on-land GPS stations to constrain near-trench slip (Loveless and Meade, 2011) and the absence of small repeating earthquakes near the trench.
Based on such observations, several studies have already discussed the generation mechanism of the Tohoku earthquake in terms of earthquake cyclicity (Hori and Miyazaki, 2011; Kato and Yoshida, 2011; Mitsui and Iio, 2011; Ohtani et al., 2011; Shibazaki et al., 2011; Mitsui et al., 2012). However, these studies did not properly consider the dynamic rupture process of the 2011 Tohoku earthquake. In addition, note that the (quasi-)cyclic occurrence of megaquakes is not guaranteed in this region. Clear evidence of previous M 9 earthquakes in this region has not been found. A geological study of tsunami deposits (Minoura et al., 2001) proposed that the 869 Jo-gan tsunami was caused by an M 8.3 earthquake, probably smaller than the M 9 Tohoku earthquake. Many tensile cracks at the ocean bottom around the trench were induced by the Tohoku earthquake (Tsuji et al., 2011), but no evidence has been found of similar cracks induced by a previous megaquake.
The rupture process of the Tohoku earthquake has also been investigated by many researchers. We now have a qualitative common image of the rupture process, although the details have not yet been clarified. The rupture started from the hypocenter at approximately 38°N, 143°E. In the first several seconds, the rupture behaved like an ordinary small earthquake (Chu et al., 2011), but it eventually propagated through a wide region of the subduction plate boundary east of northern Honshu. In many studies, the area of largest slip, which is as much as 50 m or more, was located to the east of the hypocenter, possibly near the Japan Trench (e.g., Fujii et al., 2011; Ide et al., 2011; Ito et al., 2011; Yagi and Fukahata, 2011). The slip around the hypocenter amounts to 20 m or more (e.g., Yagi and Fukahata, 2011; Yoshida et al., 2011), far greater than that of recent M 7-class earthquakes in this region.
In this paper, we consider why the Tohoku earthquake became a giant event and, in parallel, why previous earthquakes did not grow to the same extent. In terms of mechanics, we note that significant changes in the stress field due to the four M 7-class earthquakes and their afterslip before the 2011 Tohoku earthquake (Fukushima-oki on October 31, 2003, Miyagi-oki on August 16, 2005, Fukushima-oki on July 19, 2008, and Miyagi-oki on March 9, 2011) caused stress accumulation around the hypocenter of the M 9 To-hoku earthquake (Iio et al., 2011; Iio and Matsuzawa, 2012). This stress concentration is likely to have affected rupture growth in the Tohoku earthquake, especially for the eastward rupture. In addition to this, many seismic inversion results (e.g., Ide et al., 2011; Yagi and Fukahata, 2011) suggest that seismic slip drastically increased after the arrival of the rupture in the near-trench area. The large slip in the near-trench area seems to have resulted in large horizontal extensional deformation within the overlying plate (Kato et al., 2011). We adopt a hypothesis that dynamically thermal pressurization of pore fluid (hereinafter called TP) in the fault zone played a key role in the release of stress (Mitsui et al., 2012).
To evaluate the effects of stress accumulation and TP quantitatively, we must use a three-dimensional mechanical model for elastodynamic rupture propagation on a fault where friction changes with slip and pore pressure evolution. This experiment leads to a better understanding of the processes of the M 9 earthquake occurrence. Using the constructed simulation model, we are able to examine a possible scenario for the 2011 Tohoku earthquake and return to the initial questions of why the M 9 earthquake occurred on March 11, 2011, and why the previous earthquakes did not grow to the same extent.
2. Model and Method
2.1 Fault model
The shear stress on the plate interface were influenced on the order of MPa by the M 7-class earthquakes. In particular, we should note the increase in shear stress around the hypocenter of the Tohoku earthquake in four directions (Fig. 2). Such a situation had not been experienced in the previous 100 years, although stress accumulation in two directions occurred in the early 1980s in relation to two M 7-class earthquakes (Miyagi-oki on June 12, 1978, and Miyagi-oki on January 19, 1981). The four-direction stress increase is likely to be associated with the M 9 Tohoku earthquake.
The size of this simplified fault is almost half of the actually ruptured fault. In particular, the southern edge of the simplified fault corresponds to the introduction of an artificial boundary. However, the simplified fault seems sufficient to model the important features of the earthquake occurrence because a large portion of the coseismic moment release in the Tohoku earthquake was concentrated on the northern half of the ruptured fault (e.g., Yoshida et al., 2011). We first use this simplified fault model to construct a basic scenario, and then set a larger fault to perform more a realistic calculation (see Appendix).
2.2 Frictional system and parameters
For dynamic rupture simulations, we use a boundary integral equation method in the frequency domain (Geubelle and Rice, 1995; Day et al., 2005). The bulk elastic properties are as follows: the Poisson’s ratio is 0.25, the S-wave velocity cu is 3.3 km/s, and the rigidity G is 30.5 GPa. Fault motion at each cell is controlled by the difference between loading stress τ lo and frictional strength τfs, obeying the equation of motion for a three-dimensional homogeneous elastic body. τ lo at each point changes due to slip at other points. In the conventional treatment, slipping points are distinguished from sticking points. When τ lo < τfs, the slip velocity v is fixed to be zero (sticking) and the shear stress τsh is fixed to be equal to τ lo. Once τ lo tends to exceed τfs, the slip velocity v takes a positive value and τ sh is equal to τfs. As an initial condition on each fault cell, τ sh = τi is set a priori. The distribution of τi used in the simulation is explained in the next subsection. To reduce computational costs, we fix the slip vector perpendicularly to the trench, because seismic inversion studies show that the To-hoku earthquake is basically a pure dip-slip event (e.g., Yagi and Fukahata, 2011).
We assume that the Coulomb friction τfs = µ(σ - p) on the fault, where µ is the frictional coefficient, σ is the normal stress, and p is the pore fluid pressure, evolves following two processes. One is the slip-dependent evolution of the frictional coefficient µ = µ y + F(D), where µ y represents the normalized static strength and D is the slip amount. F(D) represents linear slip-weakening, simply given by (µ f - µ y )D/D c when D < D c , and by µ f - µ y when D ≥ D c (Andrews, 1976a, b). D c is 3 m in this study. Note that the linear slip-weakening model is an idealization of many elementary physical processes (Andrews, 1976a), and may be too simple, as discussed in the literature (see the review by Bizzarri (2011a)). The other is the thermal pressurization of pore fluid (TP) via frictional heating, heat flow, and fluid flow, i.e., the law of energy conservation, the Fourier law, the law of mass conservation, and the Darcy law (Sibson, 1973; Lachenbruch, 1980; Mase and Smith, 1987; Bizzarri and Cocco, 2006). The latter process does not have a characteristic slip distance D c and a priori stress drop value, leading to continuous decrease of friction τfs with slip D (Lachenbruch, 1980; Abercrombie and Rice, 2005).
The basal effective normal stress σ − p0 is assumed to be 40 MPa over the whole fault. This assumption is based on a weak fault hypothesis at the subduction plate interface (e.g., Seno, 2009). The actual plate interface would have a more complex distribution of basal effective normal stress. In fact, a recent study (Zhao et al., 2011) revealed that the ruptured region in the Tohoku earthquake is inho-mogeneous by seismic wave tomography (materials in the area subjected to large coseismic slip in the Tohoku earthquake are relatively stiff compared with the northern and southern area).
We set w = 0.04 m and χ = 1.1× 10−6 m2/s as typical values for rocks (e.g., Noda and Shimamoto, 2005; Tanaka et al., 2007). We further assume A = 0.036, which is on the small side as a typical value, leading to weaker effects of TP. The reason we assume weak TP will be described in a later section.
2.3 Calculation set-ups
Figure 3 shows the fault model used in the numerical simulation. We first describe conditions of the fault parameters in constructing the rupture simulation model.
In area (a) for rupture nucleation, the initial shear stress τi must slightly exceed the static strength µy (σ-p0) to start dynamic rupture. The shape, size, and stress drop value for area (a) are set to enable outward rupture propagation (the energy release rate at the tip of area (a) must reach local fracture energy).
We assume identical frictional properties, but a different initial stress, for areas (b) and (c) following the estimation of the stress accumulation around the hypocenter (Fig. 2). We set a shear stress increase of 1.4 MPa in area (b), and no increase in area (c). Areas (b) and (c) roughly correspond to the ruptured area of the M 7-class earthquake on October 31, 2003 (ERI, 2003). Although we do not fully understand what caused the differences between the M 7 earthquake in 2003 and the M 9 earthquake in 2011, a nonlinear mechanism of frictional weakening might play an important role in rupture propagation in these areas. Thus, we assume an effective TP in areas (b) and (c).
In determining the frictional properties in areas (d)–(f), we cannot refer to previous M 7-class earthquakes. We are also unable to consult interseismic slip behavior because of the power lack of on-land GPS stations to constrain slip behavior at the plate interface (Loveless and Meade, 2011). The constraint on these areas is the coseismic moment release for the Tohoku earthquake, estimated from slip inversions. Among areas (d)–(f), area (d) is a stress-accumulated region like area (b). We assume that TP operates effectively in area (f) to cause an extremely large slip after the rupture reaches there.
For area (g), we set the initial stress τi to be the same as the residual strength µf(σ - p0). This assumption is not strange since the source areas of the recent M 7-class earthquakes must have had low stress and static strength. The assumption of the same level of τi /(σ - p0) and µf also corresponds to a conditional stability in terms of a rate-and state-dependent friction (Ruina, 1983; Boatwright and Cocco, 1996), which might be adequate for the non-source areas of the M 7-class earthquakes (Mitsui and Iio, 2011).
The boundary conditions at the edges of the model fault are fixed ends, except at the east edge. For the east edge, to represent the effect of the free surface at the Japan Trench, we assume a free end condition by using a mirror method about the displacement vector to eliminate strain.
The parameter distributions: the normalized initial shear stress τi /(σ - p0), the normalized static strength μy, the normalized residual strength μf, and the hydraulic diffusivity ϖ. Area (s) is set only for Model-3.
7.0 × 10−3
7.0 × 10−3
4.7 × 10−4
To solve the nonlinear equation system, we first use a second-order Runge-Kutta algorithm to obtain the evolution of v and τ lo and to obtain a value for p from Eq. (1). Then, in the next time step, using the obtained values, all of the variables are calculated again. The calculation process continues until seismic slip is halted in the whole model region.
Figure 7(iii) presents the evolution of the frictional strength τfs, the loading stress τ lo,, and the shear stress τsh, at P2; the meanings of the parameters were described in Section 2.2. At the moment when τ lo = τsh exceeds τfs, the fault starts to slip. During the fault slip, τsh is equal to τfs. For instance, to discuss stress histories on the fault, we should refer to the values of τsh. The simulation result in Fig. 7(iii) shows a local dynamic undershoot (τsh increases after the finish of the fault slip). This tendency corresponds to an earlier result presented by Mitsui and Hirahara (2009), although their fault model with one-degree-of-freedom was rather simpler than that in this study. (Note that the definitions of the parameters are confusing. “τ” and “τf ” in Mitsui and Hirahara (2009) respectively correspond to τ lo and τsh in this study. Since the fault system with the rate-and state-dependent friction of Mitsui and Hirahara (2009) does not stop slipping, τfs does not appear explicitly in their system.)
In order to investigate the effects of the stress accumulation caused by the M 7-class earthquakes, we then set Model-2 that only considers the northern and western parts of the stress concentration in Model-1 (Fig. 4).
4.1 Role of dynamic weakening
By comparing Model-1 with Model-2, we illuminated the notable role of the initial stress conditions on the growth of earthquakes (M 8-class or M 9-class), in which triggering the extremely large slip owing to TP holds the key. Parametric experiments with a simpler setting of the model fault (Fukahata et al., 2012) also show that the stress field before the earthquake controls the rupture propagation in the area between the hypocenter and the trench. It should be noted that a four-direction stress accumulation occurred before the Tohoku earthquake, whereas only a two-direction, or smaller, stress increase had occurred in the case of the previous M 7-class earthquakes, at least in the last one hundred years. This study suggests that a rare condition of the stress field, as well as TP, near the trench may be necessary for the occurrence of M 9 earthquakes in this region. The hydraulic parameters for TP may also change in time. Acute sensitivity to initial stress conditions, prescribed by local seismic activities and changeable TP parameters, raise a question about the concept of (quasi-)cyclic occurrences of M 9 earthquakes, as discussed in Yagi and Fukahata (2011).
The dynamic-weakening hypothesis is further reinforced by observations of many normal-fault-type aftershocks (e.g., Asano et al., 2011), which imply that the absolute stress in this region was almost completely released by the Tohoku earthquake (e.g., Hasegawa et al., 2011). It is notable that the fault-slip models utilizing the rate- and state-dependent friction law (e.g., Hori and Miyazaki, 2011; Kato and Yoshida, 2011) do not explain such phenomena, since they do not include the effects of absolute stress level. TP is probably a dominant physical mechanism of dynamic weakening causing the release of absolute stress in such a wide region, since frictional melting, which is another popular dynamic-weakening mechanism, may act as an opposite dynamic-strengthening mechanism due to viscous braking in shallow zones (e.g., Otsuki et al., 2003; Ujiie et al., 2009).
If the extremely large slip of the Tohoku earthquake in the shallow part is associated with dynamic fault weakening, as demonstrated in this study, a strongly locked area during interseismic periods is not necessarily needed for the occurrence of megaquakes. In fact, an observation study (Tohoku University, 2010) suggested the occurrence of a slow earthquake in 2008, whose source region was located within the large slip area of the Tohoku earthquake. This finding may indicate that the fault in the whole shallow part had not been strongly locked in an interseismic phase.
4.2 Effective but moderate TP
As indicated in Section 2.2, we set A = 0.036 as a parameter for TP, which is on the small side within the typical range, because an effective, but moderate, TP is needed to assimilate the moment release rate in the seismic inversions. If the effect of TP is stronger, the moment release in the near-trench region grows too fast. A small A is not the only way to restrain the TP effect in numerical calculations. Primarily, the specific amount of the moment release depends on the assumed area for TP; there is a trade-off between the degree of TP and the dimensions of the TP region. The degree of TP also depends on the absolute stress level: for instance, the basal effective normal stress σ - p0. Next, as several numerical studies (e.g., Suzuki and Yamashita, 2007; Mitsui and Cocco, 2010; Bizzarri, 2012) have mentioned, pore pressure decrease owing to pore dilatation could counteract TP efficiently. In this context, one study (Mitsui and Cocco, 2010) proposed that more fric-tionally unstable systems lead to a more effective counteraction of porosity evolution to the TP effect. This result corresponds to our assumption of the existence of an interseismi-cally stable (not strongly locked) region near the trench, part of which is the TP region. However, since the restraining effect strongly depends on the constitutive law of porosity evolution, this issue is still under debate. More experimental support, such as that provided by Tanikawa et al. (2010), is necessary to clarify this problem. Lastly, the effect of off-fault plastic yielding can restrain the dynamic-weakening effect (e.g., Andrews, 2005; Dunham et al., 2011). By contrast, the effect of material contrast across the fault (e.g., Ben-Zion, 2001; Ma and Beroza, 2008) may not counteract the dynamic-weakening effect in this case, since a hanging wall in a subduction fault is considered to be more compliant than a footwall.
Of course, the results in this study also depend on the evolution of the frictional coefficient. The linear slip-weakening law assumed in this study might be responsible for the moderate TP hypothesis. For example, we found that the temperature on the model fault could exceed a typical melting temperature (1200°C) transiently, although the amount of the temperature increase strongly depends on the assumed absolute stress level. The melting of rock leads to viscous rheology, not the Coulomb friction (e.g., Nielsen et al., 2008; Bizzarri, 2011b). Such complicated rheology should be implemented to represent actual fault behavior more precisely.
We constructed a fault model for the dynamic rupture simulation of the 2011 Tohoku earthquake. The model included the dynamic weakening mechanism of TP. It also reflected the estimation of the shear stress changes before the Tohoku earthquake, due to the four M 7-class earthquakes which occurred during 2003–2011.
Based on the model, we succeeded in simulating the rupture process of the Tohoku earthquake, to some extent, by assimilating the moment-rate function in the seismic slip inversions, and proposed a possible scenario for its generation process, as follows. The rupture around the hypocenter was enhanced by the stress accumulation due to the four M 7-class earthquakes. The enhanced rupture triggered the TP mechanism in the near-trench area resulting in an extremely large slip. The near-trench effective TP, which should be effective but moderate, did not operate during the initial 40 s. The rupture was further promoted by the near-trench TP to propagate across a wide region, including the source areas of the M 7-class earthquakes and a surrounding conditionally-stable area. If the stress concentration around the hypocenter was insufficient, the Tohoku earthquake could end as an M 8-class earthquake. The TP effect around the hypocenter area might play a major role in this process. The nonlinear effects of TP and the stress conditions on the earthquake magnitude would lead to a greatly fluctuating occurrence time of following large earthquakes. Thus, our model implies that the concept of (quasi-)cyclic earthquake occurrences may not be applicable to M 9 earthquakes in this region.
We thank Yasuhiro Yoshida and Hiroshi Ueno for providing us with the results of their seismic inversions. We also thank Masao Nakatani, Takeshi Tsuji, Hiroyuki Goto, Kazuro Hirahara and Naoyuki Kato for comments and discussions. We used the Generic Mapping Tools (Wessel and Smith, 1995) to draw the figures.
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