A shallow strong patch model for the 2011 great Tohoku oki earthquake: A numerical simulation

GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi:10.1029/2011gl048565, 2011 A shallow strong patch model for the 2011 great Tohoku oki earthquake: A numerical simulation Naoyuki Kato 1 and Shingo Yoshida 1 Received 16 June 2011; accepted 28 June 2011; published 9 August 2011. [1] A numerical simulation is conducted for understanding the mechanics of the 2011 great Tohoku oki earthquake (Mw = 9.0), which widely broke the plate interface at the Pacific plate subducting beneath northern Honshu (Tohoku), Japan. In the model, frictional stress on the plate interface is assumed to obey a rate and state dependent friction law. A strong patch (asperity) with higher ective normal stress and a large value of characteristic slip distance is assumed at a shallower part of the plate interface. This strong patch controls the occurrence of great earthquakes that broke the entire seismogenic plate interface with recurrence intervals of several hundred years. The present model explains large coseismic slip at a shallower part of the 2011 great earthquake and accumulation of slip deficit at deeper parts, where smaller M7 class earthquakes repeatedly occurred before the great earthquake. Citation: Kato, N., and S. Yoshida (2011), A shallow strong patch model for the 2011 great Tohoku oki earthquake: A numerical simulation, Geophys. Res. Lett., 38,, doi:10.1029/2011gl048565. 1. Introduction [2] The Mw = 9.0 great Tohoku oki earthquake on March 11, 2011, broke the entire seismogenic depths of the plate interface between the subducting Pacific plate and the overriding plate at northern Honshu, Japan. Finite fault models estimated from seismic waveforms and tsunami indicate that seismic slip extended about 500 km along the Japan trench and about 200 km along the dip direction and the largest seismic slip greater than 30 m occurred at a shallower part of the fault off Miyagi prefecture and seismic slip tended to decrease with increasing depth [Ammon et al., 2011; Fujii et al., 2011; Hayes, 2011]. Large slip near the hypocenter was supported also by seafloor geodetic observations [Sato et al., 2011]. [3] Large earthquakes of M7 class repeatedly occurred on the plate interface off Miyagi including M7.5 event in 1978 and M7.2 event in 2005 at a deeper part and M7.1 event in 1981 and M7.3 event on March 9, 2011 at a shallower part of the seismogenic zone [e.g., Earthquake Research Committee, 2000; Yamanaka and Kikuchi, 2004; Umino et al., 2006; Wu et al., 2008]. Yamanaka and Kikuchi [2004] pointed out that the sum of moment release by the M7 class interplate earthquakes was significantly smaller than that expected from relative plate motion. Observations of Global Positioning System (GPS) indicate that the plate interface off Miyagi was firmly locked and the slip deficit rate was close to the relative 1 Earthquake Research Institute, University of Tokyo, Tokyo, Japan. Copyright 2011 by the American Geophysical Union. 0094 8276/11/2011GL048565 plate rate [Nishimura et al., 2004; Suwa et al., 2006; Wallace et al., 2009]. Igarashi [2010] estimated the average aseismic slip rate from small repeating earthquakes at the plate interface to find that it was significantly smaller than the relative plate rate. These observations indicate that slip deficit was accumulated off Miyagi and it was not released by M7 class earthquakes or continuous stable sliding. The accumulated slip deficit was expected to be released by larger earthquakes or slow earthquakes [Kanamori et al., 2006]. Historical documents and tsunami deposits indicate that a great earthquake of M > 8 took place off Miyagi in 869 [Minoura et al., 2001; Satake et al., 2008; Sawaietal., 2007, 2008]. Sawai et al. [2007] found four sand sheets due to great tsunamis and estimated the recurrence intervals of great tsunamis as 600 to 1300 years. [4] Numerical simulations with rate and state dependent friction (RSF) laws well describe some important characteristics of earthquake cycles at plate interfaces [Tse and Rice, 1986]. By introducing heterogeneous ective normal stress or frictional properties in the models with RSF laws, Ben Zion and Rice [1995], Kato and Hirasawa [1999], Hillers et al. [2006], and Kato [2008] succeeded in simulating complex earthquake cycles, where regions of high ective normal stress, larger characteristic slip distance, or velocitystrengthening frictional property may be barriers to rupture propagation. [5] We consider a cross section off Miyagi to build a twodimensional (2D) mechanical model for cycles of great earthquakes because it may be assumed that the region of the largest seismic slip controls the great earthquake occurrence. In the present short article, detailed fit to observation data is not intended. We consider a mechanical model that can explain some features of observations as follows: (1) Recurrence of great earthquakes that break the entire seismogenic plate interface, (2) the largest seismic slip at the shallower part of the plate interface during each great earthquake, and (3) accumulation of slip deficit at the deeper seismogenic plate interface where many M7 class earthquakes took place. 2. The Model [6] We consider a thrust fault of a dip angle of 20 in a 2D uniform elastic half space with rigidity of 40 GPa and Poisson s ratio of 0.25 (Figure 1). Stable sliding with a sliding rate V pl of 85 mm/y [DeMets et al., 1990] is assumed on the fault at depths deeper than 103 km, and frictional stress obeys the composite rate and state dependent friction law [Kato and Tullis, 2003]. Static equilibrium between frictional stress and slip induced shear stress is assumed and quasi dynamic approximation during high speed slip [Rice, 1993] is applied. Refer to Kato and Tullis [2003] for details of simulation method. 1 of 5

Figure 1. A 2D model for earthquake recurrence on a plate interface at a subduction zone. Frictional stress obeys a rate and state dependent friction law at depths shallower than 103 km on the plate interface with a dip angle of 20, and stable sliding with the sliding rate V pl of 85 mm/y is assumed at depths deeper than 103 km. Velocity weakening frictional property is assigned for thick line parts of the plate interface. Numerals 1 to 12 indicate locations where simulated slip histories are shown in Figures 3 and 5. See text for regions A to E. [7] Several tens of cases of simulation are done by varying model parameters to obtain a model that can explain some important characteristics of observations. Below we examine an example case that can explain some important features of observations. Figure 2 shows depth dependence of friction parameters a and b, which represent the direct velocity ect and the evolution ect on friction, respectively, characteristic slip distance L, and the ective normal stress s n assumed in the present model. On the assumption of lithostatic pressure and hydrostatic pore pressure, s n is given by (r r w )gy for y 20 km, where y is depth, r = 2.8 10 3 kg/m 3, r w = 1.0 10 3 kg/m 3, and g = 9.8 m/s 2. Due to excess pore pressure [Rice, 1992] s n is assumed to be 88.2 MPa for y > 20 km. A large L value of 0.5 m for y 20 km is assumed for the shallow strong patch, while the L value for y > 20 km is 0.02 m. Seismic slip may be nucleated for a region of velocity weakening frictional property, which is characterized by a b < 0. A region of velocitystrengthening friction (a b > 0), where aseismic sliding is expected to occur, is assumed for 32.5 km < y 42.5 km because shallower and deeper seismic zones exist on the plate interface off Miyagi [Earthquake Research Committee, 2000]. The existence of velocity strengthening region is supported by occurrence of small repeating earthquakes [Igarashi, 2010] and afterslip of the 2005 Miyagi oki earthquake (M7.2) estimated from GPS observation [Miura et al., 2006]. We refer to region A as the shallow patch with higher s n and large L (y 20.0 km), region B as the intermediate depth velocity weakening friction region (20.0 km < y 32.5 km), region C as the intermediate depth velocitystrengthening friction region (32.5 km < y 42.5 km), region D as the deep velocity weakening friction region (42.5 km < y 52.5 km), and region E as the deep velocity strengthening friction region (y > 52.5 km) as shown in Figure 1. 3. Simulation Results [8] After some transient slip behavior due to an artificial initial condition, great earthquakes that break the entire seismogenic plate interface repeatedly occur at a recurrence interval of 716 years. If the 2011 great earthquake is the recurrence of the 869 great earthquake, the time interval is 1142 years. Since the largest seismic slip was estimated to be larger than 30 m, the recurrence interval should be longer than several hundred years. Figure 3 shows simulated slip histories for a great earthquake cycle at twelve points on the plate interface (Figure 1). The largest seismic slip occurs at the shallow pacth (region A) during each great earthquake and seismic slip decreases with increasing depth as shown in Figure 4, where seismic slip is defined as slip with the slip rate equal to or greater than 10 mm/s. Note that seismic slip occurs in region C with velocity strengthening frictional property, because of large accumulated slip deficit and large stress concentration due to large seismic slip. The simulated depth distribution of seismic slip is similar to that estimated from seismic and tsunami data as described in section 1. Seismic rupture of a simulated great earthquake starts in region B, a little deeper than the boundary between regions A and B Figure 2. The variation with depth of (top) friction parameters a (thin solid line), b (broken line), (top) a b (thick solid line), (middle) L, and (bottom) the ective normal stress s n assumed in the present model. 2of5

Figure 5. Simulated slip histories for smaller earthquakes at points 4 to 9 on the plate interface. The locations of the observation points are shown in Figure 1. Figure 3. Simulated slip histories at twelve points on the plate interface for a great earthquake cycle. The locations of the observation points are shown in Figure 1. Broken lines denote stable sliding with sliding rate of 0.3V pl (28.3 mm/y), 0.5 V pl (42.5 mm/y), and V pl (85.0 mm/y) for reference. (Figure 4). From various simulation results, we find that the seismic rupture start point is significantly influenced by the depth profiles of friction parameters and ective normal stress and it may be located in a deeper part in region B. A smaller earthquake occurs in region B at 2.2 days before the simulated great earthquake (Figure S1 of the auxiliary material). 1 This seems to correspond to the M7.3 event on March 9, which took place near the hypocenter of the March 11 great earthquake. It is remarked that the March 9 foreshock occurred slightly up dip from the main shock hypocenter, which cannot be explained by the present simple 2D model. 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2011GL048565. Figure 4. Depth distribution of seismic slip of a simulated great earthquake. The star denotes the depth of simulated seismic rupture start. We simply point out here that an earthquake at a weaker region may trigger a larger earthquake at a stronger region. [9] Figure 3 indicates that smaller earthquakes repeatedly occur in regions B and D during an interseismic period of great earthquakes, though the average slip rates are significantly smaller than the relative plate rate. The plate interface is virtually locked in regions B and D during each interseismic period of smaller earthquakes. These simulation results are consistent with observations that the average seismic slip rate estimated from repeated occurrence of M7 class earthquakes was much lower than the average plate rate and the geodetically estimated slip deficit rate was close to the average plate rate. The average recurrence interval of M 7.5 earthquakes at the deeper plate interface off Miyagi was 37 years [Earthquake Research Committee, 2000], which is about half of the present simulation value, suggesting that the assumed L or s n at these depths is too large. Figure 5 shows that simulated slip histories on the plate interface when smaller earthquakes take place in regions B and D. An earthquake in region D is followed by an earthquake at the shallower part with a delay of 196 days. This delay time is a typical value but it is variable in the simulation. [10] Figure 3 indicates further that the aseismic slip rates at the deep velocity strengthening regions (region E) during an interseismic period are significantly smaller than the relative plate rate, which is consistent with geodetically estimated slip deficit extended deeper than the seismogenic plate interface [Suwa et al., 2006]. Although each simulated smaller earthquake in region D is followed by afterslip in the simulation, the afterslip amplitude is much smaller than the seismic slip amplitude (Figure 5). This is also consistent with GPS observation for the afterslip of the 2005 Miyagi oki earthquake [Miura et al., 2006]. Note that the afterslip amplitude depends on friction parameters in region C. The accumulated slip deficit in region E is released as significant afterslip of each great earthquake in the simulation. This significant afterslip causes rapid loading at seismogenic plate interface, leading to earlier occurrence of smaller earthquakes in regions B and D (Figure S2 of the auxiliary material). 4. Summary and Discussion [11] We conduct a numerical simulation of earthquake cycle at a subduction zone using a 2D model with the rateand state dependent friction law in order to understand the mechanics of the 2011 great Tohoku oki earthquake. On the 3of5

assumption of a strong patch at a shallow part of the plate interface, we can explain some important features of observations as follows: (1) Great earthquakes that break the entire seismogenic plate interface repeatedly occur at a recurrence interval longer than several hundred years. (2) The largest seismic slip of a great earthquake occurs at the shallow part of the plate interface. (3) Slip deficit is accumulated at the deeper part of the plate interface where M7 class earthquakes repeatedly occur during an interseismic period of the great earthquakes. The present simulation result suggests that significant afterslip occurs at a deep aseismic plate interface, where slip deficit is accumulated, leading to rapid loading at the deep seismogenic plate interface, probably resulting in earlier occurrence of M7 class earthquakes. The present model assumed that the shallow strong patch controls the occurrence of great earthquakes and the accumulation of slip deficit at the entire seismogenic plate interface. Each M7 class earthquake at the deeper seismogenic plate interface may relax stress though slip deficit is accumulated because the locked shallow patch prohibits from free slippage. When the strong patch is broken, large seismic slip occurs at the patch and slip occurs also at the deeper plate interface where slip deficit is accumulated. Note that the above features may be reproduced not only by the shallow strong patch but also by large coseismic slip at the shallow patch due to other mechanisms such as thermal pressurization [e.g., Mitsui and Hirahara, 2009; Noda and Lapusta, 2010]. If thermal pressurization occurs only at the shallow patch during each great earthquake, the recurrence interval of earthquakes at the patch would be long and smaller earthquakes would repeatedly occur at deeper parts during the interval. [12] In the present simulation, great earthquakes of the same magnitude occur repeatedly at a constant recurrence interval. Recurrence of great earthquakes along the Pacific coast of northern Honshu is not sufficiently understood. Recurrence of same magnitude earthquakes in the present model may be too simple to explain possible complex earthquake recurrence in the region. [13] The strong patch is realized by higher values of ective normal stress s n and/or large values of characteristic slip distance L in the simulation. Because s n is given by the difference between lithostatic pressure and hydrostatic pore pressure at depths 20 km, the large contrast of s n comes from higher pore pressure at deeper parts. Higher pore pressure may be justified by pore compaction and sealing in the fault zone at the deeper parts. Larger L values assumed at the shallower depths may be justified by the unconsolidated material at the shallow plate boundary. Moreover, large L may lead to slow source process, which seems to be consistent with less short period seismic radiation from the shallow patch of the earthquake [Koper et al., 2011]. The maximum L value of 0.5 m assumed in the present model corresponds to the critical slip weakening distance D c of 1.5 2.0 m [Kato and Tullis, 2003]. Although some uncertainty is included in estimates of D c for earthquakes, D c of 1.5 2.0 m seems to be permissible [e.g., Zhang et al., 2003]. The recurrence interval T r of great earthquakes is controlled by fracture energy at the strong patch and the ptheoretical ffiffiffiffiffiffiffiffiffiffi consideration suggests that T r is proportional to n L [Kato, 2010]. 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