Stress changes due to the March 11, 2011, Tohoku Off-shore megathrust earthquake (Mw9.0) and related seismicity patterns.

The great Tohoku earthquake occured on March 11, 2011, 14:46 Japan Time, at the subduction of the Pacific Plate unde the North American Plate. Besidees the "traditional" aftershock activity, this major event triggered significant earthquake activity in the inland areas of Japan as well, at relatively large distances. We are mostly concerned here in estimating the static stress changes in various inland and off-shore regions in and around the Japanese islands and estimate the potential of future triggered seismicity. Moreover, the stress change patterns are compared with the seismicity patterns after the major earthquake to establish possible correlations. However, the JMA catalog is still under continuos improvement for the time period immediately following the major earthquake, therefore we focus at this moment on analysing mainly the stress changes. Updates will be made continuosly. Moreover, we have detected some interesting seismicity patterns before the mega-earthquake, which we also present below.

A) Seismicity patterns with a possible precursory character

Starting about one month before the major Tohoku earthquake (Mw8.9) a seismic swarm, which shows a clear space-time migration pattern (see page 2 of our pdf file), occurred in the region. The swarm appears to stop shortly and is then followed by the Mw7.2 large event on March 9. After only two days the Mw8.9 earthquake struck.

The pdf file can be downloaded from:

http://www.rcep.dpri.kyoto-u.ac.jp/~benescu/seismicity_pattern_2011_Tohoku.pdf

Please check Marsan and Enescu (JGR, 2012), for a thorough discussion of the pre-Tohoku earthquake (M9.0) seismicity pattern:

http://www.rcep.dpri.kyoto-u.ac.jp/~benescu/Work/Marsan_Enescu_JGR_2012.pdf

B) Coulomb static stress changes and seismicity

The Coulomb static stress changes due to the recent Mw9.0 earthquake are calculated on various receiver planes, using the slip distribution of Suzuki et al. (2011), obtained from the inversion of regional waveform data. For more information about this slip model we refer to the web page below (in Japanese):

http://www.bosai.go.jp/news/oshirase/20110315_01.pdf 

The calculations have been done using the Coulomb 3.1 software (Toda et al., 2005). We have used an apperent coefficient of friction of 0.4, if not otherwise specified.

Fault slip model (NIED, Suzuki et al., 2011) used for the stress change calculations:


Ammount of net slip on the fault (m); Suzuki et al., 2011

1. Effect on the Tokai plane

One of the regions in Japan that is of great concern for seimic hazard, is the so-called Tokai region, located at the subduction of the Philippine Sea Plate under the Eurasian Plate. Our analysis shows that there is no significant influence on the Tokai fault plane due to the great Tohoku earthquake.



Coulomb stress changes (Bars) on the presumed Tokai fault plane, due to the 2011 Tohoku Off-shore earthquake (slip distribution by Suzuki et al., 2011). For detailes related to the location and geometry  of the Tokai plane, see Aoi et al. (Nature Geoscince, 2011). The six rectangles show presumed asperities on the Tokai plane (Matsumura et al., 1998; 2008).

The stress has an increase on most of the Tokai plane, however the maximum increase is less than ~0.1 Bars (0.01 Mpa). It was observed that in general stresses larger than 0.1 Bars can trigger earthquakes, with delays ranging from seconds to decades (e.g., Parson et al., Nature, 2008; Aoi et al., Nature Geoscience, 2010). Thus, only the increase of stress due to the 2011 Tohoku earthquake is unlikely to trigger significant earthquake activity on the Tokai plane. However, the increase of seismicity in several crustal areas around the Tokai gap, likely due to a stress increase by the Tohoku earthquake, can have indirect consequences on the stress state at the plate boundary. The Shizuoka earthquake (Mw5.9), occurred on March 15, 2011 (i.e., after about 4 days from the Tohoku earthquake) is such an example of earthquake likely triggered by the stress increase due to the Tohoku earthquake (see point 2 below).

2. Effect on the fault plane of the Shizuoka earthquake (Mw5.9), occurred on March 15, 2011

On March 15, 2011, a moderate earthquake occurred in the Shizoka region, at bout 20 km from the northern, shallow edge of the Tokai plane . The stress shows a  significant relative increase, which suggests the possiblity of static stress triggering.



Coulomb stress change (bars), resolved on receiver faults of Strike =  25.4 deg, Dip = 80 deg and Rake = 10.2 deg, at 17 km depth. These parameters have been chosen based on the CMT focal mechanism solution for the Shizuoka mainshock and the hypocentral distribution of its aftershocks.
(NIED; http://www.hinet.bosai.go.jp/topics/e-shizuoka110315/). The star shows the epicenter of the Shizuoka earthquake (Mw5.9). The increase of stress at the hypocenter is of about 0.4 bars. Stresses of more than 0.1 bar have been shown to bring faults to failure.

3. Stress changes on various receiver planes

Stress changes in various regions of Japan, using as receiver faults those determined by Toda & Enescu (EPS, 2011), for the CSEP forecasting project. The receiver faults are based on focal mechanism solutions determined by F-net, NIED, as well as geologic information (e.g., type of faults). The paper of Toda & Enescu (EPS, 2011) can be downloaded from the EPS journal web site (for those with subscription) or from the following link:

http://www.rcep.dpri.kyoto-u.ac.jp/~benescu/Work/Toda_Enescu_EPS_2011.pdf



The results show areas of possible activation or decreased earthquake activity. Most of the reverse faults in the Tohoku region show a stress shadow (blue). Relatively large positive stress changes can be seen at the north and south extremities of the mainshock fault, as well as in the outer rise characterized by normal faulting. Some regions in southwest Japan have been also brought closer to failure (positive stress changes of up to about 0.6 Bar).

3. Stress changes resolved on the nodal planes of aftershocks

We used the CMT focal mechanism solutions determined by Asano et al. (2011), who used the approach of Ito et al. (2007). The figures reveal a clear correlation betwwen the positive stress changes and the occurence of the aftershocks, in particular for the case of focal mechanism solutions that are not consistent with the focal mechanism of the mainshock.







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Note: The results reported here are continusly updated. First preliminary calculations were performed on March 12, 2011. Due to electric power supply problems in Tukuba city, which was also affected by the major recent event, we had to delay posting these results.