A normal-faulting seismic sequence triggered by the 2011 off the Pacific coast of Tohoku Earthquake: Wholesale stress regime changes in the upper plate
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We here show relocated hypocenters within a seismic sequence with normal faults in the northern part of Ibaraki Prefecture, triggered after the M 9.0 2011 off the Pacific coast of Tohoku Earthquake. Depth-sections of the hypocenters from the center of the northern Ibaraki region to the north show an earthquake alignment dipping westwards at 40° to 50° at depths shallower than 10 km. On the other hand, hypocenters from the center to the south show a cross-cutting geometry consisting of conjugate westward- and eastward-dipping planes at the same depths. The dip angles of the hypocenter alignments are roughly consistent with the nodal planes of focal mechanisms of large normal earthquakes, and exhibit optimal-orientations in terms of the frictional failure criterion. Furthermore, comparison of the focal mechanisms recorded before and after the 2011 Tohoku Earthquake suggests that the stress field abruptly changed from horizontal compression to extension in the study area. The most plausible explanation of the drastic stress changes is a significant reduction in trench-normal compressive stress compared with reduction in trench-parallel stress accompanying large horizontal extensional deformation within the overlying plate.
Key wordsNormal-faulting earthquake alignments stress changes the 2011 Tohoku Earthquake double-difference relocation
The 2011 off the Pacific coast of Tohoku Earthquake M 9.0 ruptured a fault area nearly 500 km along-strike length and 200 km down-dip width of the fault off-shore of the Tohoku region along the Japan Trench subduction zone as a huge thrust earthquake at 14:46 (JST = UT + 9 hours) on March 11, 2011 (Japan Meteorological Agency (JMA)). The focal mechanism of this earthquake is a reverse fault type with a low-dipping angle, suggesting that the fault is located along the interface between the subducting Pacific plate and the overlying plate. This huge thrust earthquake has caused devastating seismic and tsunami’s damage in NE Japan. Ide et al. (2011) imaged a spatio-temporal slip distribution of the 2011 Tohoku Earthquake applying finite-source modeling to global broadband seismograms. The highest cumulative slip in their model is about 30 m and is strongly concentrated in the shallow part of the fault. They have documented that normal faulting aftershocks in the area of high slip indicate total stress drop or dynamic overshoot on the mainshock fault plane (e.g., Hasegawa et al., 2011).
The 2011 Tohoku Earthquake triggered widespread seismicity throughout the Japanese island arc including Hokkaido and Kyushu regions. In particular, a significant increase in the shallow seismicity was observed in the days following the mainshock along the Pacific coast of NE Japan, notably the northern part of Ibaraki Prefecture, the southern part of Fukushima Prefecture and off Choshi (Hirose et al., 2011).
Temporal stress changes above a subducting plate can provide important information on stress interactions between the subducting and overlying plates (e.g., Wesson and Boyd, 2007; Saiga et al., 2011). This is therefore crucial to understand the unusual sequence of normal faults. In this paper, we have relocated earthquakes in the northern part of Ibaraki Prefecture that occurred immediately after the 2011 Tohoku Earthquake using the double-difference relocation method (Waldhauser and Ellsworth, 2000) and report here on a drastic change in the stress field induced by large extensional deformation within the overlying plate.
2. Data and Methods
In this analysis, we used the 764 earthquakes, which are listed in the JMA catalog (for the period between 18:00 14 March and 23:59 31 March (JST)); the number of P-and S-wave arrival times for each event exceeded 12 and 6, respectively.
To determine the detailed hypocenter distributions in the source region, the double-difference relocation method (Waldhauser and Ellsworth, 2000; Zhang and Thurber, 2003) was applied to the P- and S-wave data. ‘Double difference’ denotes the difference between the observed and calculated travel time of two events obtained at a particular station. The double-difference method enables us to accurately determine the relative locations of the hypocenters. The relocated hypocenters were calculated assuming a one-dimensional velocity structure estimated by JMA (Ueno et al., 2002).
We used differential arrival times obtained for the manually-picked and waveform correlation data. The differential arrival times for the manually picked P- and S-waves were 79,026 and 68,408, respectively. Waveform cross correlation in the time-domain (Schaff et al., 2004; Kato et al., 2006) yield a more accurate data set of differential arrival times containing 297,080 P-wave observations and 10,6288 S-wave observations; all had normalized cross correlation coefficients larger than 0.85. The correlation measurements were conducted using 1.2 s window lengths beginning 0.6 s before the manually picked arrival time for 1–10 Hz filtered waveforms.
3. Hypocenter Alignments
Figure 2 illustrates the distributions of relocated hypocenters at depths shallower than 12 km. The epicentral distributions are fairly heterogeneous along the seismic sequence (Fig. 2(a)): from the center to the south (E-F, G-H), the epicenters exhibit relatively wide distributions, whereas the epicenters from the center to the north (A-B, C-D) are tightly oriented in a N-S direction.
Depth-sections of the relocated hypocenters from the center to the north reveal an earthquake alignment dipping westwards at an angle from 40° to 50° (A-B, C-D in Fig. 2(b)). On the other hand, the hypocenters from the center to the south show a cross-cutting geometry consisting of westward- and eastward-dipping conjugate planes (E-F, G-H in Fig. 2(b)). The dip angles of the hypocenter alignments are roughly consistent with the nodal planes of focal mechanisms of the large events determined by NEID (Figs. 1(b) and 2(b)). The down-dip width of these hypocenter alignments ranges from 6 to 8 km, which is consistent with the typical rupture dimension of magnitude 6 earthquakes (Hanks, 1977).
The focal mechanisms of the large events represent normal faulting for which the expected orientation of the maximum compressive stress axis is nearly vertical (Fig. 1(b)). The dip angles of each hypocenter alignment (from 40° to 50°) are close to the optimal-orientations with respect to maximum principle stress axis (σ1).
4. Drastic Stress Change
We have calculated co-seismic strain changes associated with the 2011 Tokohu earthquake in the study area from continuous GPS network data. The horizontal extensional strain exhibits a step-like increase by approximately 10−5. Prior to the 2011 Tokohu earthquake, the northern part of Ibaraki Prefecture exhibited E-W contractional principle strains produced by forearc deformation accompanying of the Pacific plate. As the secular contractional strain rate increase due to plate convergence is estimated to be about 10−7/yr (e.g., Kato et al., 1998), a straightforward calculation suggests that it will take more than one hundred years to recover the stress change from horizontal extension to compression, during the re-strengthening (healing) stage of the plate boundary fault. It is thus quite important to monitor the temporal change or recover of focal mechanisms in the study area to examine the re-strengthening process on the fault of huge earthquakes.
We thank J. Townend and T. Takeda for useful and constructive comments, which led to substantial improvement to the original manuscript. We are thankful to J. Fukuda for fruitful discussion about strain change. We also thank NIED for use of their waveform data, and JMA for use of their waveform data and unified earthquake catalog. Some figures have been produced using the GMT mapping codes (Wessel and Smith, 1991).
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