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Static strain and stress changes in eastern Japan due to the 2011 off the Pacific coast of Tohoku Earthquake, as derived from GPS data
© 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. 2011
Received: 7 April 2011
Accepted: 26 June 2011
Published: 27 September 2011
The 2011 M 9.0 Tohoku earthquake induced regional crustal deformation not only in the Japanese Islands but also in north-eastern Asia. Strain release due to mainshock faulting should cause strain redistribution in the overriding plates. The dense GPS network in Japan enables us to calculate co-seismic strain and stress changes from observed data. Strain is a more objective indicator than displacement because no reference frame is required. The co-seismic strain field clearly indicates island-scale strain redistribution. Huge extensional strain changes were concentrated in the southern Iwate and northern Miyagi regions, with a maximum value of 45 × 10−6, which might correspond to approximately 225 to 450 years of strain accumulation. This implies relatively large strain accumulation and release in these regions. Small strain decay was observed in the northernmost NiigataKobe tectonic zone and a possible anomalous Coulomb failure stress change was observed in the Mt. Fuji region. Earthquakes triggered in the above regions might be associated with these anomalies, and/or these non-uniform crustal deformations may reflect crustal heterogeneity.
Large earthquakes often induce remote seismicity and several actual examples have been reported (e.g., Protti et al., 1995; Brodsky et al., 2000; Ueda and Takahashi, 2005). Earthquakes possibly triggered by this M 9.0 main-shock have been observed in the region from north to central Japan. Static strain-stress change is a candidate for the driving force and so may become an indicator in the evaluation of triggering earthquake activity (e.g., Stein and Lisowski, 1983; King et al., 1994). The static strain-stress change at each target point is usually calculated indirectly, through a model based on assumed mainshock fault parameters. This procedure is very convenient and can provide arbitrary density data. On the other hand, this method is strongly affected by the uncertainly of a priori fault parameters. The heterogeneity of the medium is also not considered. In other words, the strain-stress values obtained by the above method are only estimations, rather than actual observations.
The GEONET dense GPS network with a spacing of 20 to 30 km allows us to calculate strain tensor inner triangles from observed displacement data. The static stress change at each triangle can then be estimated from the strain tensor using proper elastic constants. In order to investigate the characteristics of the co-seismic strain-stress redistribution due to this great earthquake, we calculated strain-stress changes from GEONET GPS displacement data.
2. Data and Method
3. Results and Discussion
3.1 Spatial characteristics of strain change
The distribution of maximum shear strain has clear spatial characteristics (Fig. 2(b)). Triangles with strains of more than 30 × 10−6 were concentrated along the coastlines of southern Iwate and northern Miyagi. Concentric strain decay was observed at these locations. No triangles with anomalously large strains were observed in Fukushima or Ibaraki. These features might indicate that dominant moment release was limited off southern Iwate and northern Miyagi.
The compressional strain rate during recent decades along the Miyagi coastline region prior to the mainshock
was approximately 0.1×10−6 to 0.2×10−6 per year (Kato et al., 1998; Sagiya et al., 2000; Miura et al., 2002). If we assume that this rate was constant during the entire interseis-mic period, the largest values of released strain (45 × 10−6) correspond to approximately 225 to 450 years of strain accumulation. The occurrence of several M 7-class earthquakes (the 1897, 1936 and 1978 earthquakes of magnitude 7.4) might allow an even longer strain accumulation period. The absence of M 8-class events during the past several hundred years on the plate interface, however, implies that the impact of interseismic events on strain accumulation was less significant. Although the estimated earthquake magnitudes were smaller than that of the latest event, candidates for previous massive events were the 1611 Keicho (M 8.1) and the 869 Jogan (M 8.3) earthquakes (Usami, 2003). The recurrence interval between these three events (742 and 400 years) appears to agree somewhat with the accumulation period estimated from strain data. Tsunami deposit data, however, implies longer periods between giant tsunamis (Minoura et al., 2001). In order to evaluate this hypothesis, more detailed investigations of previous great earthquakes are required.
Note that the anomaly at the south-western edge in Fig. 2 is the co-seismic effect of a M 6.7 earthquake that occurred during the observation period.
3.2 Small strain attenuation in the northernmost Niigata-Kobe tectonic zone
3.3 Strain and stress anomalies in the Mt. Fuji area
A M 6.4 crustal earthquake occurred just beneath the active Mt. Fuji volcanic mountain on 15 March 2011, four days after the mainshock. This earthquake might be the largest earthquake in the Mt. Fuji region during historical times.
However, identification of this strain-stress anomaly is strongly dependent on the GPS station data at the summit of Mt. Fuji. In order to confirm the reliability of this result, investigation of data quality using a different data source should be performed in the future.
Co-seismic strain redistribution on the overriding plate due to the 2011 Tohoku earthquake was evaluated by observed GPS displacement data. Enormous strain changes exceeding 45 × 10−6 were computed in the southern Iwate and northern Miyagi regions, but these values did not extend to the south. This might indicate a significant release of concentrated strain in these regions. A strain accumulation period of 225 to 450 years was estimated from the interseis-mic strain rate and the co-seismic strain magnitude. A small amount of strain attenuation was found in the northernmost Niigata-Kobe tectonic zone, which might reflect subsurface heterogeneity. Possible anomalous strain and stress changes were detected around the Mt. Fuji region, where a M 6.4 crustal earthquake occurred after the mainshock. This implies that this remote earthquake may have been a triggering event due to strain redistribution.
The authors would like to thank Drs. Kaj Johnson and Sigeru Nakao for reviewing the manuscript. We obtained GEONET GPS F3 coordinate data from the Geospa-tial Information Authority of Japan (http://www.gsi.go.jp). Focal mechanism data were obtained from the NIED F-net home page (http://www.fnet.bosai.go.jp) and the GCMT homepage (http://www.globalcmt.org). Hypocenter data were determined by JMA in cooperation with the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in order to analyze seismic wave data provided by cooperating organizations, including Hokkaido Univ., Hirosaki Univ., Tohoku Univ., the Univ. of Tokyo, Nagoya Univ., Kyoto Univ., Kochi Univ., Kyushu Univ., Kagoshima Univ., the Nat. Res. Inst. Earth Sci. Disa. Prev., the Nat. Inst. Adv. Ind. Sci. Tech., the Tokyo Metropolitan Govern., Shizuoka Pref., the Hot Springs Res. Inst. Kanagawa Pref., Yokohama City, the GSI and the JAMSTEC. The present study was supported by the MEXT of Japan through the Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions and KAKENHI (21253005, 22253005).
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