[SSS17-P14] The 3-stage earthquake generation process observed during 3 months before the 2011 Tohoku earthquake (4)
Keywords:Tohoku Earthquake, Precursor, Electromagnetic anomaly, Thermal anomaly
1. Introduction
The researcher has conducted the following research on the subject earthquake.
At first, the operational situation of the F-net broadband seismic network was examined, and it was found that number of stations with missing data increased from December 22, 2010 to January 18, 2011 and from February 16, 2011 to March 2, 2011. Especially from February 19 to March 2, they reached the maximum number of 4 stations (Sapporo(HSS), Iwateyamagata(IYG), Kesennuma(KSN), Shiramine(SRN)), of which 3 of them are close to epicenter of the earthquake. (Sue, 2013)
Then, to understand such behavior of the F-net, various reports were collected, and it was found that seismic activity for three months before the earthquake was consisted of three stages shown below.
First stage: This is the final stage of strain accumulation (i.e. compression) of the land-side plate, and the strain accumulation reached the upper limit at the end of this stage.
Second stage: Slow slip started to occur near the epicenter, also the direction of the movement of land-side plate reversed.
Third stage: This is the final stage before the main shock. (Sue, 2017)
As a next step, detailed investigation focused on the events of the first stage (late November, 2011 - around January 28, 2011) was conducted. Large vibrations were found by such instruments as the F-net from January 3 to 18, which preceded the last day of strain accumulation. (Sue, 2018)
The start date of this stage shall be changed to the end of November to include the West off Chichijima island earthquake (M6.9) on November 30, 2010.
Then, the second stage (around January 29, 2011 - around March 2, 2011) was examined. A slow slip started to occur near the epicenter around January 29, and rebound of the land-side plate was also observed. From mid-February to late February, there were strong seismic activities at off Sanriku region. Expansion of the slow-slip area, and increase in F-net stations with missing data were also observed. (Sue, 2019)
As a follow-on step, this paper reports behavior of the third stage (March 6, 2011 - March 11, 2011: the day of main shock) . Note, based on the latest research, the start date of this stage shall be March 6.
2. Analysis
The GEONET shows various parts of the Tohoku region have moved independently since January 29, but from around March 6, a large area in the Tohoku region has moved toward the trench axis simultaneously. At the same time, electromagnetic and thermal abnormal were reported. These movements continued the main shock after the March 9 off Sanriku earthquake M7.3, which is understood as a foreshock. The source of the precursors is presumed to be the boundary of plates near the epicenter.
The events observed at this stage are shown below (items, period, special notes).
GEONET (wide area sliding), 3/6 and later, (Kamiyama, 2012)
VLF / LF electromagnetic anomaly, 3/5-6, (Hayakawa et al., 2012)
ULF / ELF electromagnetic anomaly, 3/6, (Ohta et al., 2013)
ULF electromagnetic anomaly, 3/6, (Hayakawa et al., 2012)
GPSTEC electromagnetic anomaly, 3/7-8, (Hirooka et al., 2016)
GPSTEC electromagnetic anomaly, 3/11, (Heki, 2011)
OLR radiation heat anomaly, 3/7-10, (Ouzounov et al., 2011)
3. Researches to be conducted
In the six days before the earthquake, crustal movements, electromagnetic and thermal changes were observed simultaneously. Therefore, it is necessary to conduct research from view points of each area.
References:
Hayakawa, M. et al., 2012, Ann. Geophysics (Italy), 55, N. 1, 95-99.
Heki, K., 2011, Geophys. Res. Lett, 38, L17312.
Kamiyama, M., 2012, Japanese Geotechnical Society, 2012 General assembly of Tohoku branch.
Hirooka, S. et al., 2016, IEEJ Trans. on Fundamentals and Materials, 136, 5, 265-271.
Sue, Y., 2013, JpGU2013, SSS30-P01.
Sue, Y., 2017, JpGU2017, SSS14-P08.
Sue, Y., 2018, JpGU2018, SSS13-05.
Sue, Y., 2019, JpGU2019, SSS09-P03.
Ohta, K. et al., 2013, Radio Science, 48, 5, 589-596.
Ouzounov, D. et al., 2011, Earthquake Science, 24, 6, 557-564.
The researcher has conducted the following research on the subject earthquake.
At first, the operational situation of the F-net broadband seismic network was examined, and it was found that number of stations with missing data increased from December 22, 2010 to January 18, 2011 and from February 16, 2011 to March 2, 2011. Especially from February 19 to March 2, they reached the maximum number of 4 stations (Sapporo(HSS), Iwateyamagata(IYG), Kesennuma(KSN), Shiramine(SRN)), of which 3 of them are close to epicenter of the earthquake. (Sue, 2013)
Then, to understand such behavior of the F-net, various reports were collected, and it was found that seismic activity for three months before the earthquake was consisted of three stages shown below.
First stage: This is the final stage of strain accumulation (i.e. compression) of the land-side plate, and the strain accumulation reached the upper limit at the end of this stage.
Second stage: Slow slip started to occur near the epicenter, also the direction of the movement of land-side plate reversed.
Third stage: This is the final stage before the main shock. (Sue, 2017)
As a next step, detailed investigation focused on the events of the first stage (late November, 2011 - around January 28, 2011) was conducted. Large vibrations were found by such instruments as the F-net from January 3 to 18, which preceded the last day of strain accumulation. (Sue, 2018)
The start date of this stage shall be changed to the end of November to include the West off Chichijima island earthquake (M6.9) on November 30, 2010.
Then, the second stage (around January 29, 2011 - around March 2, 2011) was examined. A slow slip started to occur near the epicenter around January 29, and rebound of the land-side plate was also observed. From mid-February to late February, there were strong seismic activities at off Sanriku region. Expansion of the slow-slip area, and increase in F-net stations with missing data were also observed. (Sue, 2019)
As a follow-on step, this paper reports behavior of the third stage (March 6, 2011 - March 11, 2011: the day of main shock) . Note, based on the latest research, the start date of this stage shall be March 6.
2. Analysis
The GEONET shows various parts of the Tohoku region have moved independently since January 29, but from around March 6, a large area in the Tohoku region has moved toward the trench axis simultaneously. At the same time, electromagnetic and thermal abnormal were reported. These movements continued the main shock after the March 9 off Sanriku earthquake M7.3, which is understood as a foreshock. The source of the precursors is presumed to be the boundary of plates near the epicenter.
The events observed at this stage are shown below (items, period, special notes).
GEONET (wide area sliding), 3/6 and later, (Kamiyama, 2012)
VLF / LF electromagnetic anomaly, 3/5-6, (Hayakawa et al., 2012)
ULF / ELF electromagnetic anomaly, 3/6, (Ohta et al., 2013)
ULF electromagnetic anomaly, 3/6, (Hayakawa et al., 2012)
GPSTEC electromagnetic anomaly, 3/7-8, (Hirooka et al., 2016)
GPSTEC electromagnetic anomaly, 3/11, (Heki, 2011)
OLR radiation heat anomaly, 3/7-10, (Ouzounov et al., 2011)
3. Researches to be conducted
In the six days before the earthquake, crustal movements, electromagnetic and thermal changes were observed simultaneously. Therefore, it is necessary to conduct research from view points of each area.
References:
Hayakawa, M. et al., 2012, Ann. Geophysics (Italy), 55, N. 1, 95-99.
Heki, K., 2011, Geophys. Res. Lett, 38, L17312.
Kamiyama, M., 2012, Japanese Geotechnical Society, 2012 General assembly of Tohoku branch.
Hirooka, S. et al., 2016, IEEJ Trans. on Fundamentals and Materials, 136, 5, 265-271.
Sue, Y., 2013, JpGU2013, SSS30-P01.
Sue, Y., 2017, JpGU2017, SSS14-P08.
Sue, Y., 2018, JpGU2018, SSS13-05.
Sue, Y., 2019, JpGU2019, SSS09-P03.
Ohta, K. et al., 2013, Radio Science, 48, 5, 589-596.
Ouzounov, D. et al., 2011, Earthquake Science, 24, 6, 557-564.