[SSS07-02] Crustal deformations precursory to the Tohoku M9 earthquake and tsunami
キーワード：Deterministic prediction of megathrust earthquake & tsunami, Crustal deformations evolving into megathrust events, GPS time series data analyses with physical wavelets, GPS time series data analyses with phase space paths
The Geospatial Information Authority of Japan (GSI) has more than 1240 Global Positioning System (GPS) stations laid throughout Japan. The northern part of Honshu is Tohoku that is at the eastern edge of a continental tectonic plate overriding the subducting Pacific Plate. A large fault surface between the plates held them together. The fault had a length of 500 km and a width of 200 km. The GPS stations in Tohoku and the Pacific Ocean observed three distinctive phases of the crustal deformations evolving into the March 11 2011 Tohoku M9 earthquake (EQ) and tsunami to follow. Their cross-sectional deformations are in Fig. 1.
The tectonic plate-driving forces as shown with arrows in Fig.1 had constantly compressed the Tohoku area. The compression kept pushing the west and east coasts of Tohoku upward and downward, respectively. This crustal deformation is a normal distortion for which the fault surface couples the two plates together as in Fig. 1a. A GPS station in the subducting western Pacific Plate (in the Pacific Ocean) was moving west at the speed of 3.8 cm per year that is about 0.1 mm/day averaged over 10 years.
A bulge of 1 – 3 mm began forming from the normal distortion around December 8, 2009, which is about 16 months prior to the March 2011 M9 EQ. The formation is schematically drawn in Fig. 1b. As the bulge grew, the east coast of Tohoku began to pull the subducting plate further down by their fault coupling, which started around July 11, 2010. Its pulling action by the eastern edge of the overriding plate is shown by the dotted arrow over the fault in Fig. 1b. The pulling action accelerated the motion of the subducting plate. On December 22, 2010, the westward speed of the subducting motion reached its highest of 0.65 mm/day three times faster than that on July 11, 2010. On the same day as the highest westward speed is observed, the M7.9 event raptured at the epicenter in the Pacific about 668 km away from the GPS station to the east. The event had a normal faulting, which suggests the westward anomalous acceleration triggered the event. After the highest westward speed, the reaction against the pulling began decelerating the motion.
The reaction completely stopped the subducting motion on February 21, 2011, which kept stopping for four more days. The reaction then began pushing back the subducting motion to the east on February 26, 2011. The eastward speed reached 0.06 mm/day by only three days prior to the March 11 2011 M9 event. The reaction grew to the megathrust on the fault surface between the two tectonic plates, which generated the M9 EQ and tsunami as schematically drawn in Fig. 1c.
The three phases of the deformations in Fig. 1 are quantified by analyzing GPS displacement time series with a mathematical tool named Physical Wavelets (P-Ws). The tool has the time reversal property of the position and derivatives satisfied to define the position, velocity, and acceleration for the non-differentiable displacement time series data [1, 2]. Thus, the phase space analyses of displacements with P-Ws are a key to quantifying the bulge coupling of the two tectonic plates. The analyses have the resolutions of 0.1 mm and 0.0001 mm/day 5 orders of magnitude greater than the daily vertical displacement noise of about ± 20 mm. The analyses assure that the short-term deterministic predictions within about two to three months prior to the ruptures of such impending megathrust events as suggested in Fig. 1 are available [1].
Fig. 1. Three phases of the crustal deformations. (a) Normal slow distortion. (b) Precursory bulge formation and its pulling action to accelerate the subducting Pacific Plate motion. (c) The reaction against the pulling grows to the megathrust to generate the M9 EQ and tsunami to follow.
[1] F. Takeda, Japanese Patent JP5798545B2 (2012), to be in arXiv.org
https://patents.google.com/patent/JP5798545B2/en
[2] F. Takeda, submitted to PRL (2020),
https://www.nat-hazards-earth-syst-sci-discuss.net/nhess-2017-454/
The tectonic plate-driving forces as shown with arrows in Fig.1 had constantly compressed the Tohoku area. The compression kept pushing the west and east coasts of Tohoku upward and downward, respectively. This crustal deformation is a normal distortion for which the fault surface couples the two plates together as in Fig. 1a. A GPS station in the subducting western Pacific Plate (in the Pacific Ocean) was moving west at the speed of 3.8 cm per year that is about 0.1 mm/day averaged over 10 years.
A bulge of 1 – 3 mm began forming from the normal distortion around December 8, 2009, which is about 16 months prior to the March 2011 M9 EQ. The formation is schematically drawn in Fig. 1b. As the bulge grew, the east coast of Tohoku began to pull the subducting plate further down by their fault coupling, which started around July 11, 2010. Its pulling action by the eastern edge of the overriding plate is shown by the dotted arrow over the fault in Fig. 1b. The pulling action accelerated the motion of the subducting plate. On December 22, 2010, the westward speed of the subducting motion reached its highest of 0.65 mm/day three times faster than that on July 11, 2010. On the same day as the highest westward speed is observed, the M7.9 event raptured at the epicenter in the Pacific about 668 km away from the GPS station to the east. The event had a normal faulting, which suggests the westward anomalous acceleration triggered the event. After the highest westward speed, the reaction against the pulling began decelerating the motion.
The reaction completely stopped the subducting motion on February 21, 2011, which kept stopping for four more days. The reaction then began pushing back the subducting motion to the east on February 26, 2011. The eastward speed reached 0.06 mm/day by only three days prior to the March 11 2011 M9 event. The reaction grew to the megathrust on the fault surface between the two tectonic plates, which generated the M9 EQ and tsunami as schematically drawn in Fig. 1c.
The three phases of the deformations in Fig. 1 are quantified by analyzing GPS displacement time series with a mathematical tool named Physical Wavelets (P-Ws). The tool has the time reversal property of the position and derivatives satisfied to define the position, velocity, and acceleration for the non-differentiable displacement time series data [1, 2]. Thus, the phase space analyses of displacements with P-Ws are a key to quantifying the bulge coupling of the two tectonic plates. The analyses have the resolutions of 0.1 mm and 0.0001 mm/day 5 orders of magnitude greater than the daily vertical displacement noise of about ± 20 mm. The analyses assure that the short-term deterministic predictions within about two to three months prior to the ruptures of such impending megathrust events as suggested in Fig. 1 are available [1].
Fig. 1. Three phases of the crustal deformations. (a) Normal slow distortion. (b) Precursory bulge formation and its pulling action to accelerate the subducting Pacific Plate motion. (c) The reaction against the pulling grows to the megathrust to generate the M9 EQ and tsunami to follow.
[1] F. Takeda, Japanese Patent JP5798545B2 (2012), to be in arXiv.org
https://patents.google.com/patent/JP5798545B2/en
[2] F. Takeda, submitted to PRL (2020),
https://www.nat-hazards-earth-syst-sci-discuss.net/nhess-2017-454/