Geodetic Evidence of Post-2011 Acceleration of the Pacific Plate ?1.Dept. Natural History Sci., Hokkaido Univ., N10 W8, Kita-ku, Sapporo 060-0810, JAPAN, 2.Inst. Geosci., Shizuoka Univ, Shizuoka 422-8529, JAPAN G11B-0481

20145-9

Abstract Oceanic plates may accelerate after megathrust earthquakes (D.Anderson, Science 1975). This was indirectly substantiated by Heki and Mitsui (EPSL 2013), who analyzed crustal deformation of an island arc after large inter-plate earthquakes. Here we present data suggesting postseismic velocity change of the Minami-torishima (Marcus) station, the only GNSS station on the Pacific Plate, ~2000 km southeast of NE Japan. There, continuous GNSS observations started in 2002, and showed linear movement toward WNW of ~7.7 cm/year (nnr-NUVEL1 frame). Here we show two stories based on the two data sets showing different post-3.11 velocity changes, a baseline-approach solution (F3 solution of GSI), and the PPP approach (JPL). Geophysical interpretation depends on which solution is correct.Story #1 According to the F3 solution, the station showed coseismic jump of ~1 cm toward the epicenter in the 2011 Tohoku-oki earthquake. At the same time, the velocity showed distinct increase of ~10 percent without changing the azimuth, resulting in post-2011 speed of ~8.5 cm/year. On the other hand, station movements on the stable part of the Philippine Sea Plate changed little. This suggests the post-3.11 acceleration of the Pacific Plate.Story #2 According to the Gipsy PPP solutions by JPL, acceleration is smaller and is toward the NEJ. This would be explained easily by viscous relaxation of the upper mantle.

1Kosuke Heki 日置幸介 (heki@mail.sci.hokudai.ac.jp) & 2Yuta Mitsui 三井雄太

Forces acting on the subducting slab. Downward forces (Fsp: slab pull, Frp: ridge push) are balanced by upward forces (Fc: interplate coupling, Fsr: side resistance, Fer: edge resistance). Sudden decrease of Fc after an earthquake needs to be compensated by the increase of Fsrenabled by the acceleration of u.

Anderson (1975 Science)

Slab pull

Side resistance

Ridge push

Coupling

(constant)

(constant)

(proportional to u)

(lost by megathrust)

Heki & Mitsui (2013 EPSL)

Coseismic step and sudden velocity change associated with the 2011 Tohoku-oki earthquake are clearly seen (left). Coseismic step (below) is consistent with the calculated vector. Velocity change (acceleration) is ~10 percent of the secular velocity and is parallel with the secular velocity. In the F3 solution (GSI), they fix Tsukuba-1 and the position of Tsukuba-1 is derived from 11 IGS stations in ITRF2005 around Japan (Tsuji et al. 2014).

Coseismic step

Minami-torishima (Marcus Is.)

PH

PAEUMinami-Torishima (Marcus) Island is an only Japanese island on the Pacific Plate

(PA). A GEONET GNSS station 021098 has been

continuously operational since 2002.

Movement of the VLBI station in 80s(Koyama et al., 1994)

Once upon a time in Marcus …

Errors of the reference-frame origin would emerge as the translation, rotation, and scale change of the whole network. We examined the existence of unexpected velocity changes for stations within the stable interior of the Philippine Sea Plate.

Kita-Daito (960746) and Okino-Torishima island (051140) GNSS stations show little velocity changes after the 2011 Tohoku-oki earthquake. This leaves the possibility that the velocity change in the Minami-Torishima (Marcus) island may reflect the real acceleration.

Kita-daito

Okino-torishima

Okino-torishimaHigashi-Kojima

Secular velocities of three GEONET stations on PH on the Mercator’s projection with the PH pole

Okino-torishima GNSS station

GNSS station

Kita-daito (PH)

Okino-torishima(PH)

Non PA stable stations in GEONET

∆V for stations on stable part of PH Precise Point Positioning (PPP) method is little influenced by the movement of reference stations (although it suffers from errors in satellite coordinates and satellite clocks). Here we show time series of Marcus, Shanghai, and Hawaii (Oahu), available from the website of Jet Propulsion Laboratory (JPL). They are derived by PPP using GIPSY software.

Analyses by JPL with GIPSY/PPPsecular

acceleration

step

JPL solution for a station in Hawaii show no steps nor velocity changes associated with the Tohoku-oki earthquake, while those for Shanghai show significant steps and postseismic transient.

Honolulu (Hawaii)

Story #1 (GSI F3 solution) Baseline approach PPP approach (sideshow.jpl.nasa.gov/post/series.html) Story #2

2004 Sumatra (Mw9.2)

2010 Chile (Mw8.8)

2011 Tohoku-Oki (Mw9.0)

Due to three M9-class earthquakes, global scale disturbances of “secular” velocities of GNSS stations worldwide are going on and

may distort the kinematic reference frame

Geophysical model: Acceleration of Pacific Plate

Geophysical model: Viscous relaxation

Postseismic velocities at three stations by viscous relaxation of upper mantle (62-670 km) of viscosity 5 x 1018 Pa s, calculated using the viscoelastic Green’s function after Pollitz (2007). Earth model: PREM, Fault model: GSI (2011)

Shanghai (China)

∆v