We measured seafloor movement using a Global Navigation Satellite Systems (GNSS)/Acoustic technique at the south of the rifting valley in the western end of the Okinawa Trough back‐arc basin, 60 km east of northeastern corner of Taiwan. The horizontal position of the seafloor benchmark, measured eight times between July 2012 and May 2016, showed a southeastward movement suggesting a back‐arc opening of the Okinawa Trough. The average velocity of the seafloor benchmark shows a block motion together with Yonaguni Island. The westernmost part of the Ryukyu Arc rotates clockwise and is pulled apart from the Taiwan Island, which should cause the expansion of the Yilan Plain, Taiwan. Comparing the motion of the seafloor benchmark with adjacent seismicity, we suggest a gentle episodic opening of the rifting valley accompanying a moderate seismic activation, which differs from the case in the segment north off‐Yonaguni Island where a rapid dyke intrusion occurs with a significant seismic activity.

Here we test the precursory enhancement in ionospheric total electron content (TEC) which has been reported by Heki (2011, https://doi.org/10.1029/2011GL047908) and numerous Global Navigation Satellite System (GNSS) TEC observational studies before the 2011 Mw9.0 Tohoku‐Oki and many great earthquakes. We verify the frequency of this TEC enhancement via analysis of a 2‐month vertical TEC (VTEC) time series that includes the Tohoku‐Oki earthquake using the procedure, based on Akaike's information criterion, and threshold of Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353). The averaged occurrence rate of the TEC enhancement is much larger than that reported by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) when all of the visible GPS satellites at a given station are taken into account. The frequency assessment by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) using only one satellite underestimates the frequency of the TEC enhancement. In fact, the TEC enhancement is sufficiently frequent to explain all the reported enhancements prior to the great earthquakes as a product of chance. We also analyze the spatial distribution of the preseismic TEC enhancement and coseismic TEC depletion for the Tohoku‐Oki earthquake with the data after reducing intertrace biases. We observe significant postseismic depletion that lasted at least 2 h after the earthquake and extended at least 500 km around the center of the large‐slip area. This means that evaluation of the enhancements using reference curves which was adopted by Heki (2011, https://doi.org/10.1029/2011GL047908) and even by the recent papers (e.g., He & Heki 2016, https://doi.org/10.1002/2016GL069863; 2017, https://doi.org/10.1002/2017JA024012; 2018, https://doi.org/10.1029/2017JA024871) is in danger of mistaking a large and long‐lasting postseismic TEC depletion for a preseismic enhancement.

Long-term seafloor geodetic measurements are important for constraining submarine crustal deformation near plate boundaries. Here we present an integrated analysis of a decade of GNSS/acoustic data collected at a site 60 km to the east of northeast Taiwan near the axis of the Okinawa Trough back-arc basin. We obtained a time-series of horizontal and vertical positions based on 18 measurements from 2009 to 2019. These data reveal a southeastward movement at a rate of 43 ± 5 mm/yr since 2012 with respect to the Yangtze Plate. The horizontal motion can be explained by the clockwise rotation of the Yonaguni Block and northern Central Range. In addition, the vertical displacement of the transponder array shows rapid subsidence of 22 ± 9 mm/yr from 2012 to 2019. The fast subsidence rate and negative free-air gravity anomaly in this region indicate that crustal thinning is compensated mainly by surface deformation rather than upward migration of the Moho. Taking into account the offset in 2012 owing to the replacement of the transponder array, the horizontal position time series of our site are best explained by two linear lines with a slope change in July 2013. The timing of the velocity change coincides broadly with a change in the nearby seismicity rate and dike intrusion 150 km away from the site. Our results highlight the potential of seafloor geodesy in assessing temporal changes in deformation near the spreading center of the Okinawa Trough, which cannot be one using data from onland GNSS stations.

We compared the cumulative seismic slip of interplate earthquakes (≥M5.5) with relative plate motion at subduction zones. By assuming that each interplate earthquake occurred on a stick–slip patch, we used the slip history of each patch to calculate the interplate slip in the surrounding area. We considered that areas in which interplate earthquakes occurred but that had small cumulative slips compared with relative plate motion were accumulating strain, and we calculated the size of these areas. We first used this method to test the rupture areas of six M9-class interplate earthquakes that have occurred during the past 100 years. The cumulative seismic slip preceding and following the six earthquakes was smaller than the relative plate motion in the rupture areas of the earthquakes. We interpret the areas of slip-deficient stick–slip patches to be the rupture areas of future huge earthquakes. We applied the same procedure to global subduction zones and found that slip-deficient stick–slip patches with large spatial extents (equivalent to the rupture area of M9-class earthquakes) occur in an additional 25 locations. Considering that six M9-class earthquakes have occurred in the past 110 years and that the recurrence interval in each case is probably between a few hundred and a thousand years, it is not surprising that 25 regions globally are capable of producing M9-class earthquakes. These regions may be the most likely candidates for the rupture areas of future M9-class interplate earthquakes.