2020 Volume 129 Issue 4 Pages 529-563
GNSS data analyses reveal that recent large inland earthquakes in the Northeast (NE) Japan occurred in strain concentration zones. Seismic low-velocity anomalies, indicative of mechanically weak materials (weak zones), are estimated below the strain concentration zones at depths corresponding to the lower crust. Such crustal structures with weak zones have been formed as an accumulation of tectonic movements and igneous activities since early Miocene. Volcanic activity in the NE Japan during the Late Cenozoic Era can be subdivided into three prominent stages: continental margin volcanism stage, back-arc basin opening stage, and island-arc volcanism stage. The crustal structure of the NE Japan arc is characterized by many rift structures and large transcurrent faults formed during the back-arc basin opening stage, and by many large caldera volcanoes formed during the island-arc volcanism stage. The relationships among mechanically weak crustal structures, present strain localizations, earthquake distributions, and geological characteristics including rift structures, large transcurrent faults, volcanic belts, and caldera volcanoes, are clarified using various geophysical data such as gravity anomalies, seismic velocity structures, strain rates, and epicenter distributions. The results show that strain concentration zones and inland earthquake epicenters have close spatial relationships with geological structures such as rift boundary faults, large transcurrent faults, caldera structures, and volcanic belts. It can be interpreted that fluids migrating upwards from lower crustal weak zones below rifts, volcanic belts, or calderas, effectively weakened the crust due to its high pore fluid pressure, and caused earthquake ruptures under horizontal compression.