Dynamic Leveling of the Kanto Basin, Japan: A Unified Model Linking the Kanto Fragment and Mw 8 Deep Earthquakes

Yoshikazu Hiraoka

The Kanto Basin in central Japan has preserved an anomalously flat surface for more than one million years, defying the intense compressional tectonic regime of the Japanese island arc. Conventional static sedimentation and sea‑level fluctuation models cannot adequately explain this long‑term planar stability. This study proposes a dynamic leveling mechanism driven by two interacting deep tectonic processes: the stress‑shielding effect of the rigid Kanto Fragment, an isolated Pacific Plate slab embedded beneath the basin, and the episodic occurrence of Mw 8–class deep earthquakes along its boundary. By integrating GNSS‑derived rigid‑body motion of the basin with geotechnical principles—including liquefaction, lateral spreading, and differential consolidation—we construct a unified conceptual model for basin‑scale dynamic reorganization. Large deep earthquakes periodically induce forced fluidization of the 3–4 km thick unconsolidated sedimentary sequence, three‑dimensional lateral flow governed by gravitational potential gradients, and differential consolidation driven by sediment‑load contrasts. Acting synergistically, these processes reset topographic irregularities produced by static sedimentation, functioning as a long‑term natural leveling engine. The model provides a coherent explanation for the preservation of elevated tablelands as remnants of former leveled surfaces and reframes the geomorphic evolution of the Kanto Basin as a dynamic, earthquake‑driven system rather than a static depositional environment. This framework also carries significant implications for seismic hazard assessment in the Tokyo metropolitan region.