Active faults and coastal landscapes in the back arc of Central Japan; Lessons from the Mw 7.5 Noto Peninsula earthquake

中部日本背弧の活断層と海岸地形; Mw7.5能登半島地震による教訓

Malatesta, L.*; Weiss, N.-M.*; 塚本 すみ子*; 末岡 茂   ; 石村 大輔*; Gailleton, B.*; 西村 卓也*; 高橋 直也*; 片岡 香子*; 小松 哲也  ; 岩佐 佳哉*

Malatesta, L.*; Weiss, N.-M.*; Tsukamoto, Sumiko*; Sueoka, Shigeru; Ishimura, Daisuke*; Gailleton, B.*; Nishimura, Takuya*; Takahashi, Naoya*; Kataoka, Kyoko S.*; Komatsu, Tetsuya; Iwasa, Yoshiya*

On January 1st, 2024, the Mw 7.5 Noto Peninsula earthquake ruptured on a series of coastal offshore reverse faults in the back arc of central Japan. The coastal rocks uplifted as much as 4.4 m. The coastline accordingly moved seaward by up to 200 m creating new wide bedrock platforms. Holocene terraces mapped along the northern coast suggest similar past ruptures. Many of the ruptured faults follow the coast at a depth of ca. 60 m below modern sea level, which strongly suggests that these faults define the extent of the continental domain. The Peninsula itself hosts 4767 unique mapped terraces ranging in age from Holocene to 1.02 Ma. The terraces associated with the last two interglacial high stands record a tectonic SE-tilting similar to that of the Mw 7.5 earthquake. Older terraces all record a spatially uniform rate of uplift across the Peninsula. We conclude that the faults that caused the most recent earthquake became the dominant structures on the Peninsula around 250 ka and that the Peninsula is in a state of transient equilibration. 80 km to the northeast of the Noto Peninsula lies the Island of Sado. The Island is made of two mountain ranges oriented SW-NE along the main tectonic lineation of the back arc, roughly parallel to the northern coast of Noto Peninsula. The marine terraces of the northern range, Oosado, record a strong southeast tilting synchronous and similar to that observed on the Noto Peninsula. The landscape morphology is not equilibrated to this pattern of deformation either. Earlier work suggested a hypothesis that the tilt is driven by a fault lying just offshore of the Oosado coast. Closer inspection of the bathymetry reveals a ramp at around -60 m reflecting a geometry similar to the Noto Peninsula. The lessons from the Noto Peninsula earthquake can be applied to Sado Island where information about the seismic cycle is lacking. It confirms the hypothesis and highlights a potential seismogenic source close to the shore.