The Tohoku earthquake's tsunami—which reached over 40 metres in height in some areas—was made possible by slippery clay deposits on the fault that allowed the rupture to propagate all the way to the seafloor trench. That earthquake was not an anomaly: multiple giant tsunamigenic earthquakes of similar magnitude have hit Japan in recent centuries, with evidence of paleotsunamis stretching back thousands of years. Yet the subducting plate's sediment cover is not uniform. Seismic observations suggest patches of much thinner sediment, interpreted as areas of "petit-spot" magmatism—small volcanic features that may fundamentally alter how the fault behaves. This project will use drilling results from IODP expedition 502 offshore Japan to map the scale and extent of petit-spot magmatism across the wider Japan Trench region, compare deformation styles in areas with and without these features, and run fault nucleation experiments on physically altered sedimentary rock samples from the core. If successful, the work will clarify what controls earthquake nucleation and slip in subduction zones, giving policymakers and coastal communities a clearer picture of where the next giant tsunami might originate.
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Megathrust earthquakes and consequent tsunamis are one of the most important hazards that affect human populations in tectonically active regions. The Mw 9.0 Tohoku earthquake offshore Japan was one of the largest in recorded history and resulted in over 20,000 fatalities and ~$235 billion in economic damage. The majority of the damage was not caused by earthquake shaking but by the tsunami, which exceeded 40 m in height in some areas. This tsunami occurred because of shallow co-seismic slip of the plate-bounding fault, which propagated to the trench. This shallow slip was accommodated by smectite-rich clay deposits at the plate-boundary fault zone, which had little frictional resistance to slip. This earthquake is not unique in Japan, with multiple such giant tsunamigenic (Mw 8.5) earthquakes recorded over the last few hundred years, and evidence of paleotsunamis extending back several thousand years. However, the lithological and frictional properties of the subducting plate in Japan are not all equally uniform, with significant differences in sediment thickness and composition. In addition, based on seismic observations, there appear to be areas of significantly thinner sediment cover on the downgoing plate, which are interpreted to correspond to areas of “petit-spot” magmatism. These are likely to significantly affect subduction dynamics but the physical characteristics of magmatic features and alteration of adjacent sedimentary are poorly constrained. IODP expedition 502 offshore Japan will test the Petit spot hypothesis, and seeks to understand the impact of this magmatism on subduction dynamics. This study will extend the drilling results from expedition 502 to understand the scale and extent of petit-spot magmatism in the wider Japan Trench region, and compare and contrast deformation styles in the frontal prism in areas with and without subducting petit-spot features. In addition, we will use some physical samples of altered sedimentary rocks from the core to carry out fault nucleation experiments, to compare fault nucleation characteristics with unaltered samples from Expedition 405. This will directly address Objective 7 of the 2050 Science Framework: Natural hazards impacting society: Understanding natural hazards in the marine environment. This work will provide important new insights into the processes that control earthquake nucleation and slip in subduction zones, and the hazards that these represent, with implications for policy makers and the general public living in these areas.
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