Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes

Abstract

The nature of incoming sediments is a key controlling factor for the occurrence of megathrust earthquakes in subduction zones. In the 2011 Mw 9 Tohoku earthquake (offshore Japan), smectite-rich clay minerals transported by the subducting oceanic plate played a critical role in the development of giant interplate coseismic slip near the trench. Recently, we conducted intensive controlled-source seismic surveys at the northwestern part of the Pacific plate to investigate the nature of the incoming oceanic plate. Our seismic reflection data reveal that the thickness of the sediment layer between the seafloor and the acoustic basement is a few hundred meters in most areas, but there are a few areas where the sediments appear to be extremely thin. Our wide-angle seismic data suggest that the acoustic basement in these thin-sediment areas is not the top of the oceanic crust, but instead a magmatic intrusion within the sediments associated with recent volcanic activity. This means that the lower part of the sediments, including the smectite-rich pelagic red-brown clay layer, has been heavily disturbed and thermally metamorphosed in these places. The giant coseismic slip of the 2011 Tohoku earthquake stopped in the vicinity of a thin-sediment area that is just beginning to subduct. Based on these observations, we propose that post-spreading volcanic activity on the oceanic plate prior to subduction is a factor that can shape the size and distribution of interplate earthquakes after subduction through its disturbance and thermal metamorphism of the local sediment layer.

Correlation of frontal prism structures and slope failures near the trench axis with shallow megathrust slip at the Japan Trench

“…The structure and property of the incoming plate are important controlling factors of the seismogenesis and structural evolution in the subduction zones. A recent study 30 found that the post-spreading volcanism altered the structure of the upper most part of the incoming Pacific plate in the vicinity of the trench axis around 39°N, which corresponds to the area where slope failures were identified in the seismic profiles. Magmatic intrusions and thermal metamorphism associated with post-spreading volcanism were suggested to disturb the smectite-rich pelagic clay layer in incoming sediments, and the subduction of this disturbed area was suggested to prevent giant near-trench interplate coseismic slip in this region 30 .…”

“…A recent study 30 found that the post-spreading volcanism altered the structure of the upper most part of the incoming Pacific plate in the vicinity of the trench axis around 39°N, which corresponds to the area where slope failures were identified in the seismic profiles. Magmatic intrusions and thermal metamorphism associated with post-spreading volcanism were suggested to disturb the smectite-rich pelagic clay layer in incoming sediments, and the subduction of this disturbed area was suggested to prevent giant near-trench interplate coseismic slip in this region 30 . Most of the seismic profiles around 39°30′N were interpreted to lack the chert layer (SU3) and suggest that the pelagic clay layer was disturbed in the shallow most part of the subduction zone.…”

“…3 c). The absence of the smectite-rich clay around the plate boundary fault could have increased the basal friction 30 . The increased basal friction could have contributed to building higher critical taper angle and steeper slopes of the frontal prism 31 .…”

Seismic energy radiation and along-strike heterogeneities of shallow tectonic tremors at the Nankai Trough and Japan Trench

“…The southern edge of the tremor zone corresponds to the thin-sediment region due to petit-spot volcanism (Hirano et al, 2006). The northern edge of the tremor zone off Iwate seems to correspond to a region of thick sediments, although it is unfortunately located at the edge of the area analyzed by Fujie et al (2020). Similar to the Nankai Trough, lithological differences may affect the seismicity of slow earthquakes.…”

“…This may also contribute to the generation of favorable physical conditions for shallow slow earthquakes in the regions off Fukushima and Ibaraki with high pore fluid pressures. Fujie et al (2020) investigated the spatial variations in the incoming sediments on the Pacific plate. Although their results for the trench cover only shallow tremors in the region off Iwate, we can observe spatial correlations between the sediment thickness and slow earthquakes.…”