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April 2, 2020

Petit-Spot Volcanoes May Have Determined the Northern Limit of the 2011 Tohoku
Earthquake: Discovery of Heterogeneity in Subducting Oceanic Plates Offshore of Tohoku

1. Key Points

An area with extremely thin sediments was found on the Pacific Plate before subduction from the Japan Trench.
Recent volcanic activities related to petit-spot volcanoes was a cause of sediment thinning; this finding reveals that the effects of petit-spot activities are substantially more wide-ranging than previously thought.
Changes in the sedimentary layer caused by igneous activity naturally have a strong effect on the occurrence of earthquakes along plate boundaries after plate subduction, and such changes may have determined the northern limit of a huge coseismic slip near the trench that precipitated the 2011 Tohoku Earthquake.


Researchers at the Japan Agency of Marine–Earth Science and Technology (JAMSTEC) Ocean Earthquake and Volcano Department investigated the seismic structure of the oceanic plate offshore of Tohoku. They found extensive thinning of the sedimentary layer caused by recent volcanic activities and suggested that the resultant subduction could have influenced the massive earthquake that occurred along the plate boundary in this area in 2011 (Figures 1 and 2). Although earthquakes occur along plate boundaries in places where oceanic plates subducts, the properties of plate boundary faces (shape, physical properties, etc.) determine the scale and form with which they occur. The most important factor in determining such surface properties is the condition of the surface layer of the oceanic plate; the alteration of sedimentary layers on oceanic plates by igneous activity is thought to strongly influence the occurrence of earthquakes along plate boundaries.

The reason that the 2011 Tohoku Earthquake, which occurred below the Pacific Ocean, caused a massive tsunami is that a huge seismic slip of over 50 meters occurred near the axis of the Japan Trench. This massive slip did not extend further than 39ºN. In this location, a marine area in which the sedimentary layer has been extensively altered by igneous activity, known as petit-spot volcanoes, is subducting. Prior to subduction, igneous activity altered the sedimentary layer, significantly reducing its coefficient of friction through the disturbance and loss of weak clay and mineral formations. It is therefore possible that this could have determined the northern limit of the 2011 Tohoku Earthquake (Figures 3 and 4).

This work was supported by a Grant-in-Aid for Scientific Research [grant no. JP15H05718] provided by the Japan Society for the Promotion of Science (JSPS).

The results of the study summarized here are scheduled to be published in Geology on March 31, 2020 (JST).

Title:Spatial variations of incoming sediments at the Northeastern Japan arc and their implications for megathrust earthquakes

Authors:Gou Fujie1, Shuichi Kodaira1, Yasuyuki Nakamura1, J. P. Morgan2, A. Dannowski3, M. Thorwart4, I. Grevemeyer3, Seiichi Miura1

1Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
2Department of Ocean Science and Engineering, SUSTech
3GEOMAR/Helmholtz-Center for Ocean Research Kiel
4Christian-Alberchts University, Kiel

Figure 1

Figure 1: (a) Seismic reflection survey lines. Figure 2 shows the reflection cross-section of the survey lines A4 and R2 (shown in red). (b) Sediment thickness derived from the reflection cross-section; the thickness is indicated by the time (in milliseconds) required for a round-trip from the acoustic base to the seafloor and back. The sedimentary layers are thinner in areas A and C. The thin-sedimentary areas in the south and east of area C correspond to seamounts. (c) Distributions of chert layer observed on the reflection cross-section (red) and the coseismic slip distribution of the 2011 Tohoku Earthquake (black, after Iinuma et al., 2012). The area where the chert layer is missing is consistent with the area with a thin-sediment; it is clear that no major coseismic slip related to the Tohoku Earthquake occurred where area A is subducting.

Figure 2

Figure 2: (a) Seismic reflection cross-section of the A4 survey line. The trench axis is near the western side of the survey line (left), and plate bending-related faults are observed near the trench. The sedimentary layer is thinned in the area shown in blue. (b) Seismic reflection cross-section of the R2 survey line. (c) Enlarged view of the thick-sediment area. (d) Enlarged view of the thin-sediment area. The thickness of the sedimentary layer in Figure 1(b) was measured using the strongly reflective surface (black arrow) as an acoustic base.

Figure 3

Figure 3: (a) Structural boundaries below the seafloor in a thick-sediment area, as detected by an ocean bottom seismometer (OBS) data with use of receiver function analysis technique. The vertical axis is the difference between the arrival times of S- and P- waves, which corresponded to the depth at which the structural boundaries was located. Only one boundary was imaged at approximately 2 seconds and was interpreted to be the upper surface of the oceanic crust. (b) Structural boundary in the thin-sediment area, as identified by the OBS data. Multiple boundaries were imaged at varying depths. (c) Structural model of P-wave velocity along the A4 survey line derived from OBS data; the left end is the trench axis. (d) One-dimensional structure of P-wave velocity pulling out every 10 km, based on the structural model in (c). This shows that the P-wave velocity was only low directly below the acoustic base in the thin-sediment area.

Figure 4

Figure 4: Schematics of the structures of oceanic sedimentary layers. (Left) A marine area with thick sediments. (Right) A marine area with thin sediments, depicted based on the findings of this study. In the thin-sediment area, the original sediment, including chert, is disturbed, and it is considered to have undergone thermal metamorphism. The red lines indicate the locations of the acoustic bases, where significant acoustic reflections can be observed.


(For this study)
Gou Fujie, Senior Researcher, Marine Seismology Research Group, Subduction Dynamics Research Center, Research Institute for Marine Geodynamics, JAMSTEC
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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