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

Shuichi Kodaira,Anke Dannowski,Gou Fujie,Yasuyuki Nakamura,Martin Thorwart,Ingo Grevemeyer,Seiichi Miura,Jason P Morgan

doi:10.1130/g46757.1

Abstract

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.

Highlights

The occurrence and magnitude of thrust earthquakes in subduction zones is closely linked to interplate seismic coupling
Because smectite is an extremely weak mineral whose presence can dramatically change both the static and dynamic friction along a fault, the presence of an ultraweak smectite-rich clay layer is thought to be a prerequisite for giant coseismic slip (Ujiie et al, 2013) like that observed at Tohoku
The absence of the chert unit implies that the lower part of the sediments, including the pelagic red-brown clay, is missing because the chert unit is the lowermost part of the sediments and the the clay layer is located immediately above the chert (Moore et al, 2015)

Summary

INTRODUCTION

The occurrence and magnitude of thrust earthquakes in subduction zones is closely linked to interplate seismic coupling This coupling, in turn, is generally thought to be related to the surface topography and surface materials that form the incoming oceanic plate. Results from the Integrated Ocean Drilling Program (IODP) Expedition 343 after the 2011 Mw 9 Tohoku earthquake (offshore Japan) showed that the giant coseismic slip near the trench (>50 m) occurred within a thin smectiterich clay layer at the plate boundary (Chester et al, 2013). We used a small-offset MCS system (1.2-km-long, 192-channel hydrophone streamer cable, total airgun array volume of 380 in3) and collected data along >100 densely aligned short survey lines across the Japan Trench (Fig. 1A, thin black lines). Despite significant differences in the survey configuration, spatial variations in sediment thickness are well constrained by both surveys after applying standard post-stack time migration (Figs. 2 and 3)

RESULTS

D Thin-sediment area

DISCUSSION