14:30 〜 14:45
[SCG45-35] Spatial gap of slow earthquakes and structural features revealed by seismic reflection profiles in the Nankai Trough
キーワード：スロー地震ギャップ、デコルマ、海洋地殻、ラフネス、堆積層厚、反射方地震探査
To investigate relationship between spatial variation of the earthquake activity and geological structures, we have conducted dense seismic surveys in the Nankai Trough. In the offshore southeast of the Kii Peninsula, a significant gap of the activities of very-low-frequency earthquakes (VLFEs) and tremors is recognized from the last decades seismological observation. We collected 2D multi-channel seismic (MCS) data during the cruise KM18-10 using R/V Kaimei in an approximately 7200 km2 area including the slow earthquake gap off the Kii Peninsula. The collected MCS data along 23 NW-SE lines and 4 WE-SW lines were processed to produce reflection profiles in the depth domain. Then, we interpreted key horizons of the décollement and the top of the oceanic crust in addition to internal structures of the accretionary prism.
In this study, we divided the study area into three parts based on the spatial distribution of slow earthquakes: I) west part with active VLFEs and tremors, II) middle part with quiet or sporadic activity, and III) east part with active tremors and sporadic VLFEs. We compared the slow earthquake activity distribution with the geological features derived from the reflection profiles. We estimate roughness for the oceanic crust and décollement based on linear regression of the interpreted horizons of them at 0-30 km from the deformation front along each NW-SE seismic line. We calculated thicknesses of the underthrust and overburden layers below and above the décollement, respectively.
We observed clear lateral variation of surface topography on the oceanic crust with significant NNW-SSE trending depression and several local topographic highs. These features are likely rift structures and paleo-seamounts that originally developed through the past seafloor spreading on the oceanic crust in the Shikoku Basin. The décollement surface is smoother than the oceanic crust, especially smoothest in the middle part (II), and partly rough as well as the oceanic crust surface in the west part (I) and the east part (III). The thickness of the underthrust layer between the décollement and the oceanic crust shows spatial variation: thin sediment in the west part (I), thick sediments in the middle part (II), and wide variety of thickness in the east part (III). The variation of the topographic roughness and the underthrust layer thickness suggest that the Shikoku basin sediments filled the rift basin sufficiently to smooth the potential décollement surface in the middle part (II) and thinly covered the topographically high areas with keeping the roughness of the oceanic crust in the other parts (I) and (III).
We also observed the thickness variation of overburden layers, which containing the typical fold-and-thrust structures and slope basins in the accretionary prism above the décollement. Although the variation of the overburden thickness is less than 1 km at 3–11 km from the deformation front, approximately 1-1.5 km difference of the thickness between the middle part (II) (thicker) and the other parts (I) and (III) (thinner) at 11–20 km from the deformation front. The fold-and-thrust features also seem variable with corresponding to the topographic variation on the oceanic crust. These characteristics could be affected by the evolution process through the subduction of the oceanic crust with ridges as simulated by previous experimental studies. The lateral variations of the overburden layer thickness may be effective on stress conditions with dipping angles along the décollement. Additionally, we identified continuously positive dominant polarity reflections along the décollement in the middle part (II), and discontinuous positive or mixed with negative polarity reflections in the other parts (I) and (III). The variation of reflection characteristics possibly implies physical property condition and/or lithological changes along the décollement, which might control the frictional behavior in addition to the roughness of the décollement surface and overburden stress conditions.
In this study, we divided the study area into three parts based on the spatial distribution of slow earthquakes: I) west part with active VLFEs and tremors, II) middle part with quiet or sporadic activity, and III) east part with active tremors and sporadic VLFEs. We compared the slow earthquake activity distribution with the geological features derived from the reflection profiles. We estimate roughness for the oceanic crust and décollement based on linear regression of the interpreted horizons of them at 0-30 km from the deformation front along each NW-SE seismic line. We calculated thicknesses of the underthrust and overburden layers below and above the décollement, respectively.
We observed clear lateral variation of surface topography on the oceanic crust with significant NNW-SSE trending depression and several local topographic highs. These features are likely rift structures and paleo-seamounts that originally developed through the past seafloor spreading on the oceanic crust in the Shikoku Basin. The décollement surface is smoother than the oceanic crust, especially smoothest in the middle part (II), and partly rough as well as the oceanic crust surface in the west part (I) and the east part (III). The thickness of the underthrust layer between the décollement and the oceanic crust shows spatial variation: thin sediment in the west part (I), thick sediments in the middle part (II), and wide variety of thickness in the east part (III). The variation of the topographic roughness and the underthrust layer thickness suggest that the Shikoku basin sediments filled the rift basin sufficiently to smooth the potential décollement surface in the middle part (II) and thinly covered the topographically high areas with keeping the roughness of the oceanic crust in the other parts (I) and (III).
We also observed the thickness variation of overburden layers, which containing the typical fold-and-thrust structures and slope basins in the accretionary prism above the décollement. Although the variation of the overburden thickness is less than 1 km at 3–11 km from the deformation front, approximately 1-1.5 km difference of the thickness between the middle part (II) (thicker) and the other parts (I) and (III) (thinner) at 11–20 km from the deformation front. The fold-and-thrust features also seem variable with corresponding to the topographic variation on the oceanic crust. These characteristics could be affected by the evolution process through the subduction of the oceanic crust with ridges as simulated by previous experimental studies. The lateral variations of the overburden layer thickness may be effective on stress conditions with dipping angles along the décollement. Additionally, we identified continuously positive dominant polarity reflections along the décollement in the middle part (II), and discontinuous positive or mixed with negative polarity reflections in the other parts (I) and (III). The variation of reflection characteristics possibly implies physical property condition and/or lithological changes along the décollement, which might control the frictional behavior in addition to the roughness of the décollement surface and overburden stress conditions.