Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

Abstract

The Ryukyu (Nansei-Shoto) island arc-trench system, southwest of Japan, is formed by the subduction of the Philippine Sea (PHS) plate. Among the subduction zones surrounding the Japan Islands, the Ryukyu arc-trench system is unique in that its backarc basin, the Okinawa Trough, is the area with current extensively active rifting. The length of the trench is around 1400 km, and the geological and geophysical characteristics vary significantly along the trench axis. We conducted multichannel seismic (MCS) reflection and wide-angle seismic surveys to elucidate the along-arc variation in seismic structures from the island arc to the trench regions, shooting seven seismic lines across the arctrench system and two along-arc lines in the island arc and the forearc areas. The obtained P-wave velocity models of the Ryukyu arc crust were found to be heterogeneous (depending on the seismic lines), but they basically consist of upper, middle, and lower crusts, indicating a typical island arc structure. Beneath the bathymetric depressions cutting the island arc-for example, the Kerama Gap and the Miyako Saddle-the MCS record shows many across-arc normal faults, which indicates the presence of an extensional regime along the island arc. In the areas from the forearc to the trench, the subduction of the characteristic seafloor features on the PHS plate affects seismic structures; the subducted bathymetric high of the Amami Plateau is detected in the northern trench: the Luzon-Okinawa fracture zone beneath the middle and southern trenches. There are low-velocity (<~4.5 km/s) wedges along the forearc areas, except for off Miyako-jima Island. The characteristic high gravity anomaly at the forearc off Miyako-jima Island is caused not by a bathymetric high of a large-scale accretionary wedge but by shallower materials with a high P-wave velocity of ~4.5 km/s.

Modeling the Geometry of Plate Boundary and Seismic Structure in the Southern Ryukyu Trench Subduction Zone, Japan, Using Amphibious Seismic Observations

“…The spatial extent of this thick low‐ V p area is about 30 km in the trench‐parallel direction, and we observed no other comparable layers in other eastern and western profiles. Nishizawa et al () reported that the crust of the subducting PHS has a considerable variation in both thickness and velocity structure along the entire Ryukyu Trench subduction zone. Since there are many seamounts with spatial extents similar to this convex structure on the incoming PHS, it is possible that this thick crust corresponds to a subducted seamount or topographic high (gray circle in Figure ).…”

“…In addition, the outer rise is not developed along the Ryukyu Trench based on the seafloor topography. In reality, there are no seismic refraction images showing the occurrence of hydration (i.e., velocity reduction in the slab crust and mantle) seaward of the trench axis along the Ryukyu Trench (Nishizawa et al, ). Therefore, we should consider other causes of slab mantle hydration in this study area, such as the magmatic effects of plumes (Seno & Yamanaka, ).…”

Interplate Coupling State at the Nansei‐Shoto (Ryukyu) Trench, Japan, Deduced From Seafloor Crustal Deformation Measurements

“…Furthermore, Müller and Landgrebe () reported a strong correlation between subducting fracture zones and areas of megathrust earthquake resulting from strong interplate coupling. A zonal high free‐air gravity anomaly is recognized between 125°E and 129°E along the forearc of the Nansei‐Shoto Trench (Hsu et al, ; Nishizawa et al, ; Sandwell et al, ). The gravity high is associated with the mound‐like submarine topography, which may be due to compressional deformation through the subduction of the stacked oceanic plate (Hsu et al, ).…”

Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

“…1). Early and recent active-source seismic surveys confirmed that despite having been stretched through the rifting process, the Ryukyu arc exhibits structural characteristics of typical continental lithospheres, such as thick felsic upper crust with P wave velocities of ~ 6.0 km/s, ensuring that the Ryukyu arc still preserves original continental materials and is thus essentially characterized as a continental arc (Murauchi et al 1968;Iwasaki et al 1990;Arai et al 2016Arai et al , 2017aNishizawa et al 2017) (Fig. 1c).…”

“…Another geochemical study shows that the current volcanic front has been active since approximately 2 Ma and that the magma production there is enhanced by fluids dehydrated from the subducting slab (Shinjo et al 2000). On the contrary to these advances, internal architectures of the volcanic arc based on geophysical approaches are poorly documented: Multiple seismic surveys have been carried out in the study area (Nash 1979;Iwasaki et al 1990;Nakahigashi et al 2004;Gungor et al 2012;Nishizawa et al 2017), but unfortunately none of them have discussed the volcanic structure in detail.…”