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

Self Cite

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region.

Section: Discussionsupporting

confidence: 71%

“…This is similar to typical continental crust and thus suitable to the southern Okinawa Trough, which still holds continental crust (Arai et al 2017). (Yamamoto et al 2018) and may suggest a potential volcanic front. Contour lines for b values are drawn every 0.2.…”

Section: Discussionmentioning

confidence: 58%

Characteristics of seismicity in the southern Okinawa Trough and their relation to back-arc rifting processes

Arai

2021

Self Cite

“…4) was located ~ 50 km east of Ishigaki Island, the plate geometry does not change significantly in the along-strike direction, and thus, the fault slip style along the plate interface may be related to the structure in the overriding plate. A similar relation between the structure in the overriding plate and fault slip style was indicated by seismic tomography (Yamamoto et al 2018). Yamamoto et al (2018) discussed the high Vp/Vs of oceanic crust inferred below the SSE region, indicating the existence of fluid that contributes to SSE activities.…”

Section: Spatial Distribution Of Slip Deficit Ratesupporting

confidence: 60%

Potential of megathrust earthquakes along the southern Ryukyu Trench inferred from GNSS data

Kano

Ikeuchi

Nishimura

et al. 2021

The southern part of the Ryukyu subduction zone has recorded tsunami events with a recurrence interval of several hundred years. Although their source is controversial, one model suggests that the last 1771 Yaeyama tsunami was caused by a shallow megathrust earthquake with a magnitude of 8. However, the current knowledge on interplate coupling based on recent geodetic data is limited. Here, a time series of Global Navigation Satellite System data from January 2010 to February 2021 was analyzed, including newly installed stations by Kyoto and Kyushu Universities, to obtain the distance changes between stations and vertical secular velocities. The distance changes ranged from 2.4 mm/year in contraction and to 4.7 mm/year in extension, and the vertical velocities exhibited no clear uplift or subsidence, with − 2.4 to 1.1 mm/year. The back slip inversion results indicated a slip deficit of 17–47 mm/year to the south of the Yaeyama Islands. The large slip deficit area is complementarily intervened between the shallower source area of low-frequency earthquakes and the deeper slow slip region, suggesting the spatial heterogeneity of frictional properties along the plate interface. If the large slip deficit area accumulates stress in the same rate since the last 1771 earthquake, it could result in a megathrust event with a moment magnitude greater than 7.5. Because the limited onshore data cannot resolve the slip deficit on the shallow plate interface, seafloor geodetic observations are essential to clarify the detailed spatial distribution of the slip deficit and discuss its earthquake and tsunami potential. Graphical Abstract

“…When the above observations are combined, the SSE areas with slips greater than 5 cm do not overlap the areas of the seismogenic zone of the great Kanto earthquakes and the regular interplate earthquakes. A spatial separation of the slow slip area and the regular earthquakes was also reported for the Hikrangi subduction zone in New Zealand (Bartlow et al 2014) and the Ryukyu Trench (Yamamoto et al 2018), likely indicating spatial differences in the fault friction properties on plate boundaries.…”

Section: Comparison Of the Structure To Sses Off Bososupporting

confidence: 52%

Configuration and structure of the Philippine Sea Plate off Boso, Japan: constraints on the shallow subduction kinematics, seismicity, and slow slip events

Ito

Tonegawa

Uchida

et al. 2019

We applied tomographic inversion and receiver function analysis to seismic data from ocean-bottom seismometers and land-based stations to understand the structure and its relationship with slow slip events off Boso, Japan. First, we delineated the upper boundary of the Philippine Sea Plate based on both the velocity structure and the locations of the low-angle thrust-faulting earthquakes. The upper boundary of the Philippine Sea Plate is distorted upward by a few kilometers between 140.5 and 141.0°E. We also determined the eastern edge of the Philippine Sea Plate based on the delineated upper boundary and the results of the receiver function analysis. The eastern edge has a northwest-southeast trend between the triple junction and 141.6°E, which changes to a north-south trend north of 34.7°N. The change in the subduction direction at 1-3 Ma might have resulted in the inflection of the eastern edge of the subducted Philippine Sea Plate. Second, we compared the subduction zone structure and hypocenter locations and the area of the Boso slow slip events. Most of the low-angle thrust-faulting earthquakes identified in this study occurred outside the areas of recurrent Boso slow slip events, which indicates that the slow slip area and regular lowangle thrust earthquakes are spatially separated in the offshore area. In addition, the slow slip areas are located only at the contact zone between the crustal parts of the North American Plate and the subducting Philippine Sea Plate. The localization of the slow slip events in the crust-crust contact zone off Boso is examined for the first time in this study. Finally, we detected a relatively low-velocity region in the mantle of the Philippine Sea Plate. The low-velocity mantle can be interpreted as serpentinized peridotite, which is also found in the Philippine Sea Plate prior to subduction. The serpentinized peridotite zone remains after the subduction of the Philippine Sea Plate and is likely distributed over a wide area along the subducted slab.