4:15 PM - 4:30 PM
[SSS03-09] Low-Velocity Anomalies beneath the Outer-Rise Regions: A New Type of Abnormal Oceanic Mantle
Keywords:seismology, Earth's internal structure, plate tectonics, surface wave
Broadband Ocean-Bottom Seismometers (BBOBSs) are extensively installed in the Pacific regions, and data with smaller epicentral distances (<20 degrees) are now widely available. Such data provides us a unique opportunities to reveal the shallower oceanic mantle structures which should be the key to understand the fundamental framework of the plate tectonics and mantle convections.
In this study, we inverted the data from the NOMan array (e.g., Takeuchi et al. 2017, Science; Takeo et al. 2018, G3; Isse et al. 2019, EPSL; Takeuchi et al. 2020, 2023, JGR; Nagai et al. 2023, GJI) recorded for the events in the Kuril-Japan-Izu-Bonin-Mariana regions to reveal the upper mantle structures in the north-western Pacific regions. We measured the group velocities of the fundamental mode (with the periods of 20-100 sec) and the first overtone (with the period of 7.5-12 sec) and obtained the S-velocity model. The observed data (i.e., the group velocity maps) of the fundamental mode at longer periods (> 50 secs) show pronounced low-velocities in the outer-rise regions. These observations suggest low-velocity anomalies in the asthenosphere, and our final S-wave velocity model indeed shows such features. The geographical region with the observed low-velocity anomalies well agrees with the region with the anomalous residual bathymetry, which suggests that the observed anomalies affect the evolution of the oceanic plate.
Several mantle convection simulations show the broad high-temperature anomalies beneath the older plate in the convections with internal heating (e.g., Lowman et al. 2001, GJI; King et al. 2002, EPSL; Korenaga et al. 2021, JGR). Our results suggest that such higher temperatures prevent the older oceanic plate from getting cold and sinking deeper. Because the regions with past hotspot volcanisms or oceanic plateau are assumed a anomalous regions in several previous studies (e.g., Korenaga and Korenaga 2008, EPSL), the suggested region may be assumed a new type of anomalous oceanic mantle. This anomalous mantle may be able to explain several unique tectonics in this region such as petit-spot. It may also affect the subduction of the Pacific plate by altering the viscosity of the sub-slab region. Further understanding of the outer-rise dynamics appears to be one of the most important future research topics in geodynamics and marine geophysics.
In this study, we inverted the data from the NOMan array (e.g., Takeuchi et al. 2017, Science; Takeo et al. 2018, G3; Isse et al. 2019, EPSL; Takeuchi et al. 2020, 2023, JGR; Nagai et al. 2023, GJI) recorded for the events in the Kuril-Japan-Izu-Bonin-Mariana regions to reveal the upper mantle structures in the north-western Pacific regions. We measured the group velocities of the fundamental mode (with the periods of 20-100 sec) and the first overtone (with the period of 7.5-12 sec) and obtained the S-velocity model. The observed data (i.e., the group velocity maps) of the fundamental mode at longer periods (> 50 secs) show pronounced low-velocities in the outer-rise regions. These observations suggest low-velocity anomalies in the asthenosphere, and our final S-wave velocity model indeed shows such features. The geographical region with the observed low-velocity anomalies well agrees with the region with the anomalous residual bathymetry, which suggests that the observed anomalies affect the evolution of the oceanic plate.
Several mantle convection simulations show the broad high-temperature anomalies beneath the older plate in the convections with internal heating (e.g., Lowman et al. 2001, GJI; King et al. 2002, EPSL; Korenaga et al. 2021, JGR). Our results suggest that such higher temperatures prevent the older oceanic plate from getting cold and sinking deeper. Because the regions with past hotspot volcanisms or oceanic plateau are assumed a anomalous regions in several previous studies (e.g., Korenaga and Korenaga 2008, EPSL), the suggested region may be assumed a new type of anomalous oceanic mantle. This anomalous mantle may be able to explain several unique tectonics in this region such as petit-spot. It may also affect the subduction of the Pacific plate by altering the viscosity of the sub-slab region. Further understanding of the outer-rise dynamics appears to be one of the most important future research topics in geodynamics and marine geophysics.