5:15 PM - 7:15 PM
[SCG45-P19] Seamount Subduction and Its Impact on Shallow Tremor Activity in Hyuga-Nada using Reflection Full Waveform Inversion and Depth Imaging
Keywords:reflection full waveform inversion, seamount subduction, slow earthquake, Hyuga-nada, seismic imaging, FWI
The Hyuga-nada region, southwest of Japan, is an active zone for slow earthquakes, including tremors and very low-frequency earthquakes (VLFEs). In this region, the Kyushu-Palau Ridge (KPR) is subducting at ~6 mm/yr, potentially influencing tremor distribution and fluid migration along the megathrust. Seamount subduction has been linked to variations in both slow and fast earthquake activity, but its geometry and structure remain unclear. To investigate this relationship, we analyzed three intersecting seismic surveys (KPr1, KPr2, and KR0114-8). These survey lines cross to each other and surround the subducted seamount. We applied reflection full-waveform inversion (RFWI) and Kirchhoff pre-stack time/depth migration (PSTM; PSDM). RFWI provides high-resolution velocity models using seismic reflection data (>10 Hz), while migration methods provide seismic images of subsurface structures.
The seismic structures along these three surveys reveal correlations with tremor activity. Along KR0114-8 (Ma et al., 2024), low tremor activity is observed around the subducted seamount in the stress shadow. The tremor activity correlates with the amplitudes of the décollement. We observe low-velocity zones along splay faults branching from the décollement near the KPR, likely reflecting fluid accumulation and migration. Along the KPr2 line, we divide the profile into five zones from southwest to northeast (Figure 1). The seamount top is located in Zone 3, where tremor activity is low. We infer that this region corresponds to a stress shadow zone, consistent with observations from the KR0114-8 line. The BSR deepens in Zone 2, suggesting lower heat flow. Between Zones 4 and 5, diapiric wavy structures differ significantly; Zone 4 exhibits longer wavelengths and larger amplitudes, whereas Zone 5 displays shorter wavelengths and smaller amplitudes. The lower heat flow in Zone 2 and the smaller diapir amplitudes in Zone 5 suggest a relationship between fluid flow and velocity. Along the KPr1 line, near the trench, we identify the décollement and the plate interface and observe major thrusts and minor faults in the shallow portion near the slope basin. The slope basin, with faults extending into the possible past accretionary prism, indicates the absence of active compressional stress, which corresponds to low tremor activity. Moving landward, the décollement becomes unclear, probably due to diagenetic processes affecting the underthrust sediments. The physical properties of these sediments, inferred from Vp, are similar to those in the hanging wall. Further landward, where tremor activity is high, the décollement is more distinct with negative polarity, suggesting a stronger impedance contrast. This implies that lower velocity and overpressure in the underthrust sediments correspond to higher tremor activity. These observations are consistent with those of the KR0114-8 line and confirm that the frequency of tremor activity correlates with the amplitude of the negative reflector that corresponds to the décollement near the seamount.
The seismic structures along these three surveys reveal correlations with tremor activity. Along KR0114-8 (Ma et al., 2024), low tremor activity is observed around the subducted seamount in the stress shadow. The tremor activity correlates with the amplitudes of the décollement. We observe low-velocity zones along splay faults branching from the décollement near the KPR, likely reflecting fluid accumulation and migration. Along the KPr2 line, we divide the profile into five zones from southwest to northeast (Figure 1). The seamount top is located in Zone 3, where tremor activity is low. We infer that this region corresponds to a stress shadow zone, consistent with observations from the KR0114-8 line. The BSR deepens in Zone 2, suggesting lower heat flow. Between Zones 4 and 5, diapiric wavy structures differ significantly; Zone 4 exhibits longer wavelengths and larger amplitudes, whereas Zone 5 displays shorter wavelengths and smaller amplitudes. The lower heat flow in Zone 2 and the smaller diapir amplitudes in Zone 5 suggest a relationship between fluid flow and velocity. Along the KPr1 line, near the trench, we identify the décollement and the plate interface and observe major thrusts and minor faults in the shallow portion near the slope basin. The slope basin, with faults extending into the possible past accretionary prism, indicates the absence of active compressional stress, which corresponds to low tremor activity. Moving landward, the décollement becomes unclear, probably due to diagenetic processes affecting the underthrust sediments. The physical properties of these sediments, inferred from Vp, are similar to those in the hanging wall. Further landward, where tremor activity is high, the décollement is more distinct with negative polarity, suggesting a stronger impedance contrast. This implies that lower velocity and overpressure in the underthrust sediments correspond to higher tremor activity. These observations are consistent with those of the KR0114-8 line and confirm that the frequency of tremor activity correlates with the amplitude of the negative reflector that corresponds to the décollement near the seamount.