5:15 PM - 7:15 PM
[SCG59-P04] Physical properties of basalt and peperite at petit-spot volcanoes: preliminary results
Keywords:Petit-spot volcano, Mechanical strength, Seismic velocity, Japan Trench
Petit-spot volcanoes (Hirano et al., 2006; 2008), which are known to be scattered in the Northwest Pacific, cause thermal and material perturbations to the geological and petrological structures on the Pacific plate. These perturbations may arrest the slip zone of megathrust earthquakes in the Japan Trench; especially, annealed sediments (peperite) formed by intrusion of petit-spot volcanoes into sediments are hypothesized to serve as mechanical barrier on the plate boundary fault (Fujie et al., 2020; Akizawa et al., 2022). However, mechanical properties of peperite as well as basalts that form the main body of petit-spot volcanoes are not yet understood, and therefore, their effects on megathrust earthquakes in the Japan Trench remain unclear. In this study, we conducted triaxial compression tests using basalt and peperite collected from petit-spot volcanoes in the Northwest Pacific by the submersible Shinkai 6500 during YK24-10S cruise in 2024 to investigate physical properties of basalt and peperite at petit-spot volcanoes.
Experiments were conducted using intra-vessel deformation and fluid flow apparatus at Hiroshima University, with a maximum confining pressure of 25 MPa and upstream pore pressure of 1 MPa and downstream released to the atmospheric pressure. For some samples we applied differential stress and measured triaxial fracture strengths. Before and after the experiment, μX-ray CT images of each sample were taken to observe the deformation structure, and the P-wave velocity was measured under atmospheric pressure conditions before the experiment. Porosity was also measured using small fragments from the same rock sample from which the experimental samples were taken.
P-wave velocities at atmospheric pressure were 4.4-4.8 km/s for basalts and 1.9 km/s for peperites, and porosities were 43-45% for basalts and 57-59% for peperites. The P-wave velocities and porosities of the basalts are similar to those of volcanic rocks erupted from other submarine volcanoes around Japan (Akamatsu et al., 2025). Although the peperites were consolidated enough to be hand-held and cored, their P-wave velocities were not higher than those of sediments with similar porosity. Basalt fractured at an effective confining pressure of 9.5 MPa with a differential stress of 42.2 MPa. Under the same effective pressure conditions, permeability of the basalt before deformation was~0.5×10−16 m2, but it increased to ~1.5×10−16 m2 after its fracture. Permeability of undeformed basalt was almost the same at an effective confining pressure of 19.5 MPa. Peperite was plastically deformed only by applying effective pressure of 4.5 MPa. Permeability of peperite was ~0.5×10−17 m2, which was lower than that of basalt. This result suggests that peperites do not have high mechanical strength, as inferred from the P-wave velocity.
Acknowledgements: We would like to thank crews of R/V Yokosuka and submersible Shinkai 6500 and on-board scientists of YK24-10S cruise for their assistance for samplings from petit-spot volcanoes.
Experiments were conducted using intra-vessel deformation and fluid flow apparatus at Hiroshima University, with a maximum confining pressure of 25 MPa and upstream pore pressure of 1 MPa and downstream released to the atmospheric pressure. For some samples we applied differential stress and measured triaxial fracture strengths. Before and after the experiment, μX-ray CT images of each sample were taken to observe the deformation structure, and the P-wave velocity was measured under atmospheric pressure conditions before the experiment. Porosity was also measured using small fragments from the same rock sample from which the experimental samples were taken.
P-wave velocities at atmospheric pressure were 4.4-4.8 km/s for basalts and 1.9 km/s for peperites, and porosities were 43-45% for basalts and 57-59% for peperites. The P-wave velocities and porosities of the basalts are similar to those of volcanic rocks erupted from other submarine volcanoes around Japan (Akamatsu et al., 2025). Although the peperites were consolidated enough to be hand-held and cored, their P-wave velocities were not higher than those of sediments with similar porosity. Basalt fractured at an effective confining pressure of 9.5 MPa with a differential stress of 42.2 MPa. Under the same effective pressure conditions, permeability of the basalt before deformation was~0.5×10−16 m2, but it increased to ~1.5×10−16 m2 after its fracture. Permeability of undeformed basalt was almost the same at an effective confining pressure of 19.5 MPa. Peperite was plastically deformed only by applying effective pressure of 4.5 MPa. Permeability of peperite was ~0.5×10−17 m2, which was lower than that of basalt. This result suggests that peperites do not have high mechanical strength, as inferred from the P-wave velocity.
Acknowledgements: We would like to thank crews of R/V Yokosuka and submersible Shinkai 6500 and on-board scientists of YK24-10S cruise for their assistance for samplings from petit-spot volcanoes.