15:30 〜 15:45
[SCG40-16] Multiscale crack distribution in subsuction zone and its implication for scale dependence of seismic velocity
★Invited Papers
キーワード：地震波速度、南海トラフ、クラック、スケール依存性、不均質性
Numerous seismic surveys have been conducted to uncover the geophysical structure and fluid distribution in subduction zones, since seismic velocities are primarily influenced by pore fluids within rocks. As observed seismic waves typically have frequencies of a few hertz, the resulting seismic velocities reveal structures ranging from several hundred meters to several kilometers in size. Since natural geological systems contain multiple scales of porosity structure, including microscopic and mesoscopic cracks, evaluating the distributions of multiscale cracks at the scale of seismic wavelength is essential for quantifying its impact on seismic velocity.
In this study, continuous elastic wave velocities measured through laboratory and borehole logging were analyzed using core samples collected from the Susaki area in the Shimanto accretionary complex (Itaba et al. 2014), to assess the distribution of micro- and mesoscale cracks. The Susaki core primarily comprises sandstone and mudstone experienced maximum paleo-temperatures of ~200°C, which likely corresponds to the hanging wall of the megaspray fault in the Nankai Trough (Okuda et al. C000023, JpGU2024). We measured P-wave velocities of 40 m-long continuous cores at 5 cm intervals at an ultrasonic frequency of 1 MHz. The measurements were performed under dry conditions at room temperature and pressure.
The measured P-wave velocities ranged from 4–6 km/s, with a highly heterogeneous spatial distribution. Given the crack-free P-wave velocity of the Susaki core samples estimated from the mineral composition to be ~6 km/s, the variations in the measured P-wave velocities indicate the effect of microscale porosity. Effective medium theory was then employed to estimate the crack porosity in the core and to predict P-wave velocity under water-saturated conditions. Results revealed crack porosity ranging from 0.1% to 0.8%, with predicted P-wave velocities under water-saturated conditions consistently exceeding ~5.5 km/s. On the other hand, P-wave velocities measured at sonic frequencies with borehole logging exhibited 4–5.5 km/s. These values are consistently smaller than those predicted from microscopic porosity, despite that the borehole is expected to be saturated with groundwater based on low electrical resistivity. This discrepancy suggests the influence of mesoscale porosity, which may not be accounted for in the laboratory measurements. Effective medium theory was again employed to estimate the mesoscale crack distribution, yielding mesoscale crack porosity ranges from 0.5% to 2%. These values are consistently higher than microscale crack porosity at most depth intervals. Although the estimated crack porosities may not necessarily reflect their distribution at in situ seismogenic zone, our results indicate the importance of mesoscale cracks for seismic velocity interpretations and the their potential role in assessing fluid behavior at subduction zone.
Itaba, S., Y. Umeda, N. Koizumi, H. Watanabe, N. Nakayama, S. Sakai (2014), Geological Data of the GSJ boring core at the Susaki-Otani Observation Station. GSJ Openfile Report, 595, Geol. Surv. Japan, AIST.
In this study, continuous elastic wave velocities measured through laboratory and borehole logging were analyzed using core samples collected from the Susaki area in the Shimanto accretionary complex (Itaba et al. 2014), to assess the distribution of micro- and mesoscale cracks. The Susaki core primarily comprises sandstone and mudstone experienced maximum paleo-temperatures of ~200°C, which likely corresponds to the hanging wall of the megaspray fault in the Nankai Trough (Okuda et al. C000023, JpGU2024). We measured P-wave velocities of 40 m-long continuous cores at 5 cm intervals at an ultrasonic frequency of 1 MHz. The measurements were performed under dry conditions at room temperature and pressure.
The measured P-wave velocities ranged from 4–6 km/s, with a highly heterogeneous spatial distribution. Given the crack-free P-wave velocity of the Susaki core samples estimated from the mineral composition to be ~6 km/s, the variations in the measured P-wave velocities indicate the effect of microscale porosity. Effective medium theory was then employed to estimate the crack porosity in the core and to predict P-wave velocity under water-saturated conditions. Results revealed crack porosity ranging from 0.1% to 0.8%, with predicted P-wave velocities under water-saturated conditions consistently exceeding ~5.5 km/s. On the other hand, P-wave velocities measured at sonic frequencies with borehole logging exhibited 4–5.5 km/s. These values are consistently smaller than those predicted from microscopic porosity, despite that the borehole is expected to be saturated with groundwater based on low electrical resistivity. This discrepancy suggests the influence of mesoscale porosity, which may not be accounted for in the laboratory measurements. Effective medium theory was again employed to estimate the mesoscale crack distribution, yielding mesoscale crack porosity ranges from 0.5% to 2%. These values are consistently higher than microscale crack porosity at most depth intervals. Although the estimated crack porosities may not necessarily reflect their distribution at in situ seismogenic zone, our results indicate the importance of mesoscale cracks for seismic velocity interpretations and the their potential role in assessing fluid behavior at subduction zone.
Itaba, S., Y. Umeda, N. Koizumi, H. Watanabe, N. Nakayama, S. Sakai (2014), Geological Data of the GSJ boring core at the Susaki-Otani Observation Station. GSJ Openfile Report, 595, Geol. Surv. Japan, AIST.