5:15 PM - 6:45 PM
[MGI31-P01] On physical properties of sedimentary rocks in the Susaki area, Shimanto accretionary complex: possible nature of the hanging wall of décollement in the seimogenic zone
Keywords:Seismic wave velocity structure, Nankai Trough, High velocity zone
Seismic wave velocity structure is a fundamentally important information for understanding the natures of rock, stress, fluid, and seismogenesis in subduction zones. Dense seismic reflection surveys and seismological observations allowed us to precisely resolve spatial distribution and heterogeneity of seismological structures down to the seismogenic zone or sometimes deeper, although these need to assume rock physical properties to interpret the obtained seismic wave velocity structures. Scientific drilling can directly measure seismic wave velocities by logging techniques and obtain rocks directly from the depths, but reaching depths deeper than ~3 km below seafloor is technically difficult at the moment. Measurements of physical properties of rocks from on-land accretionary complexes which experienced the seismogenic zone in the past can be an effective approach for the interpretation of geophysical observations of the seismogenic zone.
In this study, we use core samples of sandstones and mudstones from the Susaki area in the Shimanto accretionary complex near Yokonami mélange (Itaba et al., 2014) where the rocks experienced ~200 °C as the paleo maximum temperature, and measured porosities of the samples and P- and S-wave velocities (Vp and Vs) in dry and water-saturated conditions at 1 MHz under atmospheric pressure. In general, sandstones have lower porosities (0.5-1.2%) and higher Vp (4.6-5.8 km/s for dry cases; 5.4-6.1 km/s for wet cases) and Vs (3.1-3.5 km/s for dry cases; 3.1-3.4 km/s for wet cases), whereas mudstones have higher porosities (1.6-3.7%) and lower Vp (4.5-5.5 km/s for dry cases; 4.9-5.9 km/s for wet cases) and Vs (2.6-3.4 km/s for dry cases; 2.5-3.3 km/s for wet cases). These obtained data are denser and stiffer than previously reported Vp and Vs data for rocks in other locations in the Shimanto accretionary complexes, and cannot be explained by the empirical Vp-porosity relationship constructed from drilled samples in the Nankai accretionary complex and the Shimanto accretionary complexes.
The obtained high Vp and Vs suggest that the rocks used in this study may correspond to the high velocity zone in the present Nankai Trough, which is located above the megasplay fault at the seismogenic zone depth. Since this high velocity zone is in the hanging wall of the megasplay fault and will store strain energy during interseismic periods, physical properties of the rocks are required to understand earthquake dynamics such that how strain energy is accumulated and released. Toward the understanding of their physical properties, precise measurements and considerations on internal structures and mineralogical information are now underway.
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, we use core samples of sandstones and mudstones from the Susaki area in the Shimanto accretionary complex near Yokonami mélange (Itaba et al., 2014) where the rocks experienced ~200 °C as the paleo maximum temperature, and measured porosities of the samples and P- and S-wave velocities (Vp and Vs) in dry and water-saturated conditions at 1 MHz under atmospheric pressure. In general, sandstones have lower porosities (0.5-1.2%) and higher Vp (4.6-5.8 km/s for dry cases; 5.4-6.1 km/s for wet cases) and Vs (3.1-3.5 km/s for dry cases; 3.1-3.4 km/s for wet cases), whereas mudstones have higher porosities (1.6-3.7%) and lower Vp (4.5-5.5 km/s for dry cases; 4.9-5.9 km/s for wet cases) and Vs (2.6-3.4 km/s for dry cases; 2.5-3.3 km/s for wet cases). These obtained data are denser and stiffer than previously reported Vp and Vs data for rocks in other locations in the Shimanto accretionary complexes, and cannot be explained by the empirical Vp-porosity relationship constructed from drilled samples in the Nankai accretionary complex and the Shimanto accretionary complexes.
The obtained high Vp and Vs suggest that the rocks used in this study may correspond to the high velocity zone in the present Nankai Trough, which is located above the megasplay fault at the seismogenic zone depth. Since this high velocity zone is in the hanging wall of the megasplay fault and will store strain energy during interseismic periods, physical properties of the rocks are required to understand earthquake dynamics such that how strain energy is accumulated and released. Toward the understanding of their physical properties, precise measurements and considerations on internal structures and mineralogical information are now underway.
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.