An Ancient >200 m Cumulative Normal Faulting Displacement Along the Futagawa Fault Dextrally Ruptured During the 2016 Kumamoto, Japan, Earthquake Identified by a Multiborehole Drilling Program

Abstract

The Southwest Japan arc and Ryukyu arc are located on the subduction zone where the Philippine Sea plate subducts beneath the Eurasian plate (or the Amurian subplate) at the Nankai Trough and Ryukyu Trench, respectively (Kamata & Kodama, 1999; Figure 1a). The Philippine Sea plate subducted beneath Kyushu from ∼6 Ma, and was rotated in the counterclockwise direction from north-northwest to west-northwest at ∼2 Ma (e.g., Kamata & Kodama, 1999;Wu et al., 2016). Volcanism in central Kyushu related to subduction of the Philippine Sea plate began at ∼5 Ma (Nakada & Kamata, 1991). At the junction of the two arcs, subduction of fluid-rich oceanic crust and sediments introduced large volumes of fluids into the Kyushu arc system, leading to voluminous volcanism and a locally extensional stress regime (Mahony et al., 2011). As a result, the Aso Volcano, one of the greatest

Estimation of Porosity for Sedimentary Rocks and Volcanic Rocks from Sonic Log Data in the Futagawa Fault Drilling

“…This value suggests that the porosity is not only due to the pores in the homogeneous massive lava but also that the porosity is slightly higher due to the open fractures, which are not contained in the measured core samples. The fracture density from core descriptions is high, with values in the range of 10-14 cracks/m, 9 ) and the porosity increases to…”

“…The geophysical logging data were obtained from the FDB-1 borehole of the FFDP conducted by Kyoto University and supported by a consignment study of the Nuclear Regulation Authority. 8 ) , 9 ) In the first stage, borehole FDB-1 was drilled once to a depth of 302 m, and then full casing and cementing were applied to fill the gap between the casing pipe and rock wall down to the bottom. In the second stage, subsequent drilling was performed to a total depth of 666 m with full core collection.…”

“…In the FDB-1 borehole, three damage/slip zones penetrate at depths of 354, 461, and 576 m, and a high fracture density is present in volcanic rock intervals. 8 ) , 9 ) The lithology, rock type (rock species), and P-wave velocity of the sedimentary and volcanic rocks identified in borehole FDB-1 based on the results of core descriptions from previous studies 8 ) , 9 ) are shown in The conglomerate formation (conglomerate I) at depths ranging from 328-354 m is a semiconsolidated conglomerate containing many andesitic rounded gravels with sizes ranging from 10-30 cm and P-wave velocities ranging from 2.5-3.0 km/s. At Fault 354, the P-wave velocity decreases distinctly below 2 km/s.…”

“…The fracture density was also measured based on core sample descriptions. 9 ) The FDB borehole consisted of the FDB-1 and FDB-1R boreholes. FDB-1 was first drilled vertically to a total depth of 666 m, and the FDB-1R borehole that branched from FDB-1 was drilled at an inclination of approximately 1° from a depth of 454 m in FDB-1 to a depth of 692 m. Geophysical logging, including sonic logging, was conducted in FDB-1 before drilling FDB-1R (FDB in Fig.…”