2:30 PM - 2:45 PM
[SSS10-20] Thermal Pressurization in Shallow Fault Zone in the Borehole Core of the Neodani Fault
Keywords:the Neodani Fautl, latest slip zone, shallow fault zone, thermal pressurization
The Neodani Fault is a left-lateral active fault located in the northwestern part of Gifu Prefecture, Central Japan and ruptured during the 1891 Nobi Earthquake, which is one of the largest inland earthquakes with a magnitude of 8.0. Recently, the drillings penetrating to the Neodani Fault were conducted in Neo-Midori, Motosu City by the Nuclear Regulation Authority. The borehole NDFD-1-S1 penetrates perpendicular to the surface trace of the Neodani fault, and penetrated the latest slip zone at a depth of 382 m. Core samples from this borehole are expected to clarify the characteristics and phenomenon occurred in the latest slip zone. We performed scanning electron microscope (SEM) observation of rock thin sections by backscattered electron (BSE) image and color mapping by electron probe microanalyzer (EMPA) to observe the occurrence and clarify phenomenon accompanied with seismic slips in the latest slip zone.
The drilling site is Neo-Midori, where the fault is divided into two in this area, and the area between these two is the compressional fault jog. The vertical displacement at the boundary of the fault jog is 6 m in the 1891 earthquake. The Mélange with a matrix of mudstone of Jurassic accretionary prism is distributed in the drilling site, and the core is composed of a matrix of mudstone, blocks of basalt and chert. The latest slip zone in the borehole core is characterized by straight and continuous gouge zone with extremely lower CT value. This is located in the lowest depth of the fault gouge zone, and mainly originated from basalt.
The results of a BSE observation by a SEM and color mapping by an EPMA show Ca concentration in and around the latest slip zone. Ca concentration in the latest slip zone is locally occurred and it shows sub-spherical shape. In fault gouges adjacent to the latest slip zone, the zone including Ca is recognized. This zone mainly contains of a lot of calcite crystals with a diameter of several μm, quartz fragments, and a matrix of gouge. Some calcite grains show weak euhedral shape. In addition, quartz fragments and fine-grained calcite intrudes into a fault gouge.
These occurrences suggest the increase of temperature-pressure and fine-grained calcite precipitationdue to thermal pressurization during seismic slips. These calcite grains located on the western side of the latest slip zone, and it exhibited relatively subsidence during the 1891 earthquake. Considering maximum 1 km of surface erosion of the geographic features near the trace of the Neodani Fault, the above phenomenon occurred shallower than 1.4 km depth, if the western side were always relatively subsidence in the past earthquakes.
The drilling site is Neo-Midori, where the fault is divided into two in this area, and the area between these two is the compressional fault jog. The vertical displacement at the boundary of the fault jog is 6 m in the 1891 earthquake. The Mélange with a matrix of mudstone of Jurassic accretionary prism is distributed in the drilling site, and the core is composed of a matrix of mudstone, blocks of basalt and chert. The latest slip zone in the borehole core is characterized by straight and continuous gouge zone with extremely lower CT value. This is located in the lowest depth of the fault gouge zone, and mainly originated from basalt.
The results of a BSE observation by a SEM and color mapping by an EPMA show Ca concentration in and around the latest slip zone. Ca concentration in the latest slip zone is locally occurred and it shows sub-spherical shape. In fault gouges adjacent to the latest slip zone, the zone including Ca is recognized. This zone mainly contains of a lot of calcite crystals with a diameter of several μm, quartz fragments, and a matrix of gouge. Some calcite grains show weak euhedral shape. In addition, quartz fragments and fine-grained calcite intrudes into a fault gouge.
These occurrences suggest the increase of temperature-pressure and fine-grained calcite precipitationdue to thermal pressurization during seismic slips. These calcite grains located on the western side of the latest slip zone, and it exhibited relatively subsidence during the 1891 earthquake. Considering maximum 1 km of surface erosion of the geographic features near the trace of the Neodani Fault, the above phenomenon occurred shallower than 1.4 km depth, if the western side were always relatively subsidence in the past earthquakes.