Co-seismic damage of the Nojima fault one year after the 1995 Nanbu-Kobe earthquake

After the devastating M_w 7.3 Nanbu-Kobe earthquake of 1995, a 750 m deep borehole was drilled by the Geological Survey of Japan to intersect the Nojima fault at depth. The Hirabayashi borehole intersected the fault core at 625 m in 1996, less than one year after the earthquake. Such a short span provides an exceptional opportunity to assess the co-seismic damage generated by this earthquake.

As part of the AlterAction project (https://anr-alteraction.osug.fr/), which assesses the interplay between the alteration and deformation of faults embedded in crystalline rocks under hydrothermal conditions, several core samples were collected at regular intervals along the Hirabyashi borehole. X-ray CT scans with a voxel size of ~50 μm  were systematically performed on the samples.

This allows to quantify the fracture pattern across the fault zone. The scans show a network of open fractures, whose density increases when approaching the principal slip zone at 625 m, suggesting that these fractures are related to fault activity. The damage appears symmetric extending ~70m above and below the fault (corresponding to an effective thickness of ~15m given the low angle between the borehole and the fault plane). The relative volume occupied by the open fractures strongly decreased in the vicinity of the principal slip zone. This is related to the sealing of the fracture network, which is assumed to have occurred during the short interval between the 1995 earthquake and the time of drilling.

Several samples exhibited a dense and diffuse fracture pattern, but with very moderate deformation. Such a damage pattern is reminiscent to the “pulverized rocks” found at the surface near active faults. This would provide the first direct evidence of pulverization at depth in the Nojima Fault and confirms that a high strain rate was achieved during the 1995 Nanbu-Kobe earthquake.