Spatially Fixed Initial Break Point and Fault-Rock Development in a Landslide Area

摘要

The relationship between slip behavior and fault-rock properties has been investigated from several perspectives, including rock mechanics, landsliding, and earthquake faulting. This includes mainly studies on land, but recently also from submarine settings, since increasing availably of new offshore technologies allow marine fault/slip zone drilling and monitoring. Here we present a detail study for a subaerial landslide in SW Japan, the conceptual and methodological approach of which may provide important means towards better understanding various friction behaviors including ocean margin slope creeping and submarine landslides.Thousand of landslides occur in the Japanese mountains during precipitation events, and monitoring systems have been installed for disaster management and mitigation. Slopes in many of these areas undergo repeated continuous stable creep and episodic acceleration without catastrophic failure. We monitored the dynamic processes of pore-fluid-induced episodic landslides during two typhoons associated with heavy rain in the 300 m×800 m Utsugi landslide area in the Jurassic Chichibu complex. A basal fault denning the landslide body has a maximum depth of approximately 30 m and is exposed at the surface at the top and bottom ends. Borehole pipes that penetrate the basal fault have been installed at 13 sites for monitoring tilt and groundwater levels. We used five of these sites for dynamic monitoring. In two cases, slip initiation occurred in the area where the groundwater level rose quickly during the rainfall. We analyzed the fault rocks at this site in a drill core obtained during the dry season and in another drill core obtained immediately after a slip event. The fracture zone at the slip depth is overlaid with low permeable gouge, inferred to have developed by shear grind. In the post-slip event core, the fracture zone has much higher water content than that in the dry season core. The gouge layer and fracture zone may thus act as an impervious cap rock and a conduit for fluid, respectively. The fault rock and fluid flow system likely developed over a long period, and thus can explain the repeated initiation of slip at the same location.