Unzen Volcano Scientific Drilling: structure and stress state inferred from downhole measurements in the flank drilling

摘要

A scientific drilling is expanding it's challenge to target the magma conduit of the Unzen volcano shortly after the 1990-95 eruption. Final goal of this project is to clarify the ascending and degassing mechanism of magma and to evaluate models propounded for the eruption. Preceding the conduit drilling, two bore-holes were drilled in the flank of the volcano to reveal the structure and the growth history of the volcano. We conducted well logging, in-situ stress measurements and an injection test in these boreholes. Variations in physical properties of rocks were revealed by the well logging, which are correlated with small-scale changes in lithology (alternating between mud flows, lava blocks, and pyroclastic flows) over depth intervals ranging from 1 to 20 m. A fluid injection and temperature recovery test indicated three layers exist: a relatively impermeable mud flow layer, an unconfined and highly permeable block-and-ash flow aquifer, and a thick and relatively impermeable sedimentary layer. The magnitudes of the maximum and minimum horizontal compressive stresses, SHmax and Shmin, were determined by the in-situ stress measruements using the hydraulic fracturing method. The magnitude of SHmax is close to the vertical stress, indicating a transitional normal to strike-slip faulting regime. In addition, the SHmax azimuth is almost parallel to the Beppu-Shimabara graben, in which the Unzen volcano resides, and coincides with the SHmax azimuth estimated from focal mechanisms of shallow earthquakes. Thus, the current orientation of Shmin is nearly optimal for driving north-south extension across the Beppu-Shimabara graben. Furthermore, since this Shmin direction is nearly perpendicular to the inferred orientation of the feeder dike for 1990-95 Unzen eruption, it appears that the current in-situ stress field plays an important role in controlling the dynamics of magma ascent beneath Unzen. These measurements are indispensable to better understanding for the dynamics of magma emplacement and degassing, the physics of volcanic seismicity, and mechanical controls on the orientation and magnitude of fracture permeability.