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首页> 外文期刊>Pure and Applied Geophysics >Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan - I. A Simple Damage Structure Inferred from Borehole Core Permeability
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Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan - I. A Simple Damage Structure Inferred from Borehole Core Permeability

机译：日本野岛平林县野岛断层的几何形状-I.从钻孔岩心渗透率推断出的简单破坏结构

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The 1995 Kobe (Hyogo-ken Nanbu) earthquake, M = 7.2, ruptured the Nojima fault in southwest Japan. We have studied core samples taken from two scientific drillholes that crossed the fault zone SW of the epicentral region on Awaji Island. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75 m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624 m. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302 m to the SE of the fault and crossed fault strands below a depth of 1140 m. We have measured strength and matrix permeability of core samples taken from these two drillholes. We find a strong correlation between permeability and proximity to the fault zone shear axes. The half-width of the high permeability zone (approximately 15 to 25 m) is in good agreement with the fault zone width inferred from trapped seismic wave analysis and other evidence. The fault zone core or shear axis contains clays with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa effective confining pressure (10 to 30 microdarcy at in situ pressures). Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 30 to 60 microdarcy at 50 MPa effective confining pressure (300 to 1000 microdarcy at in situ pressures). Outside this damage zone, matrix permeability drops below 0.01 microdarcy. The clay-rich core material has the lowest strength with a coefficient of friction of approximately 0.55. Shear strength increases with distance from the shear axis. These permeability and strength observations reveal a simple fault zone structure with a relatively weak fine-grained core surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a high-permeability conduit for vertical and horizontal flow in the plane of the fault. The fine-grained core region, however, will impede fluid flow across the fault.
机译：1995年的神户（兵库县南部南部）地震M = 7.2，使日本西南部的野岛断裂破裂。我们研究了从两个科学钻孔中获取的岩心样品，这些钻孔穿过淡路岛上震中区域的断层带SW。由日本地质调查局（GSJ）钻出的较浅的孔是从野岛断裂表面痕迹的东南方向开始75 m，并在624 m处越过该断裂。由美国地球科学与灾难预防研究所（NIED）钻出的一个更深的孔距断层东南302 m，越过断层股线的深度小于1140 m。我们已经测量了从这两个钻孔获取的岩心样品的强度和基体渗透率。我们发现渗透率与到断层带剪切轴的接近度之间有很强的相关性。高磁导率带的半峰宽度（约15至25 m）与根据陷波分析和其他证据推断出的断层带宽度非常吻合。断层带芯或剪切轴包含的粘土在有效围压为50 MPa时的渗透率约为0.1到1微达西（在原位压力下为10到30微达西）。在断层带核心几米的范围内，岩石高度破裂，但净剪切力很小。在50 MPa有效围压下，该区域的基质渗透率约为30到60微达西（在原位压力下为300到1000微达西）。在此损坏区域之外，基质渗透率降至0.01微达西以下。富含粘土的芯材具有最低的强度，摩擦系数约为0.55。剪切强度随与剪切轴的距离的增加而增加。这些渗透率和强度的观测结果揭示了一个简单的断层带结构，其相对较弱的细粒岩心被裂隙性岩石的破坏区所包围。在这种情况下，损坏区域将充当断层平面内垂直和水平流动的高渗透率管道。但是，细粒度的岩心区域将阻止流体流过断层。

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