Direct measurements of frictional heating by earthquake faulting

一组精密温度计已经部署在离即将到来的地震断层非常近的地方。该矿场位于南非一座深度为3公里的金矿中，目前正在一个主要地质断层附近进行采矿。为了找出厚度超过20m的复杂断裂带的三维结构，我们钻了许多长约30m的孔。对岩心和钻孔视频图像进行了广泛的分析。结果表明，存在明显的软弱面，只有围岩与断裂带交界处一处约10厘米厚的部分受到严重破坏。这一特征在整个测量区域(10x10m)上是连续的，相当平坦的，并有相交的钻孔。我们可以在离这架飞机1米的范围内安装许多温度计。断裂带由来自主岩的岩化磨损物组成。然而，平面型结构非常有限。除上述边界弱面外，…此外，仅有少数平面显示出局部剪切的叶状构造。然而，只有上面提到的边界部分是如此脆弱，以至于该平面上的所有钻孔交叉点都有一圈从井壁流失的物质。这种损坏只在每个井眼中与该边界的交叉处观察到。连续记录了地面温度数据，表明地面温度稳定性较好。如果在上面确定的弱面上发生M2-3级地震，则数据质量足以解决通过滑动而产生的摩擦加热，滑动的摩擦阻力低至实验室实验中测得的摩擦阻力的十分之一。这意味着我们测量的分辨率涵盖了所谓的地壳应力悖论中所争论的可能的强度范围。本观测观测了断层周围岩石温度的时间变化，这是由断裂面的摩擦加热引起的。这需要地震后大约一个月的数据。观测必须在离断层非常近的地方进行，断层很可能受到目标地震的破坏。地震破坏很有可能使人们无法接触到观测仪器。因此，采取了许多措施，包括使用无线数据传输，以确保观测系统在数据记录、遥测和电源方面的故障安全。较少

An array of precision thermometers has been deployed very close to the fault of impending earthquakes. The site was at a 3 km depth in a deep gold mine in South Africa, where mining is taking place near a major geologic fault. We drilled many holes of about a 30 m length to find out a three dimensional structure of complex fault zone of over a 20 m thickness. Extensive analysis of core and borehole video images was carried out. The reuslt has shown that there is a distinct plane of weakness ; only of an about 10 cm thickness part at one on the boundary between the host rock and fault zone is severely damaged. This feature was continuous and quite planar over the whole area (10 x 10 m) surveyed with the intersecting boreholes. We could install many thermometers in a 1 m distance from this plane. The fault zone consisted of the lithified attrition material deriving from the host rocks. However, the plane-like structures were very limited. Besides the boundary weak plane mentioned above, … More there were only a few planes showing a foliated structures indicateve of localized shear. However, it was only the boundary part mentioend above that was so friable that all the boreholes intersections on this plane has a ring of lost material from the borehole wall. Such a damage was observed only at the intersection with this boundary in each borehole. The temperature data were continuously recorded, which showed a good stability of the ground temperature. The quality of data was good enough to resolve the frictional heating by a sliding at as low as one tenth of the frictional resistance measured in laboratory experiments, if an earthquake of M2-3, which is expected in this mine, occurred on the plane of weakness we identified above. This means that the resolution of our measurements covers the possible range of strength argued in the so-called crustal stress paradox. The present observation watches the temporal changes of rock temperature around the fault, which is caused by the frictional heating of the fault plane. This requires data for about a month following the eartquake. The observation has to be done very close to the fault, which is likely to suffer the damage from the targeted earthquake. It is quite possible that access to the observational instruments is lost by the earthquake damage. Therefore, number of measures, including the use of wireless data transfer, were taken to fail-safe the observational system, in terms of data recording, telemetry, and power supply. Less