Using Temperature to Measure Fault Stress: Analysis of Data from the Fault Zone of the Mw 7.9 Wenchuan Earthquake

1999年汶川7.9级逆冲地震造成了300公里长的地表断裂，平均偏移量约为5米。断层的抗应力是否由Byerlee控制？根据摩擦定律，这种滑移应该导致大量能量以热的形式散失。汶川断裂带科学钻探工程成功测量了地震后2年内的多个温度剖面。在589米深与主要断裂带相交的导孔中，在1.5年的时间里进行了13次高质量的重复温度测井。此外，在套管完井之前进行了温度测井，并在两次下入剖面之间的几个点进行了连续测量。对数据的初步检查和建模表明，断层上的任何瞬态温度异常都小于0.05℃，这表明地震期间的等效摩擦系数小于0.05，并暗示地震完全解除了板块运动所储存的应力。然而，初步的模拟没有考虑到平流和断层结构等几个重要的影响。该项目的目标是通过以下方法确定地震时断层应力的实际值：(1)采用更复杂的数值模型，其中包括沿断层流动的水的冷却效应，以便更好地解释钻孔数据；(2)通过收集和分析第二个井眼的新温度和压力数据，该井眼与最突出的断裂带相交1247-1250米，总深度延伸至1400米，计划于2013年完工的第三个井眼。地震是构造板块运动的结果，其驱动力已被很好地理解。然而，断层上阻碍运动的阻力受到的约束要差得多。地震时的摩擦应力是地震力学中一个主要的未知参数。在高滑移速度下控制摩擦的过程与许多理论可能性进行了激烈的辩论。需要现场测量来约束实际故障上的主导过程。该项目将利用在穿透导致汶川地震的断层的钻孔中进行的新的和计划中的温度测量，从而确定地震期间的摩擦系数。

The devastating 1999 Mw 7.9 Wenchuan thrust earthquake generated a 300 kilometers-long surface rupture with an average offset of about 5 meters. If the resisting stress of faults is controlled by Byerlee?s friction law, this slip should have resulted in significant energy dissipated as heat. The Wenchuan Fault Zone Scientific Drilling Project has successfully measured multiple temperature profiles within 2 years after the earthquake. In a pilot hole intersecting the major fault zone at 589 m depth, 13 high-quality repeated temperature logs were taken over a 1.5 year period. In addition, temperature logs were performed before casing completion and continuous measurements were made at a few points between profile runs. Preliminary inspection and modeling of the data indicates that any transient temperature anomaly over the fault is less than 0.05 degrees C, which suggests that the equivalent coefficient of friction during the earthquake is less than 0.05 and, by implication, that the earthquake completely relieved the stress stored up from plate motion. However, the preliminary modeling did not account for several important effects such as advection and fault structure. The goal of this project is to determine the actual value of the stress on the fault during the earthquake by: (1) employing a more sophisticated numerical model that includes the cooling effect of water flow along the fault in order to better interpret the borehole data; and (2) by collection and analysis of new temperature and pressure data in a second borehole that intersects the most prominent fault zone at 1247-1250 meters and extends to 1400 meters total depth and in a third borehole planned for completion in 2013.Earthquakes are the result of motions of the tectonic plates and the driving forces are well understood. However, the resisting force impeding the motion on the faults is much more poorly constrained. This frictional stress during an earthquake is one of the major unknown parameters in earthquake mechanics. The processes controlling friction at high slip velocities are vigorously debated with many theoretical possibilities. Field measurements are needed to constrain the dominant processes on actual faults. This project would take advantage of new and planned temperature measurements in boreholes that penetrate the fault that generated the Wenchuan earthquake that would allow determination of the coefficient of friction during the earthquake.