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

1999年发生在汶川7.9级的特大逆冲地震产生了长达300公里的地表破裂，平均错距约5米。如果断层的阻力受Byerlee-S摩擦定律的控制，这种滑动应该会导致大量的能量以热的形式耗散。汶川断裂带科学钻探工程在震后2年内成功测量了多条温度剖面。在与主要断裂带相交的589米深的先导孔中，在1.5年的时间里采集了13条高质量的重复温度记录。此外，在套管完井前进行了温度测井，并在剖面运行之间的几个点进行了连续测量。数据的初步检验和模拟表明，断层上的任何瞬时温度异常都小于0.05摄氏度，这表明地震期间的当量摩擦系数小于0.05，这意味着地震完全解除了板块运动积累的应力。然而，初步的模拟没有考虑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式确定地震期间断层上应力的实际值：(1)采用更复杂的数值模型，其中包括沿断层流动的冷却效应，以便更好地解释钻孔数据；以及(2)通过收集和分析第二个钻孔的新温度和压力数据，该钻孔与最突出的断层带相交，在1247-1250米处，延伸到总深度1400米，并计划在2013年完工。地震是构造板块运动和驱动力的结果。然而，阻碍断层上运动的阻力要差得多。地震过程中的摩擦应力是地震力学中的主要未知参数之一。在高滑动速度下控制摩擦的过程被激烈地争论着，有许多理论可能性。需要现场测量来限制实际断层上的主导过程。该项目将利用新的和计划中的钻孔温度测量，这些钻孔穿透了引发汶川地震的断层，从而能够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.