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

用温度测量断层应力：汶川7.9级地震断层带数据分析

• 批准号: 1220642
• 负责人: Emily Brodsky
• 金额: $ 25.38万
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-07-15 至 2016-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1220642&HistoricalAwards=false
• 关键词: Using; Temperature; Measure; Fault; Stress

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.

毁灭性的1999年汶川逆冲地震产生了300米长的地表破裂，平均偏移约5米。断层的抵抗应力是否受拜尔利控制？根据摩擦定律，这种滑动应该会导致大量的能量以热量的形式耗散。汶川断裂带科学钻探工程在震后2年内成功测量了多个温度剖面。在589米深的主要断裂带相交的一个先导孔中，在1.5年的时间内采集了13个高质量的重复温度测井。此外，在套管完井之前进行温度测井，并在剖面运行之间的几个点进行连续测量。对数据的初步检查和建模表明，断层上的任何瞬时温度异常都小于0.05摄氏度，这表明地震期间的等效摩擦系数小于0.05，这意味着地震完全释放了板块运动所储存的应力。然而，初步的模拟没有考虑到几个重要的影响，如平流和断层结构。该项目的目标是确定地震期间断层上的应力的实际值，方法是：（1）采用更复杂的数值模型，其中包括沿着断层的水流的冷却效应，以便更好地解释钻孔数据;以及（2）通过收集和分析与最突出的断层带在1247- 1248处相交的第二钻孔中的新的温度和压力数据，1250米，并延伸到1400米的总深度，并在第三个钻孔计划在2013年完成。地震是运动的结果，构造板块和驱动力是很好的理解。然而，阻碍断层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的主要未知参数之一。在高滑动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。该项目将利用穿透汶川地震断层的钻孔中新的和计划中的温度测量，以便确定地震期间的摩擦系数。