断裂带同震温度异常机制—以集集、汶川及日本东北地震为例

The coseismic temperature responses can be recognized in the borehole temperature measurements in fault zones after earthquakes. It provides an unique idea and an effective mean for basic research about the friction features and seismogenic mechanisms of fault zones. The frictional heating is considered to be the only mechanism of coseismic temperature response. But the recent experiment results indicate that 1) coseismic stress release can result temperature decreasing and its energy magnitude may be equal to that of frictional heating; 2) the increase of surface free energy would induce temperature rising nearby the new fracture surface..In order to reveal the mechanisms of coseismic temperature responses in fault zones, firstly, we plan to systematically carry out a set of experiments, including the temperature responses of water-satuated rocks during stress changes and raptures by stretching and compressing, based on our previous understanding about the temperature response of dry rocks to stress change. Then in the light of the new understandings of these experiments, we would analyze the borehole temperature anomalies after the 1999 Chi-Chi, 2008 Wenchuan and 2011 Tohoku earthquakes by the numerical model combining the stress and temperature fields. It would be useful for identifying the ranges and magnitudes of temperature anomalies caused by different mechanisms of coseismic temperature responses, and accurately calculate the coseismic friction heat and coefficient of dynamic friction of each fault zone. This work would provide theoretical basis for better understanding friction features, the seismogenic mechanisms and seismic energy distribution. Thus, it plays a significant role for for protecting against and mitigating earthquake disasters in the future.

断裂带同震温度响应可在震后钻孔测温中识别，为摩擦特性与发震机制研究提供了独特思路和有效手段。摩擦生热被广泛视为同震温度响应的唯一机制。但近年来实验结果表明：1)同震应力释放会导致降温，能量量级可能与摩擦生热相当；2)破裂面表面自由能增大也有导致升温的迹象。.为深入研究断裂带同震温度响应机制，本申请拟基于前期干燥岩样绝热应力-温度响应实验结果，首先系统开展饱和水岩样绝热应力-温度响应实验、岩石拉伸破裂和挤压破裂过程中的温度响应实验；在实验新认识的基础上，再结合应力-温度耦合数值模型，分析1999年集集、2008年汶川及2011年日本东北地震后的温度异常，力求准确区分各机制导致的温度异常范围和大小，以便从震后各钻孔测温结果中准确提取摩擦热导致的温度正异常信息，重新估算各断裂带同震摩擦热和摩擦系数。这为深入了解活动断裂带的摩擦特性、发震机制及地震能量分配提供理论依据，对今后防震减灾具有指导意义。

围绕本项目研究目标，不仅按原计划开展了：1）饱和水岩样应力变化温度响应理论与实验研究；2）岩石破裂过程中温度响应实验；3）孕震断层发震过程中，两种同震温度响应机制（应力降导致的温度降与摩擦生热）对断裂带温度场演化的耦合模拟，基本完成原定任务。而且开展了俯冲带沉积物摩擦特性实验、嘉黎断裂带温泉长期观测。获得如下主要认识：1）当岩石孔隙度大于5%，其饱水岩样的绝热应力-温度响应系数大于干燥时，也即，βwet>βdry。其根本原因在相同温压环境下，孔隙水的绝热应力-温度响应系数βw (14~32 mK/MPa)比干燥岩样βdry(1.5~6.2 mK/MPa)高1个数量级；2）在准静态加载过程中，南海北部海底表层沉积物的摩擦系数在0.46~0.51之间，这很可能意味着俯冲带应力积累过程中，板块间的耦合强度并不低；而且沉积物呈现位移强化和速度强化特征，也即不利于破裂传播；4）嘉黎断裂带温泉长期观测结果表明：断裂带温泉是地表与深部天然的连系通道，使得温泉口相对较容易尽早观测到深部各自活动或变化；5）断层发震时，不仅会因滑移摩擦和塑性变形导致破裂面及邻区温度升高；同时在远离破裂面区也会因同震应力释放导致温降，其峰值较低，但分布范围却较宽。温度正、负异常区对应的总能量基本一致。.由此可知，本项目研究进展与结果，不仅拓宽了我们对同震温度响应机制的认知，这是后续钻孔测温数据解译的科学依据，可直接用于孕震带摩擦特性研究，而且本项目工作中意外地观测到的岩石破裂前~100 s内出现了由于蠕变生热导致的快速升温，暗示着孕震带地温长期监测，或许可捕捉到地震前兆，以便发出地震预警信号。这些基础认识，不仅将强有力的推动我国地震机制与断层动力学研究，同时可直接应用有我国防震减灾工作。