Abstract:

As one of the most serious geological disasters, earthquake is of sudden and destructive characteristic. Therefore, it is of great significance to earthquake monitoring and early warning. The phenomenon of surface thermal infrared radiation enhancement is a common precursor of moderate and strong earthquakes and has been used as an important reference information for early warning and short term prediction. A variety of explanations have been given to understand internal mechanism of the above phenomenon, in which the stress-induced heating hypothesis is widely accepted and has been confirmed in the laboratory rock mechanical loading experiments, that is, under ideal conditions in the laboratory, the rock heats up when it is pressed and cools down when it is stretched. Under field conditions in practice, however, weak seismic precursors of thermal anomalies are often interfered by various environmental factors(solar radiation, atmospheric movement and human activities, etc.), and it has not been investigated whether the corresponding relationship between the above crustal compression-extension motions and thermal radiation anomalies can be observed under field conditions. The earth's gravity field, as one of the basic physical fields of the earth, contains the density distribution of crustal structure, which can be served to study the migration of the earth's material, the deformation of the crust and the change of the stress field. In this paper, we use GRACE gravity and MODIS thermal infrared remote sensing data to verify the stress-induced heat hypothesis in the field with Wenchuan earthquake as the time node. Firstly, the crustal mass density obtained by GRACE satellite was compared with thermal infrared radiation. Then, the gravity anomalies extraction method based on maximum shear strain and in-situ temperature method were used to obtain the gravity anomalies and thermal anomalies respectively. Furthermore, the correlation between the two anomalies before the earthquake was detected from the time scale and space scale respectively, and the consistency analysis between the above anomalies and the spatial distribution of the tectonic fault zone was carried out. For this purpose, two important indicators i.e., anomaly intensity and anomaly distribution, were established in time domain and space domain, respectively. The following conclusions could be drawn: 1)The stress-induced heating hypothesis can be verified by remote sensing in field conditions. The warming zone of the crust(positive thermal offset index)corresponds to the compression zone, and the cooling zone(negative thermal offset index)corresponds to the stretching zone. The consistency of positive and negative variation between the crustal mass density and thermal offset index is 88.9%, which provides field observation evidence for the stress-induced heating hypothesis. 2)The spatio-temporal variation of gravity anomalies and thermal anomalies before earthquake has strong correlation. In the time domain, there is a strong correlation between the gravity anomalies and the thermal anomalies, which shows that the intensity of the two anomalies suddenly increases synchronously and reaches the maximum simultaneously three months before the earthquake. In the spatial domain, gravity anomalies mostly occur at the junction of positive and negative values of thermal offset index, which indicates that the spatial distribution of gravity anomalies and thermal anomalies also has a certain correlation. In addition, the two anomalies appear to be distributed along the fault zone for many times, which shows that they are closely related to tectonic activities.

Key words: Longmenshan fault zone, Wenchuan earthquake, Gravity anomalies, Thermal anomalies, Correlation analysis

摘要：

地震作为最严重的地质灾害之一, 具有突发性和巨大的破坏性, 开展地震监测预警工作具有十分重要的意义。地表热红外辐射增强的现象是普遍存在的中强地震前兆, 目前已被作为地震监测预警与短临预报的重要参考信息。学者们对其产生的内在机理给出了多种解释。其中, 应力致热假说已被广泛接受, 并已在实验室的岩石力学加载实验中得到证实, 即岩石受力挤压时升温、 拉张时降温, 但这种地壳的挤压拉张运动和热辐射异常间的对应关系能否在野外条件下被观测到, 一直以来尚未有相关研究报道。为此, 文中采用GRACE重力和MODIS热红外2种卫星遥感数据, 以构造应力变化明显的大地震——汶川地震为时间节点开展应力致热假说的野外遥感验证研究。首先, 借助GRACE卫星反演得到的地壳质量密度进行与热红外辐射增温的比对分析; 然后, 分别采用基于最大切应变的重力异常提取方法和原地温度法获得重力异常和热异常, 并分别从时间尺度和空间尺度上检测震前重力异常和热异常的关联性, 对二者与构造断裂带的空间展布进行一致性分析, 得到以下结论: 1)应力致热假说在野外条件下能够被遥感手段验证。地壳的升温区(热偏移指数为正)与挤压区(地壳质量密度增加)、 降温区(热偏移指数为负)与拉张区(地壳质量密度减少)皆高度对应, 二者正、 负变化的一致性高达88.9%, 这为应力致热假说提供了野外观测证据。2)震前重力异常和热异常的时空变化具有较强的关联性。在时间域上, 重力异常和热异常具有较强的相关性, 主要表现为在震前3个月, 2种异常的强度同步出现了突增现象, 并同时达到最大值。在空间域上, 重力异常多出现在热偏移指数值的正、 负值交界处, 这表明重力异常和热异常的空间分布亦具有一定的关联性。此外, 2种异常多次呈现出沿断裂带分布的现象, 由此可知, 二者与构造活动皆密切相关。

关键词: 龙门山断裂带, 汶川地震, 重力异常, 热异常, 关联性分析