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2011年3月11日,在日本东北以东的近海发生了M9地震,这是日本地震监测史上最大的一次,其震中位于海底下约24km处,距离海岸约130km。根据位于日本领土上距离此次震中约180~200km的两个地磁监测站——江差町和水泽的数据研究了地球主磁场的长期变化。地球主磁场的分量在过去的上百年间变化都非常平稳,然而在2000~2011年期间,在日本领土发生大地震的区域内监测到了4次主磁场长期变化的异常情况,其持续时间从半年到3年不等。这些异常出现的同时,在地磁台站周边100km的区域内有地震活跃带发展,且之后发生了6级强震。从两个地磁台站监测的相关磁场分量差可发现一些最显著的异常。最近的一次异常情况最为强烈,其出现在大地震发生的约3年前。地震前地球磁场异常的原因可能是由构造运动过程引起的地壳局部区域的导电性变动造成。在初始阶段,这个过程会导致未来震源区域温度和导电性的升高,从而导致地表地电流的重新分配以及磁场的改变。之后,构造运动过程的发展导致未来震源区域裂缝的加剧,并降低了地震来临前的电导率。
On March 11, 2011, the M9 earthquake occurred off the coast of northeastern Japan in northeastern Japan. This is the largest earthquake monitoring event in Japan. Its epicenter is about 24km below the sea surface and about 130km from the coast. Based on the data of two geomagnetic monitoring stations located in Japanese territory about 180 ~ 200km away from this epicenter, Jiangcha-cho and Mizusawa, the long-term changes of the Earth’s main magnetic field are studied. The components of the earth’s main magnetic field have changed smoothly over the past hundred years. However, during the period 2000-2011, four anomalies of long-term changes in the main magnetic field were monitored in the territory of the major earthquakes in Japan’s territory, lasting from six months To 3 years range. At the same time as these anomalies occurred, there was an active seismic zone in the area of 100km around the geomagnetic station, and a magnitude-6 strong earthquake occurred. Some of the most significant anomalies can be found in the correlation of the magnetic field components monitored from the two geomagnetic stations. The most recent anomaly was the most intense, occurring about 3 years before the earthquake. The reason for the anomaly of the Earth’s magnetic field before the earthquake may be caused by the conductivity variation of the local crust caused by tectonic movement. In the initial stage, this process leads to an increase in temperature and conductivity in the future source area, resulting in the redistribution of surface currents and changes in the magnetic field. Afterwards, the development of tectonic movement led to the increase of cracks in the future source area and the decrease of the conductivity before the earthquake.