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根据1923年以来的日本气象厅地震目录,参考以被低滑动区隔开形成两个高滑动(≥20m)片区为特征的滑动分布,研究了2011年MW9.0东北近海地震之前的地震活动。北高滑动片区的滑动峰值位于海沟附近,而南高滑动片区的滑动峰值位于主震震中东南约40km处,距海沟约70km。据估计,主震时首先使南高滑动片区发生破裂,然后扩展到更大的邻近地区,包括北高滑动片区。主震之前两天开始的前震活动的震中分布在北高滑动片区的西缘,此处过去90年曾发生过其他比较强烈的活动,例如1981年地震。根据这两个高滑动片区周围的地震活动时空分布图我们推断,2011年东北近海地震是由前震活动触发的,因为前震活动中持续增加的应力可以抵消南高滑动片区的强度,此处之前已经由于2003年以来的一系列周边大地震而变得足够脆弱了。其他主要活动,比如1981年地震,并没有触发如此巨大的地震,主要是之前类似的应力条件还没有具备。南高滑动片区峰值周围形成的环状地震活动图像表明存在极强的片区,这些片区在过去很长的时间并没有由于周围发生大地震而破裂。
According to the Japan Meteorological Agency’s earthquake catalog since 1923, the seismic activity before the northeastern MW9.0 northeastern earthquake in 2011 was studied with reference to the sliding distribution characterized by two high-slip (≥20m) zones separated by low-slip zones. The sliding peak of the North High Sliding Zone is located near the trench. The sliding peak of the South High Sliding Zone is about 40 km southeast of the epicenter and about 70 km away from the trench. It is estimated that the main shock first ruptured the South High Sliding Zone and then extended to much larger adjacent areas, including the North High Sliding Zone. The epicenter of the foreshock activity that started two days before the mainshock was located on the western edge of the North High Sliding Zone, where other more intense activities have taken place over the past 90 years, such as the 1981 earthquake. Based on the temporal and spatial distribution of seismic activity around these two high slide zones, we conclude that the 2011 northeast offshore earthquake was triggered by foreshock activity because the continuously increasing stress in the foreshock activity can counteract the strength of the southern high slip zone, Due to a series of major earthquakes since 2003, it has become sufficiently vulnerable. Other major activities, such as the 1981 earthquake, did not trigger such a huge earthquake, mainly because similar stress conditions were not available before. Cyclic seismicity images formed around the peaks of the South High Sliding Zone show the presence of extremely strong zones that have not ruptured for a long time in the past due to the occurrence of a major earthquake around them.