Shimojo Kengo, Enescu Bogdan, Yagi Yuji, Takeda Tetsuya
Seismology and Volcanology Division, Japan Meteorological Agency (JMA), Tokyo, Japan.
Department of Geophysics, Kyoto University, Kyoto, Japan.
Sci Rep. 2021 Apr 16;11(1):8143. doi: 10.1038/s41598-021-86837-4.
The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely 'modulated' by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale.
2011年日本东北冲发生里氏9.0级地震后，日本内陆地区的地震活动被激活。一个突出的例子是日本中部的长野北部6.2级地震，它在此次巨大逆冲型地震发生13小时后发生，距离其震中约400公里。导致巨大逆冲型地震发生以及随后相对较大的内陆地震被激活的物理过程尚未得到很好的理解。在此，我们利用密集的本地地震台网的波形数据，以前所未有的分辨率揭示了导致此次6.2级地震发生的复杂机制。我们发现，在东北冲巨大逆冲型地震发生后的相对短时间内，长野地震源断层上开始出现此前未被检测到的小地震，并且局部地震活动一直断断续续持续到6.2级地震发生，可能受到来自东北近海大型远场余震的面波到达的“调制”。在长野地震发生前约1小时，微震活动向其震源迁移加速。迁移速度表明可能存在局部慢滑，最终引发了这次大型内陆地震，流体在区域尺度上发挥了激活地震活动的作用。
