Seismicity in the source areas of the 1896 and 1933 Sanriku earthquakes and implications for large near-trench earthquake faults
Seismicity in the source areas of the 1896 and 1933 Sanriku earthquakes and implications for large near-trench earthquake faults
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DOI：
10.1093/gji/ggx532
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发表时间：
2018-03-01
影响因子：
2.8
通讯作者：
Miura, Seiichi
中科院分区：
文献类型：
--
作者：
Obana, Koichiro;Nakamura, Yasuyuki;Miura, Seiichi
在日本海沟的北部，1933年昭和-三陆地震（M-w 8.4）发生在1896年明治-三陆海啸地震（M-w 8.0） 37年后，这是一次海沟外的正常断层地震，是一次浅层，近海沟，板块界面破裂。两次地震引发的海啸对三陆海岸造成了严重破坏。1896年和1933年地震震源区的精确地震位置以前没有得到，因为它们发生在距离海岸相当远的深水中，超出了常规海底地震仪的最大工作深度。2015年，我们在1896年和1933年三陆地震震源区进行了为期两个月的地震观测，并将设计用于深水操作的地震仪（超深地震仪）整合到一个地震仪阵列中，研究了三陆地震震源区地震活动性与外升正断层地震和近海沟海啸地震的关系。我们的分析表明，在我们的观测期间，地震活动沿着外海沟区域的三个大致线性的海沟平行趋势发生。沿着这些趋势的地震活动可能对应于1933年昭和三陆地震的余震，以及2011年东北大地震40分钟后发生的M-w 7.4级正断层地震。此外，在地震活动性线性趋势附近，观测到了大洋莫霍线对地震反射剖面的反射清晰度变化和大洋地幔内的低速异常。我们确定的震源机制表明，伸展应力区延伸至约40公里深度，在此深度以下应力区为挤压应力区。这些观察结果表明，1933年昭和三陆地震期间的破裂并没有延伸到海洋岩石圈的底部，而多个或分段断层的复合破裂是对那次地震的更合理的解释。1896年明治三陆海啸地震的震源区是一个向海沟轴向陆地方向的地震区。地震活动和地壳结构的空间异质性可能预示着未来可能导致类似1896年和1933年三陆地震等危险事件的近沟断层，在评估与近沟大地震相关的海啸灾害时应予以考虑。
In the northern part of the Japan Trench, the 1933 Showa-Sanriku earthquake (M-w 8.4), an outer-trench, normal-faulting earthquake, occurred 37 yr after the 1896 Meiji-Sanriku tsunami earthquake (M-w 8.0), a shallow, near-trench, plate-interface rupture. Tsunamis generated by both earthquakes caused severe damage along the Sanriku coast. Precise locations of earth-quakes in the source areas of the 1896 and 1933 earthquakes have not previously been obtained because they occurred at considerable distances from the coast in deep water beyond the maximum operational depth of conventional ocean bottom seismographs (OBSs). In 2015, we incorporated OBSs designed for operation in deep water (ultradeep OBSs) in an OBS array during two months of seismic observations in the source areas of the 1896 and 1933 Sanriku earthquakes to investigate the relationship of seismicity there to outer-rise normal-faulting earthquakes and near-trench tsunami earthquakes. Our analysis showed that seismicity during our observation period occurred along three roughly linear trench-parallel trends in the outer trench region. Seismic activity along these trends likely corresponds to aftershocks of the 1933 Showa-Sanriku earthquake and the M-w 7.4 normal-faulting earthquake that occurred 40 min after the 2011 Tohoku-Oki earthquake. Furthermore, changes of the clarity of reflections from the oceanic Moho on seismic reflection profiles and low-velocity anomalies within the oceanic mantle were observed near the linear trends of the seismicity. The focal mechanisms we determined indicate that an extensional stress regime extends to about 40 km depth, below which the stress regime is compressional. These observations suggest that rupture during the 1933 Showa-Sanriku earthquake did not extend to the base of the oceanic lithosphere and that compound rupture of multiple or segmented faults is a more plausible explanation for that earthquake. The source area of the 1896 Meiji-Sanriku tsunami earthquake is characterized by an aseismic region landward of the trench axis. Spatial heterogeneity of seismicity and crustal structure might indicate the near-trench faults that could lead to future hazardous events such as the 1896 and 1933 Sanriku earthquakes, and should be taken into account in assessment of tsunami hazards related to large near-trench earthquakes.