Spatial relationship between shallow very low frequency earthquakes and the subducted Kyushu-Palau Ridge in the Hyuga-nada region of the Nankai subduction zone
Spatial relationship between shallow very low frequency earthquakes and the subducted Kyushu-Palau Ridge in the Hyuga-nada region of the Nankai subduction zone
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南海俯冲带日向滩地区浅源甚低频地震与俯冲九州帕劳海脊的空间关系
DOI：
10.1093/gji/ggaa264
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发表时间：
2020
影响因子：
2.8
通讯作者：
Y. Kaneda
中科院分区：
文献类型：
--
作者：
T. Tonegawa;Y. Yamashita;Tsutomu Takahashi;M. Shinohara;Y. Ishihara;S. Kodaira;Y. Kaneda
日本南开俯冲带西部Hyuga-nada地区浅层板块界面反复发生浅层甚低频地震(SVLFE)。虽然利用陆上地震记录已经确定了sVLFE震中的位置，但仍有必要更精确地确定它们的位置，以探索sVLFE与其他浅层慢震的关系，并研究可能控制sVLFE活动的构造因素。在这里，我们利用Hyuga-nada地区临时部署的海底地震仪(OBSS)获得的地震记录来识别sVLFE震中。地震观测涉及部署5-13个OBS，为期约一年，2014-2016年间进行了三次部署，每次都更改了OBS编号和阵列分布。因此，每次观测可以在0.1-0.15赫兹的频带上探测到一个sVLFE事件，其中包含由于海洋沉积物而产生的连续的慢速瑞雷波脉冲，导致总共三个事件。利用地震目录压制了连续记录中普通地震的瑞利波振幅。我们使用尾波干涉法估计了每个阵列的瑞利波群速度的频散曲线，它代表了阵列内的平均群速度，并将使用群速度的包络相关方法(ECM)应用于连续记录。这些处理为sVLFE震中提供了5千米的水平距离误差。结果表明，在九州-帕劳山脊俯冲地区，由菲律宾海板块顶部等高线推算出的超大型流体的深度已从10~15千米增加到10~15千米。同样明显的是，sVLFE震中的迁移发生在2015年，沿着俯冲KPR的北缘，从15千米的深度迁移到较浅的深度。这些结果表明，俯冲的KPR是控制sVLFE活性激发条件的结构因素。
Shallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities.
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