Recently, a widespread and densely continuous-recording ocean-bottom seismograph network has been deployed in the Japan Trench subduction zone. Utilizing the offshore network data improves azimuthal station coverage for offshore earthquakes in the Japan Trench subduction zone. It has a potential to obtain centroid moment tensor (CMT) solutions more accurately than conventional analyses using onshore networks and a simple one-dimensional seismic velocity structure model. In this study, we conducted CMT inversion for subduction zone earthquakes that occurred between 1 April 2017, and 31 March 2024, with a moment magnitude range of 5.2–7.0. We used seismograms obtained from both the offshore and onshore networks. We calculated Green's functions using a three-dimensional seismic velocity structure model. Our CMT solutions with thrust-type mechanisms mostly indicated depths and dip angles consistent with the plate interface. For earthquakes in the outer-rise region, our CMT solutions were characterized as normal-fault mechanisms. The joint use of the offshore and onshore networks reduced the estimation errors of the CMT solutions compared with the only use of the onshore network, although the optimal solutions were consistent. The dip angles for the thrust earthquakes determined by our analysis were more consistent with the dip angle of the plate boundary than those determined by conventional CMT analyses. Additionally, we found that the conventional CMT analysis could introduce a systematic bias in depth and magnitude determinations. This finding highlights the importance of an offshore seismograph network and a reliable seismic velocity structure model for CMT inversions.

中文翻译：

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海上地震仪网络和三维地震速度结构模型对海上地震质心矩张量分析的影响——在日本海沟俯冲带的应用


最近，在日本海沟俯冲带部署了一个广泛且密集连续记录的海底地震仪网络。利用海上网络数据可以提高日本海沟俯冲带海上地震的方位站覆盖范围。与使用陆上网络和简单的一维地震速度结构模型的传统分析相比，它有可能更准确地获得质心矩张量 （CMT） 解。在这项研究中，我们对 2017 年 4 月 1 日至 2024 年 3 月 31 日期间发生的俯冲带地震进行了 CMT 反演，矩震级范围为 5.2-7.0。我们使用了从海上和陆上网络获得的地震图。我们使用三维地震速度结构模型计算了格林函数。我们采用推力式机构的 CMT 解决方案主要指示与板界面一致的深度和倾角。对于外隆地区的地震，我们的 CMT 解决方案被描述为正断层机制。与仅使用陆上网络相比，海上和陆上网络的联合使用减少了 CMT 解决方案的估计误差，尽管最优解决方案是一致的。与常规 CMT 分析确定的俯冲角相比，我们的分析确定的逆冲地震的倾角与板块边界的倾角更一致。此外，我们发现传统的 CMT 分析可能会在深度和量级确定方面引入系统性偏倚。这一发现强调了海上地震仪网络和可靠的地震速度结构模型对 CMT 反演的重要性。