Abstract Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this shallow seismicity. We also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity. We find that (1) implosive NDC earthquakes occur at the craters that were active during the 1990–1991 eruptions, which are currently buried by the approximately 200-m-thick Heisei-Shinzan Lava Dome; (2) the detected NDC earthquakes generally possess downward P-wave arrivals, although upward P-wave arrivals are observed at a few seismic stations; (3) the tensile axes of the NDC earthquakes show various slip directions; (4) repeated NDC earthquakes are in some cases detected within 20 s of the initial event; and (5) a high-resistivity zone exists beneath the NDC earthquakes. We conclude that the implosive NDC earthquakes were generated by the collapse of small, randomly oriented, vapor-filled voids at the two buried craters, with active volcanic conduits still present beneath these craters. The shallow DC earthquakes consist of two clusters that are located 2–3 km below the NDC earthquakes. The focal mechanisms of the shallower cluster (approximately 2 km depth) indicate various fault mechanism, whereas those of the deeper cluster (approximately 3 km depth) indicate strike-slip faulting. The top of the shallower cluster corresponds to the base of the conductive zone, which suggests that the DC earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer.

中文翻译：

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日本云仙火山平成-新山熔岩穹顶下浅层和内爆非直流和直流地震的联合地震学-大地电磁调查

摘要 日本云仙火山下的浅层内爆非双偶 (NDC) 地震已被常规运行的地震仪网络探测到。在这里，我们分析了富士山周围 14 个临时地震台站的波形。1990-1995 年喷发期间形成的 Fugen (Fugendake) 和平成-新山熔岩穹顶，以进一步调查这种浅层地震活动。我们还使用宽带大地电磁观测估计地下电阻率结构，并研究其与浅层地震活动的关系。我们发现 (1) 内爆 NDC 地震发生在 1990-1991 年喷发期间活跃的火山口，这些火山口目前被大约 200 米厚的平成-新山熔岩穹顶掩埋；(2) 检测到的 NDC 地震一般具有向下的 P 波到达，尽管在一些地震台站观测到向上的 P 波到达；(3) NDC地震的拉伸轴呈现不同的滑动方向；(4) 在某些情况下，在初始事件的 20 秒内检测到重复的 NDC 地震；(5) NDC地震下方存在高阻带。我们得出的结论是，内爆 NDC 地震是由两个埋藏的陨石坑中小的、随机取向的、充满蒸汽的空隙坍塌引起的，这些陨石坑下方仍然存在活跃的火山管道。浅层 DC 地震由位于 NDC 地震下方 2-3 公里处的两个集群组成。较浅簇（约 2 公里深度）的震源机制表明各种断层机制，而较深簇（约 3 公里深度）的震源机制表明走滑断层。