Changes in seismicity in a volcanically active region, on the eastern side of Izu Peninsula, Japan

K. Z. Nanjo, Y. Yukutake, T. Kumazawa
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Abstract

The Izu-Tobu region, on the eastern side of Izu Peninsula, in Japan, is volcanically and seismically active. In this region, earthquake swarms of ordinary earthquakes frequently occur at shallow depths, which is considered to be associated with magma intrusion. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) are infrequently observed. We conducted a timeseries analysis of both types of earthquakes during the time period 2005-2020, using a variant of the Epidemic-Type Aftershock Sequence model. For this analysis, we used the Japan Meteorological Agency catalog of ordinary earthquakes and the catalog of LFEs produced using the matched filter method. The observed result, which was common to both types of earthquakes, showed a significant change in seismicity, which became quiet, with the inflection point falling sometime between late 2009 and mid-2013, during which two out of three pronounced earthquake swarms occurred. We associated this seismic quiescence with changes in background rate to be low, where background rate, by removing the triggering effect of aftershocks, was interpreted as having been caused directly by the magma source, which can vary with time. We used surface displacement data obtained from the Geospatial Information Authority of Japan, and observed that the uplift due to magma intrusion was significant during the 1970s-1990s whereas it was in abatement or unobservable during the studied period (2005-2020). We also found that the seismic quiescence occurred without significant crustal movement during the studied period. Our implication from this finding is that magma source, which caused magma intrusion into the Izu-Tobu region, is in a transition phase, becoming less active, compared with the magma source during the 1970s-1990s.
日本伊豆半岛东侧火山活跃地区的地震变化
日本伊豆半岛东侧的伊豆东武地区火山和地震活动频繁。在这一地区,浅层经常发生普通地震群,这被认为与岩浆侵入有关。在普通地震之下,很少观测到低频地震(LFE)。我们使用流行型余震序列模型的一个变体,对 2005-2020 年期间的两种地震进行了时间序列分析。在分析中,我们使用了日本气象厅的普通地震目录和使用匹配滤波法生成的 LFE 目录。观测结果表明,两种类型的地震都出现了明显的不震性变化,地震变得平静,拐点出现在 2009 年末至 2013 年中的某个时间,在此期间,三次明显的地震群中有两次发生了地震。我们将这种地震静止与本底率的变化联系起来,认为本底率很低,因为剔除了余震的触发效应,本底率被解释为是由震源直接引起的,而震源会随时间变化。我们使用了从日本地理空间信息局获得的地表位移数据,观察到岩浆侵入导致的隆起在 20 世纪 70 年代至 90 年代期间非常显著,而在研究期间(2005 年至 2020 年)则处于减弱或不可观测的状态。我们还发现,在所研究的时期内,地震静止发生时地壳运动并不明显。我们从这一发现中得出的结论是,与 20 世纪 70 年代至 90 年代期间的岩浆源相比,导致岩浆侵入伊豆东武地区的岩浆源正处于过渡阶段,变得不那么活跃。
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