The 30 May 2015 Mw 7.9 Bonin Islands earthquake, Japan, is one of the largest deep-focus earthquakes ever recorded. Its occurrence, close to 700-km depth, in an area without any known historical seismicity, along with its magnitude, was a surprise to scientists. Deep earthquakes are generally believed to have few aftershocks and no foreshocks. Here, we explore the earthquake productivity in the hypocentral surroundings and detect 49 not previously identified earthquakes, 28 of which occurred during an accelerating preseismic phase that started 3 months prior to the main shock. This is the first time that such foreshock activity has been observed for a deep earthquake. The preseismic and postseismic activity suggests transformational faulting within a metastable olivine wedge (MOW) inside the slab at depth as the triggering principal mechanism for this deep earthquake sequence, the seismicity starting where the backward bending of the subducting Pacific plate is maximum. Plain Language Summary Deep earthquakes have been puzzling seismologists for almost a century, since their discovery in the 1920s. Earthquakes deeper than 50 km represent about 25% of the global seismicity, and deep-focus earthquakes are defined as those located at 300-km depth or more. They can be exceedingly large and occur at temperatures and pressures where sliding and fracture are inhibited, thus the brittle fracture/friction mechanism, valid in Earth's crust, cannot hold. Despite their abundance, the physical mechanism behind deep-focus earthquakes is still a subject of ongoing debate. The Bonin Islands earthquake (30 May 2015, Mw7.9) occurred at 680-km depth in a previously quiet area. In order to better understand why and how it occurred there, we searched for previously unknown earthquakes in the surroundings. For the first time, we identify a preseismic phase preceding such a very deep-focus earthquake. The location of the newly detected earthquakes reveals the geometry of the slab at depth, and their spatiotemporal distribution supports transformation of metastable olivine as the rupture initiation process.

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日本~700 km 深 M w 7.9 Bonin Islands 地震的震前和震后阶段

2015 年 5 月 30 日，日本波宁群岛发生的 7.9 级地震是有记录以来最大的深源地震之一。它的发生深度接近 700 公里，在一个没有任何已知历史地震活动的地区，以及它的震级，让科学家们感到惊讶。一般认为深地震很少有余震，也没有前震。在这里，我们探索了震源周围环境的地震生产力，并检测了 49 次以前未发现的地震，其中 28 次发生在主震前 3 个月开始的加速预震阶段。这是首次在深地震中观察到这种前震活动。震前和震后活动表明，深部板块内部亚稳橄榄石楔（MOW）内的转换断层是触发该深地震序列的主要机制，地震活动始于太平洋板块向后弯曲最大的地方。简单语言总结 深地震自 1920 年代被发现以来，近一个世纪以来一直困扰着地震学家。深度超过 50 公里的地震约占全球地震活动的 25%，深源地震被定义为位于 300 公里或以上深度的地震。它们可能非常大，并且在滑动和断裂受到抑制的温度和压力下发生，因此在地壳中有效的脆性断裂/摩擦机制无法成立。尽管它们丰富，深源地震背后的物理机制仍然是一个持续争论的话题。博宁群岛地震（2015 年 5 月 30 日，Mw7.9）发生在 680 公里深度的一个先前安静的地区。为了更好地了解它在那里发生的原因和方式，我们在周围搜索了以前未知的地震。我们第一次确定了这样一个非常深的地震之前的震前阶段。新探测到的地震的位置揭示了深部板块的几何形状，它们的时空分布支持亚稳态橄榄石的转变作为破裂起始过程。我们在周围寻找以前未知的地震。我们第一次确定了这样一个非常深的地震之前的震前阶段。新探测到的地震的位置揭示了深部板块的几何形状，它们的时空分布支持亚稳态橄榄石的转变作为破裂起始过程。我们在周围寻找以前未知的地震。我们第一次确定了这样一个非常深的地震之前的震前阶段。新探测到的地震的位置揭示了深部板块的几何形状，它们的时空分布支持亚稳态橄榄石的转变作为破裂起始过程。