Deep-focus earthquakes (DFEs) present an interesting scientific challenge in that they occur at depths where brittle failure should be impossible. The fact that their occurrence is confined to locations where subducting lithospheric slabs are crossing through the transition zone suggests that olivine phase transformations may be involved in the production of these earthquakes. Experimental studies have shown that olivine can persist metastably in subducting slabs and that olivine phase transformations can lead to faulting at high pressures. However, it has been argued that large DFEs are too large to be contained within a metastable olivine wedge preserved in the interior of subducting slabs. We demonstrate, using experiments on olivine-analog materials, that transformational faulting can continue to propagate via shear-enhanced melting into the stable high-pressure phase. We also show that transformational faulting is controlled by the ratio between strain rates and the olivine-ringwoodite transformation rates, and extrapolate this relationship to the natural conditions of DFEs. Counterintuitively, these results imply that cold and fast-subducting slabs produce transformational faulting at higher temperatures, which results in more numerous DFEs.

中文翻译：

---

深源地震：从高温实验到冷板

深源地震 (DFE) 提出了一个有趣的科学挑战，因为它们发生在不可能发生脆性破坏的深度。它们的发生仅限于俯冲岩石圈板片穿过过渡带的位置，这表明橄榄石相变可能与这些地震的产生有关。实验研究表明，橄榄石可以在俯冲板片中保持亚稳态，并且橄榄石相变可以导致高压下的断层。然而，有人认为大型 DFE 太大而不能包含在俯冲板片内部保存的亚稳橄榄石楔中。我们证明，使用橄榄石模拟材料的实验，变形断层可以通过剪切增强熔化继续传播到稳定的高压阶段。我们还表明，变形断层受应变率和橄榄石-菱镁石转化率之间的比率控制，并将这种关系外推到 DFE 的自然条件。与直觉相反，这些结果意味着冷和快速俯冲板片在较高温度下会产生转变断层，从而导致更多的 DFE。