Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. The identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards¹. The unanticipated May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai‘i² displayed exceptional chemical and thermal variability in erupted lavas, leading to unpredictable effusion rates and explosivity. Here, using an integrated analysis of seismicity and magma rheology, we show that the orientation of fault-plane solutions (which indicate a fault’s orientation and sense of movement) for earthquakes preceding and accompanying the 2018 eruption indicate a 90-degree local stress-field rotation from background, a phenomenon previously observed only at high-viscosity eruptions³, and never before at Kīlauea^4,5,6,7,8. Experimentally obtained viscosities for 2018 products and earlier lavas from the Pu‘u ‘Ō‘ō vents tightly constrain the viscosity threshold required for local stress-field reorientation. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves magma of heightened viscosity, and thus real-time monitoring of the orientations of fault-plane solutions could provide critical information about the style of an impending eruption. Furthermore, our results provide insight into the fundamental nature of coupled failure and flow in complex multiphase systems.

岩浆粘度强烈地控制着火山喷发的类型（例如，爆炸性与喷发性）及其潜在危险，但只能在喷发期间或之后进行测量。识别指示岩浆粘度的前体将能够预测喷发类型和相关危害的规模¹。夏威夷基拉韦厄火山 2018 年 5 月意外的裂谷侵入和喷发²在喷发的熔岩中显示出异常的化学和热变化，导致不可预测的渗出率和爆炸性。在这里，通过对地震活动性和岩浆流变学的综合分析，我们表明 2018 年喷发之前和伴随的地震的断层平面解的方向（表明断层的方向和运动方向）表明一个 90 度的局部应力场从背景旋转，这种现象以前仅在高粘度喷发³中观察到，以前从未在基拉韦厄火山^{4,5,6,7,8中观察到}. 实验获得的 2018 年产品和 Pu'u 'Ō'ō 喷口早期熔岩的粘度严格限制了局部应力场重新定向所需的粘度阈值。我们认为，基拉韦厄火山和世界其他火山的地震群中的旋转断层平面解决方案提供了动荡涉及高粘度岩浆的早期迹象，因此实时监测断层平面解决方案的方向可以提供有关样式的关键信息即将爆发。此外，我们的结果提供了对复杂多相系统中耦合故障和流动的基本性质的洞察。