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），从而控制了其潜在的危险，但只能在喷发期间或之后进行测量。指示岩浆黏度的前驱体的识别将能够预测喷发类型和相关危害的规模¹。2018年5月，夏威夷²基拉韦厄火山意外裂谷和喷发在熔岩喷发中显示出异常的化学和热变异性，导致不可预测的积水率和爆炸性。在这里，通过对地震活动和岩浆流变学的综合分析，我们显示了2018年爆发之前和之后发生的地震的断层平面解的方向（指示断层的方向和运动感）指示了90度局部应力场从背景旋转，以前只在高粘度喷发³才观察到这种现象，而在基劳亚^{4、5、6、7、8则}从未见过这种现象。。实验获得的2018年产品的粘度以及来自Pu'u'Ō'ō喷口的较早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，基拉韦厄火山和全球其他火山群中旋转的断层平面解提供了早期迹象，表明动乱涉及粘度升高的岩浆，因此，实时监测断层解的方向可能会提供有关样式的重要信息。即将爆发。此外，我们的结果提供了对复杂多相系统中故障和流量耦合的基本本质的了解。