Nathan Lee Andersen Annika E. Dechert Josef Dufek Helene Le Mevel 2026 <p>From 2019 to 2024, gravity surveys were conducted at the Three Sisters volcanic cluster (TSVC), measuring 246 gravity sites using a spring relative gravimeter. We calculated the residual Bouguer anomaly and identified three main zones with negative anomalies, ranging from −4 to −8 mGal, located southwest and west of South Sister, within an area that has been uplifting for the past two decades. After inversion, we obtain a 3D density model of the subsurface and identify low-density bodies extending from the surface down to 3 km. We estimate a total of 15 km³ of crustal bodies with density close to 2 g/cm³ that could store up to ~5 km³ of water, forming an extensive hydrothermal system beneath the TSVC. We explore the possible combinations of melt compositions and temperatures that could create a bulk density close to our reference crustal density (2.5 g/cm³) using MELTS thermodynamic simulations. Our results indicate that a magmatic mush with as little as 15% partial melt of bulk rhyolitic composition or as much as 52%–57% partial melt of a bulk dacitic composition could be stored in a magmatic system under TSVC without generating a detectable gravity anomaly. Episodic magma injections at the base of the magmatic system, such as the 1998–2000 intrusion at ~6 km depth, would bring heat and gas to the hydrothermal system while maintaining a low melt fraction in the magmatic mush, as imaged at other Cascade volcanoes.</p> application/pdf 10.1029/2025JB031886 en American Geophysical Union The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements article