Source: Journal of Geophysical Research: Solid Earth
The Three Sisters in central Oregon, USA, is one of the volcanic centers in the Cascade volcanic arc. Although dormant for more than 2000 years, an eruption of the volcano could affect about 280,000 thousand people living within 100 kilometers and about 2.3 million people in the Portland metropolitan area, 170 kilometers away.
Lisowski et al.  compiled ground deformation of Three Sisters in the past 25 years and found that the volcano uplifted by a maximum of about 300 millimeters. The uplift was initially fast but slowed down exponentially to the current rate of about 3 millimeter per year. The observed deformation history can either be explained by decreasing magma supply to the crustal reservoir at about 4 kilometers below sea level or an injection of magma only before 2000, followed by a viscoelastic relaxation around the magma reservoir.
This insight is possible only with long-term observations because a volcano often changes its behavior gradually. The authors recommend continuing monitoring of volcanoes in the world regardless of their current activity.
Citation: Lisowski, M., McCaffrey, R., Wicks, C. W., & Dzurisin, D. . Geodetic constraints on a 25-year magmatic inflation episode near Three Sisters, central Oregon. Journal of Geophysical Research: Solid Earth, 126, e2021JB022360. https://doi.org/10.1029/2021JB022360
―Yosuke Aoki, Associate Editor, JGR: Solid Earth