Robert McCaffrey Charles Wicks Daniel Dzurisin Michael Lisowski 2021 <div class="article-section__content en main"><p>Crustal inflation near the Three Sisters volcanic center documented since the mid-1990s has persisted for more than two decades. We update past analyses of the event through 2020 by simultaneously inverting InSAR interferograms, GPS time series, and leveling data for time-dependent volcanic deformation source parameters. We explore several source models to estimate how the deformation rate varied through time and to identify parameters that can reproduce measured deformation. Our preferred model is a Mogi source 4.1&nbsp;km below sea level (5.9&nbsp;km below the surface) about 5&nbsp;km west of the summit of South Sister. Inflation started in late 1995 or 1996; the rate increased rapidly during 1998–1999, and peaked in late 1999, resulting in maximum surface uplift of about 30&nbsp;cm by mid-2020. Since 2000, the inflation rate generally declined exponentially with a time constant of about 6&nbsp;years. Two source inflation scenarios fit the data equally well. In the first, the crust surrounding the source is elastic and the net source-volume increase, which we attribute to persistent magma input, has been about 49&nbsp;×&nbsp;10⁶&nbsp;m³. The second scenario adds a viscoelastic shell surrounding the Mogi source. In that case, an injection of about 21&nbsp;×&nbsp;10⁶&nbsp;m³<span>&nbsp;</span>of magma prior to 2000, followed by continuing relaxation of the viscoelastic shell, can account for most of the observed surface deformation. In both scenarios, modeling reveals quasiperiodic increases in the inflation rate (pulses) with a recurrence interval of 3–4&nbsp;years, both before and after 2000.</p></div> application/pdf 10.1029/2021JB022360 en American Geophysical Union Geodetic constraints on a 25-year magmatic inflation episode near Three Sisters, central Oregon article