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 km below sea level (5.9 km below the surface) about 5 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 cm by mid‐2020. Since 2000, the inflation rate generally declined exponentially with a time constant of about 6 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 × 10 6 m 3 . The second scenario adds a viscoelastic shell surrounding the Mogi source. In that case, an injection of about 21 × 10 6 m 3 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 years, both before and after 2000.

Recent activity has provided new insights into the causes of surface deformation in and around the Yellowstone Caldera, a topic that has been debated since the discovery of caldera floor uplift more than four decades ago. An episode of unusually rapid uplift (>15 cm/yr) centered near Norris Geyser Basin along the north caldera rim began in late 2013 and continued until a M w 4.9 earthquake on 30 March 2014; thereafter, uplift abruptly switched to subsidence. Uplift at rates of several centimeters per year resumed in 2016 and continued at least through the end of 2018. Modeling of Global Positioning System and interferometric synthetic aperture radar data suggests an evolving process of deep magma intrusion during 1996–2001 followed by volatile ascent and accumulation at shallow levels, perhaps as shallow as a few hundred meters depth. The depth of shallow volatile accumulation appears to have shallowed from the 2014 to the 2016 deformation episode, and frequent eruptions of Steamboat Geyser since March 2018 are likely a surface manifestation of this ongoing process. Hydrothermal explosion features are prominent in the Norris Geyser Basin area, and the apparent shallow nature of the volatile accumulation implies an increased risk of hydrothermal explosions.

The arrival of space-based imaging radar as a revolutionary land-surface mapping and monitoring tool little more than a quarter century ago enabled a spate of innovative volcano research worldwide. Soon after launch of European Space Agency’s ERS-1 spacecraft in 1991, the U.S. Geological Survey began SAR and InSAR studies of volcanoes in the Aleutian and Cascades arcs, in Hawaiʻi, and elsewhere in the western U.S. including the Yellowstone and Long Valley calderas. This paper summarizes results of that effort and presents new findings concerning: (1) prevalence of volcano deformation in the Aleutian and Cascade arcs; (2) surface-change detection and hazard assessment during eruptions at Aleutian and Hawaiian volcanoes; (3) geodetic imaging of magma storage and transport systems in Hawaiʻi; and (4) deformation sources and processes at the Yellowstone and Long Valley calderas. Surface deformation caused by a variety of processes is common in arc settings and could easily escape detection without systematic InSAR surveillance. Space-based SAR imaging of active lava flows and domes in remote or heavily vegetated settings, including during periods of bad weather and darkness, extends land-based monitoring capabilities and improves hazards assessments. At Kīlauea Volcano, comprehensive SAR and InSAR observations identify multiple magma storage zones beneath the summit area and along the East Rift Zone, and illuminate magma transport pathways. The same approach at Yellowstone tracks the ascent of magmatic volatiles from a mid-crustal intrusion to shallow depth and relates that process to increased hydrothermal activity at the surface. Together with recent and planned launches of highly capable imaging-radar satellites, these findings support an optimistic outlook for near-real time surveillance of volcanoes at global scale in the coming decade.

Earth is home to about 1500 volcanoes that have erupted in the past 10,000 years, and today volcanic activity affects the lives and livelihoods of a rapidly growing number of people around the globe. About 20 volcanoes are erupting on Earth at any given time; 50–70 erupt each year, and about 160 erupt each decade. Impressive as these statistics are, they do not include a large but unspecified number of volcanic vents along submarine midocean ridges that girdle the globe (Smithsonian Institution, Global Volcanism Program, http://www.volcano.si.edu/faq.cfm#q3). The nature of eruptive activity ranges from the quiet outpouring of fluid lava on the ocean floor and in places like Hawaii (http://hvo.wr.usgs.gov/) to the explosive ejection of volcanic ash, pumice and other fragmental material at volcanoes like Mount Fuji (Japan), Mount St. Helens (United States), Chaitén (Chile) and others along the Pacific Ring of Fire and elsewhere. Less frequent, larger events, like the 1912 eruption of Novarupta–Mount Katmai (Alaska) (Hildreth and Fierstein 2012) and the 1991 eruption of Mount Pinatubo (Philippines) (Newhall and Punongbayan 1996), produce local and regional impacts that can last for decades, and shorter-term effects on the average global temperature. At the extreme end of the spectrum, it has been suggested that a catastrophic eruption at Lake Toba on the Indonesian island of Sumatra about 74,000 years ago caused a decade-long ‘volcanic winter’, resulting in a genetic bottleneck that profoundly affected the course of human evolution—an idea that remains plausible but controversial (Robock 2013 and references therein). The products of volcanic eruptions also vary widely, giving rise to a large range of associated hazards (Myers et al. 2008). Explosive eruptions produce ballistic ejecta (solid and molten rock fragments) that can impact the surface up to several kilometres away from the vent. Smaller fragments are carried upward in eruption columns that sometimes reach the stratosphere, forming eruption clouds that pose a serious hazard to aircraft. Large eruption clouds can extend hundreds to CONTENTS

Since awakening from a 123-year repose in 1980, Mount St. Helens has provided an opportunity to study changes in crustal magma storage at an active arc volcano—a process of fundamental importance to eruption forecasting and hazards mitigation. There has been considerable progress, but important questions remain unanswered. Was the 1980 eruption triggered by an injection of magma into an upper crustal reservoir? If so, when? How did magma rise into the edifice without producing detectable seismicity deeper than 2.5 km or measurable surface deformation beyond the volcano’s north flank? Would precursory activity have been recognized earlier if current monitoring techniques had been available? Despite substantial improvements in monitoring capability, similar questions remain after the dome-forming eruption of 2004–8. Did additional magma accumulate in the reservoir between the end of the 1980–86 eruption and the start of the 2004–8 eruption? If so, when? What is the significance of a relative lull in seismicity and surface deformation for several years prior to the 2004–8 eruption onset? How did magma reach the surface without producing seismicity deeper than 2 km or measurable deformation more than a few hundred meters from the vent? Has the reservoir been replenished since the eruption ended, and is it now primed for the next eruption? What additional precursors, if any, should be expected? This paper addresses these questions, explores possible answers, and identifies unresolved issues in need of additional study. The 1980–86 and 2004–8 eruptions could have resulted from “second boiling” during crystallization of magma long-resident in an upper crustal reservoir, rather than from injection of fresh magma from below. If reservoir pressurization and magma ascent were slow enough, resulting strain might have been accommodated by viscoelastic deformation, without appreciable seismicity or surface deformation, until rising magma entered a brittle regime within 2–2.5 km of the surface. Given the remarkably gas-poor nature of the 2004–8 dome lava, future eruptive activity might require a relatively long period of quiescence and reservoir pressurization or a large injection of fresh magma—an event that arguably has not occurred since the Kalama eruptive period (C.E. 1479–1720).

ABSTRACT Meticulous field observations are a common underpinning of two landmark studies conducted by Don Swanson dealing with the rate at which magma is supplied to Kīlauea Volcano, Hawai‘i. The first combined effusion rate and ground deformation observations to show that the supply rate to Kīlauea was constant at ~0.11 km3/yr during three sustained eruptions from 1952 to 1971, a quiescent period at neighboring Mauna Loa volcano. This rate was also interpreted as the steady supply rate from the mantle to both volcanoes combined throughout historical time. The second breakthrough involved field evidence that activity at Kīlauea alternates between dominantly effusive and explosive styles over time scales of several centuries, and that the magma supply rate during explosive periods is only 1%–2% of the rate during effusive periods. For the historical period, several later studies concluded that the supply rate to Kīlauea has varied by as much as an order of magnitude, contrary to Swanson’s earlier suggestion. All such estimates are fraught with uncertainty, given the poorly known amount of magma stored within the volcano’s rift zones as a function of time—an enduring problem and active research topic. Nonetheless, Swanson’s original work remains an important touchstone that spurred many subsequent investigations and refinements. For example, there is strong evidence that Kīlauea experienced a surge in magma supply during 2003–2007 that exceeded the historical average by as much as a factor of two, and that the surge was followed by a comparable lull before the supply rate returned to “normal” by 2016. There is also evidence for supply-rate variations of similar magnitude during the latter part of the twentieth century and possibly earlier, subject to the aforementioned uncertainty in rift-zone storage. The extent to which variations in the magma supply to Kīlauea can be attributed to partitioning between Kīlauea and Mauna Loa, a long-debated topic, remains uncertain. Since Kīlauea’s inception, the net magma supply to the volcano (and also to Lō‘ihi Seamount, since it began growing) has increased, while Mauna Loa’s growth rate has slowed, suggesting that the volcanoes compete for the same magma supply. However, geochemical differences between lavas erupted at Kīlauea and Mauna Loa indicate that they do not share a homogeneous mantle source or common lithospheric magma plumbing system. Both ideas might be correct; i.e., Kīlauea and Mauna Loa magmas may be sourced in differing portions of the same melt accumulation zone and ascend through different crustal pathways, but those pathways interact through stress or pressure changes that modulate the supply to each volcano. Currently, magma supply-rate estimates are facilitated by comprehensive imaging of surface deformation and topographic change coupled with measurements of gas emissions. Physics-based models are being developed within a probabilistic framework to provide rigorous estimates of model parameters, including magma supply rate, and their uncertainties. Further refinement will require intensive multiparameter observations of the entire magmatic system—from source to surface and above, and from the volcanoes’ summits to their submerged lower flanks—in order to account fully for a complex magma budget.

Relative gravity measurements were made at 12 sites on Mount St. Helens and 4 sites far afield during the summers of 2010, 2012, 2014, and 2016. Positive residual gravity changes of 0.05-0.060.01mGal from 2010 to 2016a sign of mass additionremain at proximal sites after accounting for the effects of changes in Crater Glacier shape and mass. Modeling of the 2010-2016 monitoring data indicates mass addition in the volcano magma reservoir, the volcano conduit, and/or the shallow hydrothermal aquifer. Magma intrusion in the volcano's known reservoir is suggested by the joint inversion of GPS and gravity data (d=5800710m below sea level, V-m=49.88.6x10(6)m(3), =1930300kg/m(3)); the modeled depth and location are consistent with that of the reservoir that fed the 2004-2008 eruption, and its mass change can explain up to 19% of the residual gravity. The other two potential sourcesthe conduit and shallow aquiferare not well constrained. Magma addition along the volcano conduit can explain up to 62% of the residual gravity (V(m)31x10(6)m(3), (m)2, 300kg/m(3)). However, such an intrusion should have produced a measurable surface deformation, which is not observed in the GPS time series. Changes in the level of the volcano's shallow hydrothermal system ((w)=1, 000kg/m(3)) can explain 17% (V(recharge)9x10(6)m(3)) to 61% (V(recharge)30x10(6)m(3)) of the residual gravity. It would therefore seem that the bulk of the mass change measured at Mount St. Helens during 2010-2016 was caused by shallow accumulation of water beneath the floor of the crater. Plain Language Summary Measurements of subtle changes in the Earth's gravity field can provide information on the addition/loss of mass (e.g., magma or aqueous fluids) beneath a volcano. In this study, we measured gravity at Mount St. Helens from 2010 to 2016 to investigate possible mass changes following the 2004-2008 dome-forming eruption. The raw gravity measurements were corrected for changes in the mass and shape of Crater Glacier and for deformation of the volcanic edifice to obtain residual gravity values. We found positive residual gravity changes that we interpret as evidence for partial recharge of the magma reservoir that fed the 2004-2008 eruption and/or for accumulation of groundwater in one or more shallow aquifers. Most of the gravity signal can be explained by groundwater accumulation; magma recharge is not required. However, there is additional evidence for at least partial recharge from seismic, deformation, and geochemical observations.

Experience during historical time throughout the Cascade arc and the lack of deep-seated deformation prior to the two most recent eruptions of Mount St. Helens might lead one to infer that Cascade volcanoes are generally quiescent and, specifically, show no signs of geodetic change until they are about to erupt. Several decades of geodetic data, however, tell a different story. Ground- and space-based deformation studies have identified surface displacements at five of the 13 major Cascade arc volcanoes that lie in the USA (Mount Baker, Mount St. Helens, South Sister, Medicine Lake, and Lassen volcanic center). No deformation has been detected at five volcanoes (Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake, and Mount Shasta), and there are not sufficient data at the remaining three (Glacier Peak, Mount Adams, and Mount Jefferson) for a rigorous assessment. In addition, gravity change has been measured at two of the three locations where surveys have been repeated (Mount St. Helens and Mount Baker show changes, while South Sister does not). Broad deformation patterns associated with heavily forested and ice-clad Cascade volcanoes are generally characterized by low displacement rates, in the range of millimeters to a few centimeters per year, and are overprinted by larger tectonic motions of several centimeters per year. Continuous GPS is therefore the best means of tracking temporal changes in deformation of Cascade volcanoes and also for characterizing tectonic signals so that they may be distinguished from volcanic sources. Better spatial resolution of volcano deformation can be obtained through the use of campaign GPS, semipermanent GPS, and interferometric synthetic aperture radar observations, which leverage the accumulation of displacements over time to improve signal to noise. Deformation source mechanisms in the Cascades are diverse and include magma accumulation and withdrawal, post-emplacement cooling of recent volcanic deposits, magmatic-tectonic interactions, and loss of volatiles plus densification of magma. The Cascade Range thus offers an outstanding opportunity for investigating a wide range of volcanic processes. Indeed, there may be areas of geodetic change that have yet to be discovered, and there is good potential for addressing a number of important questions about how arc volcanoes work before, during, and after eruptions by continuing geodetic research in the Cascade Range.

Semipermanent GPS (SPGPS) is an alternative to conventional campaign or survey-mode GPS (SGPS) and to continuous GPS (CGPS) that offers several advantages for monitoring ground deformation. Unlike CGPS installations, SPGPS stations can be deployed quickly in response to changing volcanic conditions or earthquake activity such as a swarm or aftershock sequence. SPGPS networks can be more focused or more extensive than CGPS installations, because SPGPS equipment can be moved from station to station quickly to increase the total number of stations observed in a given time period. SPGPS networks are less intrusive on the landscape than CGPS installations, which makes it easier to satisfy land-use restrictions in ecologically sensitive areas. SPGPS observations are preferred over SGPS measurements because they provide better precision with only a modest increase in the amount of time, equipment, and personnel required in the field. We describe three applications of the SPGPS method that demonstrate its utility and flexibility. At the Yellowstone caldera, Wyoming, a 9-station SPGPS network serves to densify larger preexisting networks of CGPS and SGPS stations. At the Three Sisters volcanic center, Oregon, a 14-station SPGPS network complements an SGPS network and extends the geographic coverage provided by 3 CGPS stations permitted under wilderness land-use restrictions. In the Basin and Range province in northwest Nevada, a 6-station SPGPS network has been established in response to a prolonged earthquake swarm in an area with only sparse preexisting geodetic coverage. At Three Sisters, the estimated precision of station velocities based on annual ~3month summertime SPGPS occupations from 2009 to 2015 is approximately half that for nearby CGPS stations. Conversely, SPGPS-derived station velocities are about twice as precise as those based on annual ~1week SGPS measurements. After 5years of SPGPS observations at Three Sisters, the precision of velocity determinations is estimated to be 0.5mm/yr in longitude, 0.6mm/yr in latitude, and 0.8mm/yr in height. We conclude that an optimal approach to monitoring volcano deformation includes complementary CGPS and SPGPS networks, periodic InSAR observations, and measurements from in situ borehole sensors such as tiltmeters or strainmeters. This comprehensive approach provides the spatial and temporal detail necessary to adequately characterize a complex and evolving deformation pattern. Such information is essential to multi-parameter models of magmatic or tectonic processes that can help to guide research efforts, and also to inform hazards assessments and land-use planning decisions.

The 2004–2008 dome-building eruption at Mount St. Helens ended during winter 2007–2008 at a time when field observations were hampered by persistent bad weather. As a result, recognizing the end of the eruption was challenging—but important for scientists trying to understand how and why long-lived eruptions end and for public officials and land managers responsible for hazards mitigation and access restrictions. In hindsight, the end of the eruption was presaged by a slight increase in seismicity in December 2007 that culminated on January 12–13, 2008, with a burst of more than 500 events, most of which occurred in association with several tremor-like signals and a spasmodic burst of long-period earthquakes. At about the same time, a series of regular, localized, small-amplitude tilt events—thousands of which had been recorded during earlier phases of the eruption—came to an end. Thereafter, seismicity declined to 10–20 events per day until January 27–28, when a spasmodic burst of about 50 volcano-tectonic earthquakes occurred over a span of 3 h. This was followed by a brief return of repetitive “drumbeat” earthquakes that characterized much of the eruption. By January 31, however, seismicity had declined to 1–2 earthquakes per day, a rate similar to pre-eruption levels. We attribute the tilt and seismic observations to convulsive stagnation of a semisolid magma plug in the upper part of the conduit. The upward movement of the plug ceased when the excess driving pressure, which had gradually decreased throughout the eruption as a result of reservoir deflation and increasing overburden from the growing dome, was overcome by increasing friction as a result of cooling and crystallization of the plug.