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Modern Deformation and Uplift in the Sisters Region

In 2001, scientists discovered a broad area of uplift west of South Sister that had been rising a few cm (inches) per year since the 1990s.  Uplift rates have varied, with a new pulse starting in 2020–2021. Geochemistry data indicate that magmatic activity in the region predates the past few decades.

Discovery of Uplift and Geochemical Anomalies: 1990s-2001

In 2001, inspection of InSAR data (a technique that uses satellite data to measure deformation of the ground surface) revealed a broad region of uplift to the west of South Sister volcano in central Oregon.  The deformation suggested accumulation of either magma or a hot, buoyant plume of water and gas at a depth of about 6 km (4 mi) beneath the surface.  Modeling of satellite imagery suggests that the deformation started in late 1995 or 1996. 

Interferogram image made from InSAR monitoring, showing 1995-2001 ground uplift.
About 130 mm (5 inches) of peak uplift from 1995 to 2001 is shown in this interferogram in the Three Sisters Wilderness. Magma accumulating at about 7 km (about 4.5 mi) depth is the likely cause of uplift. The area of uplift, about 20 km (12 miles) in diameter, is almost entirely within the wilderness area. The two white "thumbtacks" show the location of continuous GPS instruments installed by Cascades Volcano Observatory personnel to monitor the uplift. The last volcanic eruption in the area was about 1500 years ago. (Public domain.)
Deformation and geochemical anomalies near South Sister, Oregon
(Top) 1996–2000 interferogram draped over a 30-m DEM and showing uplift centered a few kilometers (miles) west of South Sister volcano. (Bottom) Geochemical data on top of the 1996–2000 interferogram. The columns show chloride (Cl) and sulfate (SO4) concentrations at sampled springs. The lengths of the cyan columns depict Cl concentration (0.6 to 18.6 mg/L versus a regional background of 0.2 to 0.7 mg/L). The lengths of the red columns show SO4/Cl ratios (0.2 to 2.8). Blue lines highlight stream areas with steady or decreasing Cl concentration in a downstream direction. Red lines highlight stream areas within which Cl concentrations increase downstream. The color bar shows a range increase from 0 to 28.3 mm that corresponds to a continuous color change from violet to red.  Figure adapted from Wicks and others (2002).

The uplift is centered in a region where chloride values in streams and springs are unusually high.  Chloride concentrations can reflect the presence of magma in the subsurface—as the magma slowly cools and emits gases and fluids, chloride concentrations can increase in springs and streams above the magma body.  The anomalous chloride values indicate that the uplift that started in the mid-1990s is not caused by a brand-new magma body, but instead probably reflects an episode of magma or magmatic fluid accumulation in a long-lived magma reservoir complex.


Global Positioning System receiver (dome on pole) and seismometer
Global Positioning System receiver (dome on pole) and seismometer (box on ground in distance) at a volcano monitoring station for Three Sisters in Central Oregon. Solar panel mounted to pole.

Expanded Monitoring and Decaying Uplift Rates: 2001–2020 

In response to the discovery of uplift, Cascades Volcano Observatory scientists installed new monitoring stations and conducted surveys in the region starting in 2001.  GPS data were collected from benchmarks located around the uplifted region.  Comparison of these measurements over time shows that the ground was moving up and away from the center of uplift.  Additional sampling of springs and streams confirmed that water continued to contain elevated levels of chloride, but other gas and water compositions did not suggest major changes over time.

To better track activity west of South Sister in real time, a continuous GPS station and seismometer were installed on The Husband—a peak located about 2 km (1.25 mi) west of the center of uplift.  These stations were connected by radio to the Cascades Volcano Observatory so that earthquake and ground deformation activity could be detected year-round.  Despite the uplift, seismicity in the region remained low, with the exception of a small swarm of about 300 earthquakes (max M=1.9) in March 2004.

The rate of uplift, as measured by continuous GPS and InSAR, decreased slowly over time, from 3–5 cm (1–2 in) per year in 1999, to 1.4 cm (0.5 in) per year by 2006, to about 0.5 cm (0.2 in) per year by 2017.  Careful analysis of GPS and InSAR data indicate that even during the overall decay in the uplift, there were small, temporary increases in inflation rate approximately every 3–4 years.  These changes in rate suggest that minor pulses of magma or fluid accumulation were occurring, although these pulses were far smaller than the pulse that peaked in 1999 and that led to the discovery of the uplift. 

By 2020, the maximum amount of uplift measured by satellite radar data at the center of the uplift west of South Sister since 1995 was approximately 30 cm (12 in). At continuous GPS site HUSB, a few kilometers (miles) west of the center of deformation, maximum uplift since 2001, when the station was installed, was about 15 cm (6 in). 

Renewed Uplift and Seismicity: 2020–Present 
GPS time series from station HUSB, 2001-2022, showing uplift west of South Sister, Oregon
Time series showing daily vertical GPS positions from the continuous GPS station HUSB between 2001 and 2022.  Each dot represents the position of the HUSB station on that day. Data have been adjusted to remove artifacts caused by snow covering the GPS antenna, as well as seasonal fluctuations. Gray bar indicates time period spanned by 2020–2021 interferogram. 
Deformation near South Sister from GPS data, 2001-2017
Horizontal displacements from campaign (black vectors) and continuous (red vectors, with station names given) GPS stations, as well as vertical displacements (indicated by color of GPS station symbol) near South Sister, Oregon. Length of arrow gives amount of horizontal displacement, with scale in lower left showing an arrow length indicating 5 cm (2 in) of motion. Circles at end of arrows indicate uncertainty in horizontal deformation (the arrow ends are 95% certain to lie within the associated circle). Displacements are cumulative between 2001 and 2017 based on a logarithmic fit to the time series from continuous GPS site HUSB and applied to all other campaign and continuous GPS stations. Summit of South Sister is indicated by white triangle, and white circle marks location of modeled inflation source.  Figure adapted from Poland and others (2017).


Recent analyses of InSAR and GPS data indicate that a renewed pulse of uplift—the highest rate since 1999—began at South Sister starting in 2020 or 2021.  InSAR data spanning from summer 2020 to summer 2021 show that the uplift is centered in the same place as the deformation that started in the 1990s.  Continuous GPS data from the site on The Husband, HUSB, show a subtle increase in the rate of uplift since mid-summer 2021.  Starting in late 2021, several small earthquake swarms were detected in the region, with bursts of seismicity in October 2021, December 2021, and January 2022.  Most of the earthquakes were too small to be located, but those that were large enough occurred within the broad region of uplift.

What Does the Uplift Mean? 

Interferogram spanning 2020-2021 and showing uplift west of South Sister
Satellite radar interferogram spanning June 19, 2020, to August 13, 2021, and showing the ground motion in the direction of the satellite.  The maximum uplift, indicated by the red color, is about 2.2 centimeters (0.85 inches), and it is located to the west of South Sister.  The HUSB continuous GPS site is marked by the large black dot.  Earthquakes that occurred during the time period spanned by the interferogram are indicated by small black dots.

Uplift just west of South Sister is probably caused by accumulation of magma or magmatic fluids (water and gas) at about 6 km (4 mi) beneath the surface.  Geochemistry data from springs and streams indicate that magma has been present beneath the region for a long time.  The uplift measured since 1995 is probably part of a long-term sequence of magmatic activity that occurs in pulses, with rates that vary over time. 

The ultimate outcome of the activity is uncertain.  Any magmatic intrusion could eventually lead to a volcanic eruption, but that would probably be preceded by more vigorous unrest. In general, as magma moves closer to the surface it causes dramatic uplift, fractures rock to generate intense swarms of earthquakes, and releases significant amounts of volcanic gases, such as carbon dioxide. Such activity would be detected by the enhanced monitoring network that was installed in the early 2000s in response to the earlier episode of uplift. 

If an eruption were to occur, it would not necessarily be focused on one of the Three Sisters stratovolcanoes.  Central Oregon is a complex volcanic environment, with eruptions that span a range of compositions and styles.  The region has numerous cinder cones and lava shields that erupt away from the main volcanic vents of the Cascades, and this uplift may reflect a magma body that will eventually feed such an eruption.  It is also possible that no eruption from this magma body will ever occur (globally, it is common for magmatic intrusions to occur without leading to an eruption). 

The Cascades Volcano Observatory will continue to closely monitor the area for changes that might indicate a change in the style of magmatic activity. 


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