Assessing Seasonal Changes in Microgravity at Yellowstone Caldera

Abstract

Microgravity time series at active volcanoes can provide an indication of mass change related to subsurface magmatic processes, but uncertainty is often introduced by hydrologic variations and other noise sources that cannot easily be isolated. We empirically assessed seasonality and noise by conducting four surveys over the course of May-October 2017 at Yellowstone caldera, Wyoming. Yellowstone experiences frequent changes in the rates and styles of seismicity and surface deformation, but the mechanisms of these changes are poorly understood. Past gravity data from the caldera have yielded ambiguous results, possibly due to hydrologic noise. Given the strong visually observable changes in surface water and snow conditions over the course of our surveys, we expected to see significant variations in gravity. The net change in gravity, however, was less than 20 μGal at most sites, and there was no strong correlation with river and lake levels or snow conditions. Seasonal changes in gravity are therefore small compared to those that would be expected from magmatic activity, although they may be on the same order as those associated with Yellowstone's hydrothermal system. We did find that noise levels in gravity data were highly dependent on site characteristics, with bedrock sites away from trees yielding the lowest levels of noise, and thin concrete pads in forested areas the highest. These results can be used to plan future surveys at Yellowstone and to reinterpret past data, and they provide guidance in terms of best practices for repeat gravity work on volcanoes worldwide.

Post‐Collapse Gravity Increase at the Summit of Kīlauea Volcano, Hawaiʻi

“…GNSS stations in the south part of the caldera (CALS, CRIM, and OUTL) are characterized by subsidence of 3–5 cm over the time spanned by the gravity campaigns. This subsidence, which is also apparent in interferograms (Figure b), would cause a free‐air gravity increase of up to 15 μGal, which is close to the expected repeatability (Poland & de Zeeuw‐van Dalfsen, ). For station 112YY (Figure ), we used the 5‐cm subsidence of collocated GNSS site OUTL to determine a free‐air effect of 15 μGal, which lowers the observed gravity change from 23 ± 9 to 7 ± 13 μGal.…”

“…Data analysis followed Poland and de Zeeuw‐van Dalfsen (), with tilt and tide corrections provided by the instrument and daily drift rates for each gravimeter calculated using gTOOLS (Battaglia et al, ). For the single‐looped days in November 2018, we calculated the drift rates based on a linear fit to repeat occupations of the base station and then averaged the 2 days, taking the standard deviation of the measurements as the uncertainty in the gravity values.…”

“…For the single‐looped days in November 2018, we calculated the drift rates based on a linear fit to repeat occupations of the base station and then averaged the 2 days, taking the standard deviation of the measurements as the uncertainty in the gravity values. For station HVO35, which was measured on only 1 day, we assigned an uncertainty of 20 μGal, which is expected to be a worst‐case scenario (Poland & de Zeeuw‐van Dalfsen, ).…”

Dynamics of Episodic Magma Injection and Migration at Yellowstone Caldera: Revisiting the 2004–2009 Episode of Caldera Uplift With InSAR and GPS Data

“…However, this hypothesis can not be proved as either true or false because only two cycles of uplift in 1923-1984 and 1996-1997 before 2004 were recorded with instrumental observations, both with a significantly worse temporal sampling than the 2004-2009 cycle. Micro-gravity data were only measured during 2007-2012 (Farrell, 2014), and then since 2017 (Poland & Zeeuw-van Dalfsen, 2019). The 2007-2012 data did not show clear gravity changes but as the data did not include high quality elevation measurements for each gravity station, it did not provide insights on any particular geological process (Poland & Zeeuw-van Dalfsen, 2019).…”

“…Micro-gravity data were only measured during 2007-2012 (Farrell, 2014), and then since 2017 (Poland & Zeeuw-van Dalfsen, 2019). The 2007-2012 data did not show clear gravity changes but as the data did not include high quality elevation measurements for each gravity station, it did not provide insights on any particular geological process (Poland & Zeeuw-van Dalfsen, 2019). Therefore the gravity data cannot be directly compared with the InSAR and GPS observations during 2004-2009.…”