Using Seismic Waves to Image the Yellowstone Magma Storage Region
How do we know what's beneath Yellowstone, and how can we image the shallow magma? Seismologists at the University of Utah (a YVO member agency) and the Swiss Federal Institute of Technology undertook a study to image the Yellowstone magma reservoir through a technique called seismic tomography. Using improved methods and data from thousands of earthquakes; they discovered that the magma reservoir is much larger than inferred in previous studies. The results from this new approach are published in the Journal Geophysical Research Letters.
To create an "image" of the magma storage region (reservoir), the research team used data from the Yellowstone Seismic Network from 1984-2011 including 4,520 earthquakes and a total of 48,622 individual travel time measurements. They then mapped how fast the P-waves (the fastest type of earthquake wave) moved through different parts of the subsurface beneath Yellowstone. The resulting model reveals a very large region where P-waves move more slowly, interpreted to be due to the presence of warm, partially melted rock; that is the crustal magma storage region that has fueled Yellowstone's past volcanic activity. The inferred magma body is 90 km long, extends from 5 km to 17 km depth, and is 2.5 times larger than imaged in a previous study.
Analysis of the data reveals that the magma reservoir contains between about 5 and 15% molten rock (melt) that occupies pore spaces between solid (crystalline) material. Magma typically does not erupt unless it has greater than 50% melt. Importantly, the low-velocity body extends about 15 km NE of the caldera at depths shallower than 5 km. This shallow region to the northeast could contain some partial melt but is most likely due to the presence of hot water, other fluids, and gasses coming off of the main body of the magma reservoir. This region correlates with a concentrated area of low-density rocks (with a large negative "Bouguer" gravity anomaly) as measured by gravimeters, instruments that measure the local gravity field. This northeasterly progression of the magma system is consistent with the southwestern movement of the North American Plate at ~2.35 cm/yr over a stationary plume of hotter mantle material (the Yellowstone hot spot) that is located beneath about 60-90km.
The increase in the inferred size of the magma reservoir is due in part to the greater extent of coverage by the Yellowstone Seismic Network, especially in the northeast part of the park. Six new seismographs were added over the last decade to better image the northeast part of the park and explore the region with the large negative gravity anomaly.
This research and resulting updated seismic image provides important constraints on the dynamics of the Yellowstone magma system and its potential for future volcanic eruptions and earthquakes. The larger imaged size of the magma reservoir better matches the geologic record of Late Quaternary volcanic eruptions and lava flows but, importantly, does not increase the volcanic hazard in the Yellowstone region.