Controlled-Source Seismic Survey of the Yellowstone Caldera: Preliminary Results from Waves Reflected from the Top of the Magma Reservoir

Despite extensive previous work on the Yellowstone magmatic system, the depth to the top of the magma reservoir and its melt fraction remain unclear. Most previous seismic studies use surface and/or body-wave tomography to image smooth volumetrically averaged properties of the magma reservoir and estimate the top of the reservoir at ~3 to 8 km depth and a melt fraction of ~5-15%. To obtain different sensitivity to magma reservoir structure, we conducted a controlled-source seismic survey in September 2020, which was designed for recording reflections from the top of the magma reservoir. The survey was conducted with 128 vertical-axis vibroseis source locations along two transects overlying the reservoir location estimated with tomography. Each vibroseis location was sampled with 20 sweeps from 6-30 Hz over a 40-second duration, with a ~24,000 kg truck. The vibroseis signals were recorded by three-component nodal seismometers with an average spacing of ~175 m along both transects. The southwestern transect near the Mallard Lake dome is ~22 km long and the northeastern transect near the Sour Creek dome is ~32 km long. Stacks of short-term-average to long-term-average ratio of 5-25 Hz envelopes show PP and PS reflections, where both transects overlie the estimated magma reservoir. The reflector disappears southeast of the Sour Creek dome, where the magma reservoir is weak or absent in seismic tomography. If we assume mean P-wave velocities of ~4.5-4.75 km/s, based on direct P-wave arrivals from the vibroseis survey, the two-way travel time of ~1.6 s for the PP reflection suggests that the depth to the top of the reservoir is ~3.6-3.8 km. The preliminary results are consistent with new short-period surface wave tomography, which used the dense array and surrounding networks to find a concentrated low-velocity layer with its top at ~4-5 km depth. Further analysis of the ratio of PS to PP reflection amplitudes as a function of incidence angle will provide localized estimates of the velocities and melt fractions near the top of the magma reservoir.