Evidence for gas and magmatic sources beneath the Yellowstone volcanic field from seismic tomographic imaging

The 3-D P-wave velocity and P- to S-wave velocity ratio structure of the Yellowstone volcanic field, Wyoming, has been determined from local earthquake tomography using new data from the permanent Yellowstone seismic network. We selected 3374 local earthquakes between 1995 and 2001 to invert for the 3-D P-wave velocity (Vp) and P-wave to S-wave velocity ratio (Vp/Vs) structure. Vp anomalies of small size (15×15 km) are reliably imaged in the northwestern part of the model outside the Yellowstone caldera; inside the caldera only Vp anomalies of large size extending over several grid nodes are reliably imaged. The Vp/Vs solution is generally poorer due to the low number of S–P arrival times. Only the northwestern part of the model is resolved with confidence; the Vp/Vs solution also suffers from strong vertical and horizontal velocity smearing. The tomographic images confirm the existence of a low Vp-body beneath the Yellowstone caldera at depths greater than 8 km, possibly representing hot, crystallizing magma. The most striking result of our study is a volume of anomalously low Vp and Vp/Vs in the northwestern part of the Yellowstone volcanic field at shallow depths of <2.0 km. Theoretical calculations of changes in P- to S-wave velocity ratios indicate that these anomalies can be interpreted as porous, gas-filled rock. The close spatial correlation of the observed anomalies and the occurrence of the largest earthquake swarm in historic time in Yellowstone, 1985, suggest that the gas may have originated as part of magmatic fluids released by crystallization of magma beneath the Yellowstone caldera.