SEISMICS Seismic studies by Smith (1991) reveal that waves slow down as they pass beneath the caldera. This area of slower-than-normal seismic velocity is as shallow as 1 mile. From this information, we know the waves are passing through molten magma material. The shallow depth indicates the magma is in the upper part of the crust. In the area north east of the caldera, seismic velocities are even slower. However, the magma in this area is estimated to be about 2 miles from the surface. This is indicative of a continuous magma body. Miller and Smith (1997) focused on the area outside of the caldera. They used P and S velocities to find the distribution of hypocenter locations. From their evidence, they concluded that the Yellowstone volcanic field shows strong evidence for crustal magma chambers. This was determined from using the inversion of primary arrival times from 7,942 local earthquakes and 16 source explosions that were controlled. The P wave model had an rms of +/-0.09 seconds. The Vp/Vs ratio model had an rms of +/- 0.29 seconds. These high P and S wave velocities outside the caldera represent thermally undisturbed basement rocks and sedimentary rocks. A caldera-wide 15% decrease from regional P velocities at depths of 6 kilometers to 12 kilometers is coincident with a negative gravity anomaly. This information means it is a hot, subsolidus, granitic batholith with a quasi-plastic rheology. Figure from Smith & Miller showing P and S wave velocity structure of Yellowstone. Miller and Smith also found a 30% localized reduction in regional seismic velocities. The Vp/Vs ratios were also higher 8 kilometers beneath the resurgent domes. These data lead to the conclusion that there is partial melts and vestigial magma systems associated with youthful (less than 2 MA) silicic volcanism. Miller and Smith also conducted studies beneath the caldera rim. The area studied was about 4 kilometers beneath the surface. They found low P and S wave velocities and low Vp/Vs ratios. This means the area is a hydrothermal fracture zone thermally driven by underlying partial melt. Humphreys, Dueker, Schutt, and Smith (2000) used teleseismic seismology to study the Yellowstone hotspot. This was done by deploying a seismic array occupying 50 sites in a straight line across the width of the swell. Their three component broadband seismometers allow them to obtain receiver function imagining of the crust and upper mantle interfaces, S-wave splitting analysis for upper mantle anisotropy, and P and wave tomographic imaging of upper mantle velocity variations. The receiver function is used for crustal and upper mantle discontinuities. Their analysis showed 1) a mid-crustal basalt sill across the width of the Snake River Plain; 2) about a 5 kilometer thick partially molten lowermost crust across the width of the plain; and 3) a Moho that is approximately flat across the width of the seismic parabola which thickens rapidly southeast of the swell. From the P-wave velocities, the sill is half basalt and half granitic country rock. The partially molten lowermost crust is more than likely gabbroic crust. To study the anisotropy of the upper mantle, they use S-wave splitting. Waves that are not naturally polarized in the orientation of the hotspot track with the North American plate are not split.
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