P and S velocity structure of the Yellowstone volcanic field from local earthquake and controlled‐source tomography

Abstract

Abstract. The three-dimensional P and S velocity (from VF/V s ratios) distribution and improved hypocenters for the Yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled-source explosions. The P velocity model has an rms residual of ñ 0.09 s, whereas the Vp/Vs ratio model (calculated from 511 earthquakes) has an rms residual of ñ 0.29 s. High P and S velocities outside the Yellowstone caldera represent thermally undisturbed basement and sedimentary rocks. A caldera-wide 15% decrease from regional P velocities at depths of 6 to 12 km is coincident with a -60 mGal gravity anomaly and is interpreted as a hot, subsolidus, granitic batholith with a quasi-plastic rheology. Localized 30% reductions from regional seismic velocities and higher Vp/Vs ratios 8 km beneath Yellowstone's resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful (less than 2 Ma) silicic volcanism. Additional low seismic velocities and Vp/Vs ratios and a 20 mGal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt. Hypocenters relocated with the three-dimensional P velocity model show subparallel alignment with NW-SE trending postcaldera volcanic vents and normal faults northwest of the caldera. Focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera, reflecting thinning and heating of the seismogenic crust.

Geologic field-trip guide to the volcanic and hydrothermal landscape of the Yellowstone Plateau

“…Benz and Smith (1984) showed two zones of unusually low velocities in the northeast and the southwest part of the caldera. Miller and Smith (1999) found a caldera-wide 15 percent decrease in P-wave velocities compared to the surrounding area at depths of 6 to 12 km. In addition, they found a smaller but more pronounced low-velocity zone that underlies the northeast caldera rim from depths less than 2 km and explained this by the presence of a saturated fluid (gas or gas/liquid) filling fractures, and possibly the presence of hydrothermally altered volume of rock (Miller and Smith, 1999).…”

“…Miller and Smith (1999) found a caldera-wide 15 percent decrease in P-wave velocities compared to the surrounding area at depths of 6 to 12 km. In addition, they found a smaller but more pronounced low-velocity zone that underlies the northeast caldera rim from depths less than 2 km and explained this by the presence of a saturated fluid (gas or gas/liquid) filling fractures, and possibly the presence of hydrothermally altered volume of rock (Miller and Smith, 1999). A local earthquake tomographic study by Husen, Smith, and Waite (2004) confirmed the existence of a low P-wave velocity body beneath the Yellowstone Caldera at depths greater than 8 km, possibly representing hot, crystallizing magma.…”

“…Specifically, uplift related to the hot spot created an ideal situation for glacial ice accumulation, as shown by the Yellowstone geoid anomaly (Miller and Smith, 1999). The Yellowstone Plateau rises more than 1 km above the 90-to 100-km-wide topographic trough of the eastern Snake River Plain, where storms from the Pacific Northwest are channeled to the northeast.…”

The temporal evolution of chemical and physical properties of magmatic systems

“…Geophysical and geochemical evidence suggests that subvolcanic reservoirs are, in many cases, composed of multiple pockets of eruptible magma that may have been separated for most of their lifetime (Miller & Smith 1999;Bachmann & Bergantz 2008;Cashman & Giordano 2014;Cooper & Kent 2014;Ellis et al 2014;Wotzlaw et al 2014). The accumulation of residual melt in these separate reservoirs has been modelled hitherto solely as a single, large accumulation in the upper portion of a single reservoir.…”

A 3D algorithm based on the combined inversion of Rayleigh and Love waves for imaging and monitoring of shallow structures

“…In turn, such small perturbations can be detected as changes of seismic wave properties based on seismic noise (e.g. Ratdomopurbo & Poupinet 1995;Miller & Smith 1999;Grêt et al 2005;Wegler et al 2006). However, despite considerable efforts in the past, the precise monitoring of the location of subsurface changes has proven to be difficult, meaning that there is a need for novel observational methods to obtain information about the ongoing subsurface changes and their relation to volcanic processes.…”