Mushy Magma beneath YellowstoneRisheng Chu1, Don Helmberger1, Daoyuan Sun1, Jennifer Jackson1, Lupei Zhu2

California Institute of Technology1 and Saint Louis University2

Abstract: A recent prospective on the Yellowstone Caldera discounts its explosive potential based on inferences from tomographic studies on regional earthquake data which suggests a high degree of crystallization of the underlying magma body. In this study, we analyzed P-wave receiver functions recorded by broadband stations above the caldera from 100 teleseismic earthquakes between January and November 2008. After applying a number of waveform modeling tools, we obtained much lower seismic velocities than previous estimates, 2.3 km/sec (Vp) and 1.1 km/sec (Vs), with a thickness of 3.6 km in the upper crust. This shallow low velocity zone is severe enough to cause difficulties with seismic tool applications. In particular, seismologists expect teleseismic P-waves to arrive with motions up and away or down and back. Many of the observations recorded by the Yellowstone Intermountain Seismic Array, however, violate this assumption. We show that many of the first P-wave arrivals observed at seismic stations on the edge of the caldera do not travel through the magma body but have taken longer but faster paths around the edge or wrap-around phases. Three stations near the trailing edge have reversal radial-component motions, while stations near the leading edge do not. Adding our constraints on geometry, we conclude that this relatively shallow magma body has a volume of over 4,300 km3. We estimate the magma body by assuming a fluid-saturated porous material consisting of granite and a mixture of rhyolite melt and supercritical water and CO2 at temperatures of 800 0C and pressure at 5 km (0.1 GPa).Theoretical calculations of seismic wave speed suggests that the magma body beneath the Yellowstone Caldera has a porosity of 32% filled with 92% rhyolite melt and 8% water-CO2 by volume.

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Figure 3 Snapshots of wave propagation using finite difference algorithm for teleseismic wave coming from northwest (left). Seismic waves that travel through the LVZ are much slower than waves that wrap around the edge. Beginning at 17.5 sec, the wrap-around waves can be seen at the trailing edge which are shown in the synthetic waveforms (right).

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Figure 1 (A) Complete Bouguer gravity anomaly map of the Yellowstone area showing structural, hydrothermal, volcanic, geophysical and seismological features. The solid thick contour marks the boundary of the Yellowstone National Park. The dashed line denotes the boundary of the Yellowstone Caldera generated by the eruption 0.64 Ma ago. Red asters are locations of post-caldera volcanic vents and yellow stars stand for mapped hydrothermal features. The interpreted magma migration paths are shown by arrows (taken from Wicks et al. (2006) ). Thin lines are Quaternary faults. Earthquakes with Mw >= 1.0 between 2000 and 2008 are shown by gray circles whose size represents magnitude. Broadband seismic stations include H17A (red squares), LKWY (red star), and Y100, Y102, and Y103 (red triangles). The yellow contour indicates the position of Yellowstone Lake. (B) The stacked InSAR interferogram and GPS velocities between 2004 and 2006 show uplift of the caldera (taken from Chang et al., (2007)), see caption of Fig. 2 of Chang et al., (2007) for details. The rapid changes in ground uplift are compatible with about 0.1 km3 magma change in volume yearly. The shape of the magma body agrees with the purple area suggested by the low gravity anomaly.

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Figure 2 (A) Velocity structures beneath station H17A and LKWY inferred from receiver function modeling. Various ray paths from southeast (red) and northwest (blue) are identified. An ultra low velocity zone (LVZ) is embedded in the upper crust beneath the Yellowstone Caldera with a thickness of 3.6 km with Vp = 2.3 km/sec, Vs = 1.1 km/sec. On the southeast side, the LVZ is a horizontal layer at a depth of 9.0 km. On the northwest, the LVZ is dipping with an angle of 2 degree at 5.4 km . The ray paths, 1, 2 and 3, are direct P wave through the LVZ, P-to-SV converted phase from top of the LVZ and P-to-SV phase from bottom of the LVZ, respectively. Solid and dotted lines indicate P and SV waves, respectively. Gray dots represent earthquake hypocenters. The horizontal dimension of the LVZ agrees with InSAR observations. (B) Observed (solid) and synthetic (dashed) receiver functions for stations H17A and LKWY. Different ray paths in (A) are also labeled as they change mode from P to S. Phases after 3 sec are multiples trapped above the LVZ. (C) Comparison of observed data (solid) and synthetic waveforms (dashed) calculated using finite difference algorithm (Yan and Clayton, 2007) and the velocity model from (A) for the vertical and radial component velocity-field. (D) Comparison of waveform data (solid) and synthetics (dashed) for stations Y102 and Y103. If the velocity drop is severe enough (upper two rows, dVs=-66%), the radial component becomes reversed or flipped (Y102 northwest and Y103 southeast) as observed. Otherwise, radial and vertical components have the same polarities (lower two rows).

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Figure 4 Theoretical compressional (black) and shear (red) velocities in granite as a function of porosity filled with different rhyolite melt and water-CO2 contents. The various Vp curves show combined water-CO2 saturation from 0% to 10% with an inclement of 2%. The corresponding Vs curves (solid and dashed) are not distinguishable. It suggests that adding water/CO2 into the magma will not change shear velocities much. Compressional velocities, however, change significantly, especially when the porosity approaches the critical porosity. Red stars denote our modeling velocities suggesting a porosity of 32% and water/CO2 saturation of 8% by volume. For comparison, we also plotted results from previous studies (gray circles) based on tomography of regional earthquake data, inversion of surface wave dispersion, and ambient noise tomography.

Material Vp (km/sec) Vs (km/sec) ρ (g/cm3) T ( 0C) P (GPa)

granite 5.70 3.40 2.63 625 0.42rhyolite melt 2.52 0.0 2.20 1000 0.0carbon dioxide 0.675 0.0 0.37 827 0.1water 0.82 0.0 0.23 800 0.1

Table 1 Physical properties of rocks and fluids used in the calculation.

Conclusions:1) The magma chamber beneath Yellowstone has a volume of 4,300 km3 with compressional and shear velocities at 3.3 km/sec and 1.1 km/sec, respectively.2) The ultra slow velocities are severe enough to reverse radial P-wave motions for stations near the trailing edge.3) The magma body has a porosity of 32% filled with 92% rhyolite melt and 8% water-CO2 by volume.

For questions and comments, please contact: chur@gps.caltech.edu