Resolving Continental Magma Reservoirs With 3D Surface Wave Tomography

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 8
• Main Authors: Maguire, Ross, Schmandt, Brandon, Chen, Min, Jiang, Chengxin, Li, Jiaqi, Wilgus, Justin
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.08.2022; Wiley
• Subjects: Anomalies; Calderas; Imaging techniques; Inversions; Lava; Magma; Magma chambers; magmatic systems; P-waves; Phase velocity; Properties; Reservoirs; Seismic activity; Seismic tomography; Seismic waves; Shear; Surface water waves; Surface waves; Tomography; Velocity; Volcanic eruptions; Wavelength; Wavelengths
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010446

Surface wave tomography is widely used to improve our understanding of continental magma reservoirs that may be capable of fueling explosive volcanic eruptions. However, traditional surface wave tomography based on inversions for phase velocity maps and locally 1D shear velocity may have difficulty resolving strong 3D low‐velocity anomalies associated with crustal magma reservoirs. Here, we perform synthetic tomography experiments based on 3D seismic waveform simulations to understand how the limitations of surface wave tomography could affect interpretations of tomography in volcanic settings. We focus our modeling on the Yellowstone volcanic system, one of the largest and most thoroughly studied continental magmatic systems, and explore scenarios in which the maximum shear velocity anomaly associated with the crustal magma reservoir ranges between −10% and −66%. We find that even with the well‐instrumented setting near Yellowstone, the recovered shear velocity anomalies in the mid‐to‐upper crust are severely diminished due to the small spatial scale of the reservoir with respect to the seismic wavelengths that sample it. In particular, recovered VS anomalies could be reduced by a factor of two or more, implying that the inferred melt fraction of large‐scale continental magma reservoirs may be considerably underestimated. Plain Language Summary Some of the largest explosive volcanic eruptions in the geologic record, such as those that formed Yellowstone Caldera, are fueled by extensive magma reservoirs in the crust. Much of what we know about the properties of these magma reservoirs comes from a geophysical imaging technique known as seismic tomography, which measures variations of seismic wave speed in the subsurface. The wave speed is then used to infer important properties of the magma reservoir such as the distribution of magma in the crust. Here, using computer simulations of seismic waves propagating through models of crustal magma reservoirs, we explore the limitations of surface wave tomography, one of the more common methods used to study magmatic systems. We find that, largely due to the small size of magma reservoirs compared to the wavelength of surface waves used to image them, melt fractions inferred from previous tomography studies may be underestimating the amount of melt present in the crust. Key Points 3D seismic wavefield simulations are used to generate complete seismograms for testing surface wave tomography of magma reservoirs Underestimation of velocity anomalies is scale‐dependent due to seismic wavelengths and instrument spacing Underestimation of velocity anomalies is more severe for extreme structures like those expected for eruptible reservoirs