| Description |
Here I evaluate the relationship between the seismicity in the Yellowstone region, in particular the properties of the dominant earthquake swarms, and the three-dimensional Vp seismic velocity structure employing local earthquake tomography. The Yellowstone region averages ~1,500-2,000 earthquakes per year and ~40% occur in swarms. Two of the largest Yellowstone swarms have provided an important opportunity to better understand how and why swarms occur in Yellowstone and how they may be related to active volcanic and tectonic processes. The 2008-2009 Yellowstone Lake swarm consisted of ~800 events with magnitudes ranging from -0.5 ≤ MC ≤ 4.1 and was modeled by a migration at up to 1 km per day as an upper-crustal dike-intrusion of magma or magmatically-derived aqueous fluids. The 2010 Madison Plateau swarm exhibited over 2,200 earthquakes with magnitudes ranging from -0.6 ≤ MC ≤ 3.9 and may have occurred on structures at depth related to the nearby Hebgen Lake fault or may have been facilitated by the movement of hydrothermal fluids away from the Yellowstone caldera. Both swarms occurred during a period of caldera deformation reversal from uplift to subsidence and may be indicative of processes involving pressurized fluids escaping the caldera into the surrounding region, allowing the caldera to enter into a time of subsidence. These fluids are derived from the Yellowstone magma reservoir, a large body of crystallizing rhyolite magma that underlies most of the Yellowstone caldera. To better understand the extent and composition of the Yellowstone magmatic system, we have used data from the Yellowstone Seismic Network from 1984-2011 to image the P-wave velocity structure of the Yellowstone crust using local earthquake tomography using the 83-station Yellowstone seismic network. P-wave tomographic images revealed a large, low P-wave anomaly with values up to -7% change from a background normal crustal velocity structure, underlying most of the Yellowstone caldera at depths of 5-16 km, notably ~50% larger than imaged in earlier studies. The low P-wave velocity body extends ~20 km beyond the caldera to the NE at depths of less than 5 km and has aerial dimension of 30 km wide and 90 km long. |
