Seismic Swarm SVS20040901.1 in the Yellowstone Region

Earthquake swarm SVS20040901.1 occurred approximately 55 km south-southeast of Mammoth, Wyoming, within the Yellowstone volcanic system. The sequence began at 23:32 on 31 August 2004 and concluded at 09:48 on 2 September 2004, spanning 34 hours and 15 minutes. During this interval, 27 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.4 and focal depths primarily between 1 and 6 km.

The events clustered tightly in time and space, characteristic of swarm behavior where numerous small quakes occur without a distinct mainshock-aftershock pattern. Notable activity included a magnitude 2.0 event at 01:29 on 1 September and a magnitude 2.4 event at 02:25 the same day, both at depths of 3 km. Shallower events (1–2 km) dominated later phases, while one outlier reached 16 km depth.

This swarm aligns with the broader seismic character of the Yellowstone region. The area overlies a large silicic volcanic system featuring a caldera formed by major eruptions, most recently 640,000 years ago. Ongoing crustal deformation arises from a combination of tectonic extension along the Basin and Range province, magmatic fluid movement, and hydrothermal circulation. Earthquake swarms here frequently result from pressurized fluids or migrating magma at depth rather than purely tectonic stress release.

Since 2000, eight swarms have been documented in the region, with notable clusters in 2000 (one swarm), 2001 (one swarm), and 2002 (six swarms). These episodes underscore the persistent low-level unrest typical of Yellowstone, where annual earthquake counts often exceed 1,000, the vast majority below magnitude 2.0. Depths in the 1–6 km range commonly reflect brittle failure within the shallow crust influenced by geothermal gradients.

The 2004 swarm provides insight into swarm dynamics: rapid onset, short duration, and limited magnitude range suggest fluid-driven triggering. Such patterns aid in distinguishing volcanic or hydrothermal processes from tectonic events. No surface deformation or changes in hydrothermal features were associated with this particular sequence.

Continued monitoring by the Yellowstone Volcano Observatory supports hazard assessment, as swarms contribute to understanding long-term volcanic and seismic risk in this active caldera setting.

References

• U.S. Geological Survey, Yellowstone Volcano Observatory reports (usgs.gov)
• University of Utah Seismograph Stations, Yellowstone seismic data archives (quake.utah.edu)