Seismic Swarm S20090630.1 Near Mammoth, Wyoming: Geological Context and Event Analysis

Seismic swarm S20090630.1 occurred approximately 25 km south-southwest of Mammoth, Wyoming, within the Yellowstone volcanic region. The sequence began at 15:52 on 30 June 2009 and concluded at 04:40 on 2 July 2009, spanning 36 hours and 48 minutes. During this period, 34 earthquakes were recorded, with magnitudes ranging from -0.3 to 3.2 and focal depths primarily between 2 and 15 km.

The initial event reached magnitude 3.2 at a depth of 9 km, followed rapidly by smaller events clustered at shallower depths. Activity showed a typical swarm pattern, with multiple low-magnitude events occurring in close succession rather than following a classic mainshock-aftershock sequence. Depths concentrated around 2–7 km for most events, consistent with shallow crustal processes in the area. Notable larger events included a magnitude 2.2 earthquake at 16:52 on 30 June at 7 km depth and several magnitude 1.5 events distributed throughout the first day.

This swarm aligns with the broader seismicity of the Yellowstone Plateau, situated atop an active volcanic system driven by a mantle hotspot. The region features a large caldera formed by massive eruptions over the past 2.1 million years, with the most recent at approximately 631,000 years ago. Ongoing deformation, hydrothermal activity, and magma movement at depth contribute to frequent earthquake swarms. Historical records indicate elevated swarm activity in this tectonic setting, where fluid migration and crustal stress changes often trigger clustered seismicity without significant surface rupture.

Since 1 January 2000, 42 swarms have been documented in the area. Yearly counts include 10 in 2000, 5 in 2001, 6 in 2002, 2 in 2003, 1 in 2004, 1 in 2005, 5 in 2006, 3 in 2007, 7 in 2008, and 2 in 2009. These episodes underscore the persistent nature of seismic unrest linked to the underlying volcanic and hydrothermal systems.

Analysis of the 2009 sequence reveals a rapid onset followed by diminishing activity, with the majority of events below magnitude 1.0. Such patterns provide insight into localized stress release within the brittle upper crust overlying more ductile zones influenced by heat from magmatic sources. Depths remained consistently shallow, supporting interpretations of hydrothermal fluid involvement rather than deeper magmatic intrusion.

The Yellowstone region continues to be monitored closely due to its volcanic history and potential hazards. Swarms like S20090630.1 represent routine manifestations of the dynamic geological environment rather than indicators of imminent large-scale eruptions.

References:
United States Geological Survey Earthquake Hazards Program archives
Yellowstone Volcano Observatory reports on regional seismicity
SeismoSight internal swarm classification database