The Lassen region of the southernmost Cascade Range is an active volcanic area. At least 70 eruptions have occurred in the past 100,000 years, including 3 in the past 1,000 years, most recently in 1915. The record of past eruptions and the present state of the underlying magmatic and hydrothermal systems make it clear that future eruptions within the Lassen Volcanic Center are very likely. Although the annual probability of an eruption is small, the consequences of some types of eruptions could be severe. Compared to those of a typical Cascade composite volcano, eruptive vents at Lassen Volcanic Center and the surrounding area are widely dispersed, extending in a zone about 50 km wide from the southern boundary of Lassen Volcanic National Park north to the Pit River. This report presents a discussion of volcanic and other geologic hazards in the Lassen area and delineates hazards zones for different types of volcanic activity. Owing to its presence in a national park with significant visitorship, its explosive behavior, and its proximity to regional infrastructure, the Lassen Volcanic Center has been designated a "high threat volcano" in the U.S. Geological Survey National Volcano Early Warning System assessment. Volcanic eruptions are typically preceded by seismic activity and ground deformation, and the Lassen area has a network of seismometers and Global Positioning System stations in place to monitor for early warning of volcanic activity.

This report evaluates the volcano-related hazards of the Lassen region, California, which is here defined as an area between the Pit River on the north and the southern limit of active Cascade volcanism, approximately 5–10 km south of the southern boundary of Lassen Volcanic National Park. Most active volcanism occurs in a zone about 40 km wide between Viola on the west and the eastern boundary of Caribou Wilderness Area, but sparser volcanism in the west extends the width of this zone to about 75 km. The report is based primarily on knowledge of hazardous phenomena observed at other, similar volcanoes and on recently published geologic mapping and geochronology (Clynne and Muffler, 2010; Christiansen and others, 2002; Clynne and others, 2008) applied to the volcano-hazards framework established by Miller (1989).