California volcano locations, threat rank and hazard zones

The California volcano locations, threat rank and hazard zones data release contains two shapefiles for download or use as a web map service. The California Volcanic Center Locations shapefile was created to provide a generalized location of volcano hazard sources. The California Volcano Hazard Zones shapefile was created from previously published hazard zone reports. Specific details about each file can be found in the metadata included with each file and the read-me document for this data release. Together, these files were used to define California Volcano Hazards for the GIS analysis that supports conclusions in the California's exposure to volcano hazards report.

Geologists produce hazard zone maps to convey the types of hazards that may occur during future eruptions and to identify the areas of potential impact. Hazard zones are derived from detailed geologic studies that identify the type and extent of volcanic deposits created in past eruptions and on isotopic and paleomagnetic dating of the age and frequency of eruptions. Users of the information in this report should be aware that volcanic areas in California are the subject of continuing research and that refinement of volcano hazard zones are sure to come in subsequent years. The volcano hazard zones provided in this report reflect a simplified compilation of the following peer-reviewed U.S. Geological Survey reports:

1) For Lassen Volcanic Center: Clynne, M.A., Robinson, J.E., Nathenson, M., and Muffler, L.J.P., 2012, Volcano hazards assessment for the Lassen region, northern California: U.S. Geological Survey Scientific Investigations Report 2012-5176-A, 47 p., 1 plate, scale 1:200,000, [Available at http://pubs.usgs.gov/sir/2012/5176/a], and, Robinson, J.E., Clynne, M.A., 2012, Lahar hazard zones for eruption-generated lahars in the Lassen Volcanic Center, California: U.S. Geological Survey Scientific Investigations Report 2012-5176-C, [Available at http://pubs.usgs.gov/sir/2012/5176/c].
2) For Medicine Lake Volcano: Donnelly-Nolan, J.M, Nathenson, M., Champion, D.E., Ramsey, D.W., Lowenstern, J.B., and Ewert, J.W., 2007, Volcano hazards assessment for Medicine Lake volcano, northern California: U.S. Geological Scientific Investigations Report 2007-5174-A, 33 p., 1 plate, [Available at https://pubs.usgs.gov/sir/2007/5174/a, and, subsequent GIS compilation in Ramsey, D.W., Donnelly-Nolan, J.M., and Robinson, J.E., 2019, Hazard boundaries for the volcanic hazard assessment of Medicine Lake volcano, California: U.S. Geological Survey data release, available at https://doi.org/10.5066/P9SDH8E6.]
3) For Mount Shasta, Clear Lake volcanic field, Long Valley volcanic field, Ubehebe Craters, Salton Buttes: Miller, C.D., 1989, Potential hazards from future volcanic eruptions in California: U.S. Geological Survey Bulletin 1847, 17 p., 2 tables, 1 plate, scale 1:500,000. [Available at https://pubs.usgs.gov/bul/1847, and, subsequent GIS compilation in White, M.N., Ramsey, D.W., and Miller, C.D., 2011, Database for potential hazards from future volcanic eruptions in California: U.S. Geological Survey Data Series 661 (database for Bulletin 1847), available at http://pubs.usgs.gov/ds/661].

The studies above represent the work of numerous researchers occurring over a collective span of almost three decades. As a result, methodology, nomenclature, and level of geologic detail vary from one report to the next. The simplified hazard zone maps presented in this report maintain the scientific integrity of the reports listed above, while simplifying nomenclature and amalgamating information to provide a consistent, statewide portrayal of California's volcano hazard zones.

It is important to note that volcanic hazard zone boundaries are gradational in nature, with the severity of the hazard diminishing outward from the eruption site (vent), or, for the various flowage hazards, with increasing height above valley floors or basins. The simplified hazard zone maps in this report portray hazard boundaries as diffuse bands rather than as sharp lines. Diffuse boundaries give a qualitative sense of the level of uncertainty in the original data, and account for differences in geologic resolution (map scales) across the various published reports listed above.

It is unlikely that all parts of a volcanic area will be impacted during an eruption. As a volcano reawakens, real-time monitoring of earthquakes, ground deformation, and gas emissions will provide the information needed to anticipate the vent location and geographic sectors most likely to be impacted. Specific hazards to people and property will depend on the eruption style, the volume of lava erupted, the location of the eruptive vent, and the eruption duration, as well as local meteorological and hydrological conditions.