SEISMIC INVESTIGATION OF THE SAN FRANCISCO VOLCANIC FIELD

The San Francisco Volcanic Field (SFVF), located within the Northern Arizona Seismic Belt, is one of several predominantly basaltic volcanic fields of late Cenozoic age on the southern margin of the Colorado Plateau. The last volcanic event in the field was the eruption of Sunset Crater less than a 1,000 years ago, leading to its classification as potentially active.

The deployment of a temporary network consisting of 7 seismometers from June 2013 to September 2014 within the eastern portion of the field allows us to better understand the modern deformation and structure of the SFVF. Determining earthquake locations provides information about the current stress regime and possibly relates seismicity to magma movement in the subsurface beneath the SFVF. Initial analysis of seismic data detected 123 localized seismic events in a span of 5 months within the SFVF and surrounding areas. Current work involves the analysis of data from the rest of the deployment and improving the earthquake locations within the SFVF.

A 3D shear velocity model for the region using ambient noise tomography relates seismicity within the field to the Earth’s structure. Data collected by the EarthScope Transportable array has been analyzed using ambient noise tomography. Preliminary analysis indicates high-velocity zones under the SFVF at crustal depths less than 20 km. High P-wave velocities in the region have previously been attributed to igneous intrusions associated with the volcanic centers that formed the SFVF. A boundary between low velocities beneath the SFVF and higher velocities to the east is observed in the lower-mid crust of the eastern part of the SFVF and is possibly related to the emplacement of the field.

Once completed these results will be combined with receiver function data in a joint inversion in order to further improve our velocity model for the field. The combination of earthquake locations, ambient noise tomography, and receiver functions allows for a better understanding of the geological structure and evolution of the SFVF.