SEISMIC REFRACTION SURVEY OF MERRIAM CRATER, SAN FRANCISCO VOLCANIC FIELD, AZ

Geophysical techniques can be used to image subsurface features beneath volcanoes, giving insight into their eruptive processes and emplacement. A seismic refraction survey at Merriam Crater in the San Francisco volcanic field near Flagstaff, Arizona was conducted in order to better understand the transport of magma that fed the eruption. Merriam Crater is a basaltic cinder cone in the center of a cluster composed of monogenic volcanic vents trending W-NW that erupted ~20 ka. This cluster has experienced minimal erosion since eruption and lacks any significant outcrops exposing its internal structure. Two seismic lines were placed at the base of the eastern and western flanks of the crater, roughly perpendicular to the apparent trend of vents. The eastern line was 690 meters in length and the western line was 345 meters, both with geophones spaced 5 meters apart. Tomographic images were calculated by measuring the arrival times of the refracted seismic waves. Images beneath both lines show a gradual increase in seismic velocity with depth. However, prominent high-velocity features are present under both lines that are not linearly correlated. These features are interpreted to be feeder dikes associated with an en-echelon fault system. Propagating dikes drive eruptions of monogenic volcanoes and often intrude through zones of weakness, such as fault surfaces. Initial results show that shallow subsurface basaltic features can be located using seismic refraction techniques.