Seismicity and active faulting of the Kanab-Fredonia area of the southern Colorado Plateau

Detailed study of the historic seismicity of the Kanab-Fredonia area adds information to the location of the Colorado Plateau tectonic boundary in this area and to the nature of contemporary seismicity and faulting on this part of the Colorado Plateau. Historic seismicity over 45 years (1959-2004) defines a band of activity which extends through Fredonia, Arizona and into southern Utah around Kanab. The area is dominated by north trending high-angle normal faults. The spatial distribution of epicenters consists of clusters associated with the West Kaibab, Big Springs, and Toroweap-Sevier faults indicating that these faults may be active. Four focal mechanism solutions show Basin-Range style high-angle normal faulting on north to north-northwest trending surfaces that are parallel to subparallel in most cases to mapped Quaternary faults. This suggests a present east-west extension occurring on the southern Colorado Plateau for the Kanab-Fredonia area and an expansion of the adjacent Basin-Range province eastward into the plateau as far as the West Kaibab/Big Springs faults. The West Kaibab/Big Springs faults are proposed to be the tectonic boundary of the southern Colorado Plateau in this area since they mark the limit of significant seismic activity that is combined with Basin-Range type faulting.