Abstract

The 1 December 2014 moment magnitude (Mw) 4.7 Arizona earthquake occurred 13 km southwest of the city of Flagstaff, Arizona. Flagstaff is located within the northern Arizona seismic belt and the San Francisco volcanic field. The Flagstaff area is subject to infrequent moderate magnitude earthquakes, such as the 1993 Cataract Creek (Mw 5.3) and the 1912 surface‐wave magnitude (Ms) 6.2 events. The epicenter of the Mw 4.7 earthquake lies 2 km to the east of the Oak Creek normal fault. This is the most proximal event to the fault in the last 40 years since the seismographic networks were first deployed. The earthquake was recorded throughout northern Arizona by stations of the Arizona Integrated Seismic Network. Data were also acquired by the network from two foreshocks of duration magnitude (MD) 3.2 and 2.8 and 27 aftershocks (MD 1.4–3.0). Locations were obtained using HYPOINVERSE 2000, and epicenters were accurate in most cases to within 3.0 km or less. Fault‐plane solutions were obtained from the mainshock and from seven of the other events of the sequence. Analysis of the data indicates that release of the stress was controlled by the presence of pre‐existing structures including the Oak Creek fault and prominent fractures in the hanging‐wall block of the fault. This is the first well‐documented fault‐slip sequence in northern Arizona that has shown (1) active northeast (NE)‐trending fracture and (2) a pattern of alternation of stress release on NE‐ and northwest (NW)‐trending fractures. The alternation of stress release accommodates southeast (SE)‐directed slip on both NE and NW trends. The reactivation of the Oak Creek fault in northern Arizona appears to be the result of the westward motion of the Colorado plateau producing a strain‐rate field that implies NW–SE extension.

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