Monday, December 01, 2014

Moderate quake (M=4.7) between Sedona and Flagstaff

A magnitude 4.7 earthquake struck north of Sedona on Sunday night at 10:57 p.m. local time and was widely reported by residents across the region. One small rockfall was reported on highway 89A but it was quickly cleared by ADOT.  [Right top, AZGS Hazards Viewer shows active faults and historical earthquakes.    The new quake is shown in red. Bottom right, waveforms from the Arizona Broadband Seismic Network, operated by AZGS.  Prepared by Jeri Young.   Bottom left, rock fall on Highway 89A, Credit, David Mendez, Channel 10 News, Phoenix]

A number of aftershocks in the magnitude 3 range have also been felt. The main shock was preceded by a  nearby magnitude 3.5 earthquake on November 25, that itself had minor foreshocks (yellow circles abutting  southwest of last nights quake).    Our location of the epicenter is a few miles NNE of where the USGS plots the events, based Dr. Jeri Young's calculation using the stations of the Arizona Broadband Seismic Network, operated by AZGS.

Dr. Phil Pearthree at AZGS has mapped this area extensively and reports that the location reported by the USGS is quite close to the Oak Creek Canyon fault zone, a down-to-the-east normal fault with substantial late Cenozoic displacement. The AZGS fault map and data currently depict the northern part of this fault zone as "Quaternary" age, as early Pleistocene basalts are displaced there. During the recent AEG Grand Canyon field trip that he led up Oak Creek Canyon, there was discussion about adding the section of the fault that the canyon follows (where this earthquake occurred) as Quaternary. Perhaps the only reason it is not so designated is lack of units of suitable age that are clearly displaced.

The USGS focal mechanism for this event suggests primarily normal slip on a steeply SE-dipping, NE-SW striking fault zone. If correct, Phil suggests the earthquake most likely did not occur on the Oak Creek Canyon fault per se, or on the mapped NW-striking faults on the Colorado Plateau margin that are also close to the the epicenter (purple and green faults to the west and west-northwest of the epicenter).

Flagstaff-based geologist and blogger Wayne Ranney posted an extensive discussion of the Oak Creek Canyon fault and its offsets over geologic time, at

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