Sunday, July 07, 2013

Aftershocks and possible source of northern Arizona earthquake

Today's early morning earthquake north of Grand Canyon had a magnitude of 3.6 to 4 and at least three likely aftershocks have been detected (large red circles in map at right.  Small red dots are historical earthquakes), according to AZGS's Jeri Young, who manages the Arizona Broadband Seismic Network.

AZGS Section Chief Phil Pearthree reports that, based on the USGS epicenter location (36.456, 112.576), the earthquake occurred in the general vicinity of several suspected Quaternary-age faults. It was evidently located quite close to the Sinyala fault, a down-to-NW normal fault, which cuts SW-NE across the Grand Canyon just west of the Kaibab Plateau (see white line on map at right, passing just east and southeast of the quake and aftershocks). This fault was not included in Phil's 1998 Quaternary active fault compilation because evidence for Quaternary activity is not compelling. It is ~15 km west of one of the strands of the West Kaibab fault zone, which has a fairly prominent, down-to-west escarpment associated with it and was included in the 1998 compilation.

So, he would not place the earthquake on a mapped fault with any confidence, but there are several suspected Quaternary faults in the vicinity. 

We cannot say definitively that the Sinyala fault is Quaternary aged, or that the earthquake occurred on this fault, but both things are possible. Source for the location of this fault is AZGS Open File Report 83-22, by Menges and Pearthree, and originally mapped by Huntoon, as described in the following report:

Huntoon, P., 1974, The post-Paleozoic structural geology of the eastern Grand Canyon, Arizona, in Breed, W. J., and Roat, E. C., eds., Geology of the Grand Canyon: Museum of Northern Arizona, Flagstaff, and Grand Canyon Natural History Association, Grand Canyon, Arizona, pp. 82-115.

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