Friday, December 05, 2014

It Increasingly Looks Like It Was the Oak Creek Fault That Ruptured

Accumulating evidence increasingly suggests that it was the Oak Creek fault that is responsible for the November 30, 2014 M 4.7 quake in north-central Arizona. Phil Pearthree, Chief of Environmental Geology at the Arizona Geological Survey (AZGS) said, 

"The location is quite close to the Oak Creek fault zone, [and it is thought that] this fault has been active in the past 2 million years, but how recently it has ruptured in a large earthquake [is unknown]."


As reported previously on Lee Allison's blog, Dr. Pearthree has mapped the area extensively and reports that the epicenter location reported by the USGS is quite close to the Oak Creek Canyon fault. The AZGS fault map and data currently depict the northern part of this fault zone as "Quaternary" in age (the last 2 million years), as basalts of that age are displaced. 

Additionally, on December 3, I communicated with Dr. Richard Holm, a retired professor of geology at Northern Arizona University who completed a detailed mapping project of the fault in 1990 with graduate student Bob Cloud. An abstract of their paper in Geology can be found here. This is a seminal paper on the area and showed that nearly 1,000 feet of displacement occurred on the fault near Wilson Mountain (south of the recent quakes epicenter) since canyon volcanism ceased about 6.4 to 6.0 million years ago.

Dr. Holm had this to say about the recent quake and its possible relationship to the Oak Creek fault:

"Cloud and I (1990) show a dip of 77 degrees E on the Oak Creek fault at the head of the canyon, approximately 3 miles west of Sunday's epicenter. Using 3.0 [miles from the epicenter] and 6.4 miles [of depth], the trigonometry gives a dip of about 65 degrees east for a straight plane. But because there is evidence for a listric (curved) shape of the fault, the focus could reasonably be assigned to the Oak Creek fault."  Dr. Richard Holm (pers. comm.)

Figure 1 from Cloud and Holm, 1990 showing Oak Creek Fault
Obviously, incoming data and the interpretation of that data may change this scenario. But at this time, it is reasonable to assume that it was the Oak Creek fault (or the Oak Creek Canyon fault as it was initially called by Mears, 1950) that ruptured on November 30.

At a recent public event in Sedona on December 3, I was asked by an audience member what would be the significance of whether it occurred on the Oak Creek fault or some other fault. The only significance I can ascertain is that there has never been historical movement reported on this fault. For context, "historical" in this area might be 125 years or so. Since the Oak Creek fault is one of the major structural features in the Sedona/Flagstaff area, this would be of some concern if it is found that the fault is active.

1 comment:

Mark Thomas said...

Isn't there an unidentified fault that parallels I-17 from at least Kachina Village, through Newman Park, past Willard Springs a mile or so where it turns eastward and then runs just south of Munds Park. It sure looks like it on Google Earth. If so, wouldn't that be a more likely fault location for the earthquake swarm we just experienced? Mark t.