Monday, December 01, 2014

Sunday's 4.7M Earthquake - What is the Oak Creek Canyon Fault?


Last night's 4.7 magnitude earthquake in north-central Arizona has everyone wondering about earthquakes in this part of the world. Some of these comments reflect a lack of understanding about seismic activity in northern Arizona. I have an entire chapter devoted to the Oak Creek Canyon fault in my book, Sedona Through Time, but here is a summary of the fault here.
Courtesy of the Arizona Earthquake Information Center
First, it's true that Arizona lacks the seismic punch that our California neighbor does, but we do get quakes here. After the Yuma area (located within the shadow of the great San Andreas fault) the Grand Canyon region is the second most seismic area in the state. It is not uncommon for quakes to occur here and last night's event may be a precursor to more activity, or it may just be a random rumbling. There is no way to know until after fact whether this is an isolated event or a foreshock of more to come. 

The Oak Creek Canyon fault is part of a series of extensional faults that form when the earth's crust is being stretched and Arizona has been stretched a lot in the last 20 million years. This extension is what makes the central and southern portions of our state look the way they do. Oak Creek Canyon is located at the far northern end of this zone of extension and so this explains the location of the Oak Creek Canyon fault. The fault is approximately 25 miles long and trends north right into the San Francisco Mountain stratovolcano. It dies out beneath lava flows south of the Village of Oak Creek.

Oak Creek Canyon fault has a very long history. It likely began in the Precambrian Era some 1,700 million years ago when pieces of crust were sutured onto the southern edge of North America. This same scenario is responsible for many of the faults in the Grand Canyon. During the Laramide Orogeny (70 to 40 million years ago) it was active again but this time the rocks were squeezed in a compression event. Finally, Basin and Range faulting began after 20 million years ago when the crust was extended or stretched.
Google Earth image of Oak Creek Canyon annotated by Wayne Ranney
In the Google Earth image above I have placed the fault on the landscape and show the location of the epicenter on last night's quake. The landscape on the western (left) side of the fault has been uplifted relative to the eastern (right) side. The amount of offset in the past totals about 1,000 feet near the area around Grasshopper Point. 

Along the fault, lava flows that are 6 million years old are broken. That means that the 1,000 feet of displacement began after 6 million years ago. As I stated in a previous post, I am unaware of any historical movement on this fault, remembering that "historical" covers about 125 years at the most. So this quake is significant in that respect.

Below are a series of slides from a lecture I oftentimes give in Sedona. They show how the fault has behaved in the past.

Aerial view to the north of the trace of the Oak Creek Canyon fault. Oak Creek Canyon is the obvious gash that trends north into the slope of San Francisco Mountain (upper right). The epicenter of last night's earthquake is in the upper part of the canyon after it makes a jog to the right.

The fault has separated lava flows found on Wilson Mountain and Wilson Bench by about 860 feet (see this reference for more information: Holm and Cloud, 1990). These lava flows were once continuous but movement on the fault separated them after 6 million years ago.

A second strand of the fault drops the rocks another 130 feet. this strand is very near the Grasshopper Point Recreation Area. The total offset on the two strands is nearly 1,000 feet. Remember that canyon carving could have occurred only after the faulting, since the creek follows the trace of the fault. The sequence of events interpreted from these observations are: 1) lava flow emplacement between 8 and 6 million years ago; 2) nearly 1,000 feet of fault offset occurred after the last flow (6 million years); 3) the canyon was carved. This makes the canyon very young geologically.

Next, if we portray the lava flows by their thickness and graphically show where the faults break them, it looks something like the diagram above. Note the scale at which the flows are faulted, 860 feet of offset represented on the left and 130 feet of offset on the right.

Geologists can "subtract" the total offset along the two faults and place the lava flows into their original orientation. This is depicted by the green wedge above. Note that this restoration shows how the flows become thin and pinch out to the west and thicken considerably to the east to over 500 feet thick.

The restoration shows that an ancestral valley or canyon existed in the area when the lava flows were erupted beginning 8 million years ago. This valley extends eastward about 18 miles where the flows thin again against the bedrock at Hollingshead near Beaver Creek.

I hope this helps in the understanding of the Oak Creek Canyon fault. Stay tuned for updates.

2 comments:

Ron Chilston said...

Thank You! I appreciate the look into the geologic past of this region. We live in Jerome and slept through it, many felt it here.

R&K Ownby said...

Thanks for the summary!