Arizona does not experience earthquakes often, but more than 20 earthquakes having magnitudes greater than 5 have occurred in our history. The 7.4 magnitude Sonoran or Pitaycachi Earthquake of 1887 had an epicenter in Sonora, Mexico. It caused significant property damage in southeastern Arizona. Several quakes in the early 20th century near Flagstaff caused damage, including a 50 mile earthen crack near the San Francisco Peaks, and sent people fleeing the area on numerous occasions. Yuma is another area within Arizona known to produce earthquakes. To date, no earthquake in recorded history has caused deaths or injuries in Arizona.
There are nearly 100 faults identified in Arizona that probably generated earthquakes of magnitude 6 or larger during the past 2 million years or so (Quaternary Period). Faults in Arizona tend to have long recurrence rates on the order of tens of thousands of years in between major earthquakes. Areas most at risk from earthquakes include Yuma, Prescott, and the Flagstaff-Grand Canyon area.
Six of the more potentially damaging faults are: Hurricane Fault (northwestern Arizona); Big Chino and Lake Mary Faults (northern Arizona); Algodones Fault (Yuma); and the Safford and Santa Rita Faults ( southeastern Arizona).
Near the Phoenix area, there are just a handful of faults: Carefree Fault Zone; Horseshoe Fault Zone; Sugarloaf Fault Zone; and Cottonwood Basin Fault. The coordinates of this earthcache take you to the Carefree Fault Zone.
The Carefree Fault Zone was first identified in 1984. It is 11- 12 km long (depending on the source) and consists of a series of north- and northwest-trending normal faults along granite bedrock pediment located just south of Wildcat Hill and another larger fault trending mostly north except for the southernmost part of it, which trends northwest. (See accompanying map on this cache page.)
Along much of the fault zone, the fault is a contact between Precambrian granite bedrock on the upthrown (east) side and Middle Pleistocene alluvium on the downthrown (west) side. Displacement has been minimal, a few meters or less, which is consistent with other Quaternary faults in central and southern Arizona. Displaced materials are found almost entirely on the downthrown side and have red clay soils developed on top of them, as well as occasional outcroppings of white layers of caliche believed to have formed from precipitation of carbonate from fluid flow along the fault. Near Cave Creek Road, the fault has displaced older conglomerate, tuff, basalt, and younger basin fill deposits against granite bedrock. Ruptures of the fault zone are believed to be infrequent. The fault itself is not well exposed. The slip rate of this fault is small, approximately 2 mm per year. Compare that with the more active and well known San Andreas Fault with a slip rate of 37 mm per year. Using the method developed by Wells and Coppersmith, geologists have estimated that the Carefree Fault Zone could produce earthquakes in the magnitude of 6.3-6.5.
Please answer the following questions and send it to me in an email or message on this website. Please do not post the answers in your log entry.
1) If you drive or walk on 110th St between Cave Creek Rd and 110th Pl, do you notice any color changes in the soils? What colors do you notice? How do they differ, if at all, from what you see at the coordinates? What do you think that means? (Some pipe is being laid down near 110th St and Cave Creek Rd, so part of 110th St may be closed during certain hours of fall 2015. This will not affect the ability of anyone to access this earthcache or obtain the information needed to answer the questions.)
2) From the coordinates, looking towards Cave Creek Rd, where is the downthrown side of the fault?
3) Are there homes along the projected fault line between the coordinates at the first point, the coordinates at the second point and beyond both points?
4) How much of a hazard do you think this is to the people in Carefree?
5) What strikes you the most about what you learned from this earthcache?