On July 7th at 1:38 MST, a magnitude (*Mw) 3.5 earthquake shook the towns of Fredonia, Jacobs Lake, and Havasu (Figure 1). The earthquake was followed by a series of aftershocks, with two registering Md ~ 2.0 and several in the 1.0 range.
The mainshock, and several aftershocks were located just west of the western edge of the Kaibab Plateau, within Grand Canyon National Park. This area lies within the Northern Arizona Seismic Belt, which experiences tens of earthquakes per year, some quite large (Figure 2). For example, between 1906 and 1912, three ~ M > 6.0 events occurred in the zone within about 25 miles of Flagstaff, Arizona.
Figure 2: Northern Arizona Seismic Belt (NASB) shaded in gray on the left image and shown striped in orange on the right inset. The inset shows epicenters of historic earthquakes of magnitude 3.0 or greater in the NASB and environs.
The National Earthquake Information Center (NEIC) determined that the mainshock occurred on either a nearly due north-south right-lateral dip-slip fault, or a nearly east-west left lateral dip slip fault (Figure 3). A north-south to north-northeast fault is located near the quake locations. This fault is referred to as the West Kaibab fault, which is a normal fault considered to be an active fault. There is however, another fault located in the same area called the Sinyala fault (Figure 1). This fault trends NE-SW, which coincides with the trend of quake locations. Given the uncertainties in quake locations due to a lack of station coverage and the fact that quakes smaller than M 6.0 rarely rupture the ground-surface, it is not possible to precisely determine what fault the quakes actually occurred on.
Table 1. Earthquake parameters – time, location, magnitude, and depth -- for North Rim seismic episode of 7 July 2013. The smaller Md 1.0 events were not located due to poor station coverage in the area.
|
Date |
Time (MST) |
Location (Lat/Long) |
Size |
Depth (km) |
|---|---|---|---|---|
|
07/07/2013 |
01:38:59 |
36.456/-112.579 |
3.5 Mw* |
5 |
|
07/07/2013 |
01:51:28 |
36.506/-112.560 |
2.3 Md |
3 |
|
07/07/2013 |
04:06:29 |
36.524/-112.4265 |
1.8 Md |
20 |
*Mw – Moment Magnitude. The magnitude is related to the product of the area of the earthquake fault, multiplied by the average fault slip over that area and by the shear modulus of the fault rocks. From the US Geological Survey’s “Magnitudes” web page.
|
Plane |
Strike of fault |
Dip |
Rake |
|---|---|---|---|
|
Nodal Plane 1 |
106° |
65° |
-136° |
|
Nodal Plane 2 |
354° |
51° |
-33° |
Figure 3: NEIC's focal mechanism for the Mw 3.5 earthquake. The white areas represent quadrants in which the P-wave first motions are toward the source. The black dot represents the axis of maximum compressional strain and the white dot represents the axis of extensional strain. Nodal planes (shown as lines crossing the image) represent the two possible fault orientations. See the USGS website for further explanation.
The Arizona Geological Survey manages and operates the Arizona Broadband Seismic Network (ABSN) comprising 7 broadband seismometers strategically distributed throughout the state, while Northern Arizona University maintains a northern Arizona analogue network. Together, with two Arizona-based USGS seismometers and those in surrounding states, Arizona earthquakes are located and catalogued to better characterize seismic activity and hazard.
Other Resources for Arizona Earthquakes and Faults
Arizona Broadband Seismic Network interactive map
Earthquakes in Arizona – 1852 – 2011: The Movie! (90-second time-lapse YouTube video)
Lake Mary Fault (Flagstaff, Arizona) (4.5 minute YouTube video)
Earthquake Monitoring in Arizona (4 minute YouTube video)
Arizona's Earthquakes – Dr. J.J. Young PowerPoint presentation
Quaternary Fault Data and Map for Arizona by Dr. P. Pearthree
