Seismicity in Arizona Spring 2011. Over half of the activity occurred during the month of March.
Arizona seismicity started slow in 2011 but picked up in March, when more than 50% of the events to date occurred. Additionally, seismometers of the Arizona Integrated Seismic Network (AISN) were inundated by the 9.0 M Tohoku earthquake and its many larger aftershocks.
Locally, 17 events occurred throughout Arizona from January through March 2011. Most were situated between the northwest Transition Zone, north of Prescott, Arizona, and the Northern Arizona Seismic Belt (NASB). Notably, there were four events between the Sevier/Toroweap, Aubrey and Hurricane fault zones, two in the Grand Canyon area, two near the Washington and Northern Hurricane systems, and several near Lake Mead. Additionally, there were two events near Kayenta, Arizona. Activity in this relatively stable part of the Colorado Plateau is rare; the ratio of events in the northeast (excluding mining related activity) to events in the NASB is about 1:30.
In southern Arizona, an M 1.6 event occurred near Yuma, relatively close to the eastern extent of the San Andreas Fault System; this was the smallest recorded event in spring 2011.
The March event north of Clarkdale as seen on a station north of Flagstaff. The closest short period station was in William, AZ, 40 km N. The nearest broadband was in Pine, AZ, around 75 km to the SE. This event was detected by all of the active stations in the AISN and azimuthal coverage was very good.
The largest event of spring 2011, was the ML 3.7 north of Clarkdale, AZ, which was widely felt in Cottonwood, Arizona, and as far east as Winslow. The Arizona Earthquake Information Center (AEIC) received felt reports for this event from several Cottonwood residents, all of whom experienced the shaking from indoors. Based on community feedback, the US Geological Survey assigned this event a Modified Mercalli Intensity of IV -- weak shaking and no apparent damage. This event followed a ML3.6 earthquake that occurred January 23, 2011, in virtually the same location, near the mouth of Sycamore Canyon. The felt area for this second event was much smaller, and no reports were filed at the AEIC. These two Sycamore Canyon events are considerably larger than any recent or historic activity at this location. Within a 50 mile radius the other significant activity included two magnitude 2 events near Perkinsville to the northwest and a few small events near Clarkdale, possibly mining related.
January and March internet intensity maps from the USGS. Note that the March, 3.7 event generated 101 responses while the smaller 3.6 event in January elicited only 9.
In general, major faults in Arizona’s Transition Zone display SW extension and steady strain rates during the Quaternary. For the largest fault in the area, the Big Chino, maximum credible earthquakes are estimated between 7.0-7.25 with long recurrence intervals. The largest historic event on record in this zone is the 4.9 earthquake that occurred in 1976, possibly associated with the Prescott Valley Grabens near Williamson, Arizona; in the late 1990's, several lower magnitude events occurred in the area. Overall, microseismicity rates in this area are an order of magnitude lower than in the Intermountain Seismic Belt to the north. Despite this, continued seismicity indicates that low level deformation here, as well as within the NASB, is ongoing.
1. Analysis of the 4 February 1976 Chino Valley, Arizona, earthquake. Eberhart-Phillips, D, Richardson, R., Sbar, M., Herrmann, R.B. Bulletin of the Seismological Society of America, June 1981; 71: 787 – 801.
2. Big Chino Fault. Arizona Department of Transportation. Arizona Transportation Research Center.
3. Geologic Map of the Chino Valley North 7½' Quadrangle, Yavapai County, Arizona, Report. Brian F. Gootee, Charles A. Ferguson, Jon E. Spencer, and Joe P. Cook. Arizona Geological Survey Digital Geologic Map DGM-80, version 1.0. December, 2010.