Report by Lisa Linville (Arizona Earthquake Information Center)
As the Arizona Integrated Seismic Network (AISN) closes its third year in operation, we added 50 events in 2010 to our growing catalog of earthquakes. And from neighboring networks in California, New Mexico, and Mexico, we added seven more stations to augment azimuthal coverage for border events.
Figure 1. Epicenters of Arizona earthquakes of Md 1.0 and greater captured by AISN seismometers in 2010. Md is a measure of event magnitude as a function of event duration. Shown as blue lines are faults active in the past 2.6 MY (Quaternary faults).
In May, the Arizona Earthquake Information Center (AEIC) assumed responsibility for the transfer of AISN data to the Data Management Center in Washington State. Data from all AISN stations continues to be available through the IRIS/DMC web interface request tools. Additionally, a searchable catalog is available at the AEIC website. With help from Jeff Lockridge of ASU, the earthquake catalog is routinely converted into a google earth kmz file which allows users to view earthquakes in Arizona from 1830-present as a function of time. That file can be downloaded from the link above.
Figure 2. Screenshot from a google earth kmz file that shows time-slices of earthquake epicenters from Arizona and environs from 1830 to the present.
As is typical for the region, most of the seismicity for the year ranged from Md 2.0-2.5 with an average depth of 7 km. The year's largest event, Md 3.5, occurred in the Roosevelt Lake and Clifton areas. The Clifton event, which occurred in May of this year, consisted of a main large event and a series of ~17 lesser magnitude aftershocks that occurred over the next several days. All of these events were in the upper few kilometers of the crust.
The Roosevelt Lake quake at the end of June is of especial interest due to its size and proximity to the Roosevelt Lake swarm detected during the TA occupation and reported in the Summer 2010 issue of Arizona Geology. The Clifton events, like the adjacent transition zone swarm that occurred in December of 2003 are not tied directly to a known fault system (http://www.azgs.az.gov/arizona_geology/archived_issues/Fall_2007.pdf). Thus, detailed mapping of the area is required to explore the association of the seismicity to faulting, which, in turn, should fuel a better understanding of transition zone dynamics.
Figure 3. Arizona's three physiographic provinces – Colorado Plateau, Transition Zone, and Basin and Range – and the Northern Arizona Seismic Belt; site of numerous small magnitude earthquakes in northern Arizona.
A large portion of the remaining earthquakes occurred in the northwestern part of the state between the Washington Fault Zone south of St. George and the Kaibab Fault System, with an average of 1-2 events per week. Though many of those events were too small for accurate detection, the ones that were locatable were generally on the lower end of Md2. There haven't been any appreciable events here since 1962 when the area experienced a ML 4.5, and prior to that in 1959 a ML5.7. This zone is thought to be an intermediate stress field between the Colorado Plateau and the Basin and Range Province and is considered part of the Northern Arizona Seismic Belt, which is a continuation of the southern Intermountain Seismic Belt (Fig 2).
In the past, focal mechanisms for the region have shown Basin & Range style high-angle normal faulting on north to north-northwest trending surfaces1, indicating that at least in some cases, this stress regime extends beyond the physiographic western boundary of the Colorado Plateau. No stress direction analysis has yet been performed on 2010 events. These may or may not associate with known surface faults. Mapped Quaternary faults in the region are generally north-northeast trending (Fig 1).
The majority of Felt Reports filed with the Arizona Earthquake Information Center were in response to the Easter Baja quake reported on in the Spring 2010 issue of Arizona Geology. Shaking from Cucapah aftershocks continued to dominate the record well into the summer.
1. Brumbaugh, D.S. "Seismicity and active faulting of the Kanab-Fredonia area of the southern Colorado Plateau" JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B05309, 9 p., 2008.
2. Johnson, P.A., Sbar M. L. "A microearthquake study of southwest Utah-northwest Arizona: Transition between the Basin and Range Province and Intermountain Seismic Belt" Bulletin of the Seismological Society of America; April 1987; v. 77; no. 2; p. 579-596.