Summary of Arizona Earthquake Activity 2011 (L. Linville)
In 2011, 131 instrumentally detected earthquakes were recorded in Arizona (Figure 1). Most of the seismicity occurred in the north-northwest quadrant of the state (Figure 2). The largest events of the year were magnitude (Md) 3.7 and 3.6 near Clarkdale in central Arizona.1 Both events were felt by a number of residents and reported by local media outlets. The smallest event detected was a magnitude 1.6 event that occurred in the northwestern corner of the state where the combination of Northern Arizona University stations, network stations from Utah, and broadband sites from the Arizona Geological Survey afford the highest density coverage in the state and ergo the best locus for detection of very small events. The depth limit for events in 2011 was 26.0 miles (41.9 km), with an average of 5.8 miles (9.4 km) (Figure 3). This compares with the 53 events recorded in 2010 with depth ranging from 0.64 to 12.4 miles (1-20 km) and magnitude from 1.4 to 3.6, respectively; average hypocenter depth was 4.5 miles (7.2 km).2 Overall, seismic activity in 2011 is consistent with past behavior, namely, a propensity for deeper seismicity to occur in two pockets, the northwestern Utah-Arizona border and well within the Colorado Plateau in the northeast corner of the state (Figure 4). Though no events can be reliably attributed to movement on a specific fault, in general the highest concentration of energy release correlates well with the pattern of established Quaternary faulting, indicating that this portion of the crust continues to be an active area of strain release and of particular interest for hazard studies in Arizona.
As regional instrumentation has improved over the past several years, a clearer picture of Arizona seismicity is slowly emerging. Of the 131 events experienced this year, 36% were associated with swarms of small magnitude events that occurred over the space of a few days, or, as with the Clarkdale activity, were spatially clustered but occurred over a larger time window. This group of activity includes, Clarkdale, Whitmore Fault section, Lake Mary, Tusayan, and most recently, Colorado City. Though a majority of the 2011 seismic clusters occurred in the Transition Zone or on the western margin of the Colorado Plateau, this type of activity in Arizona does not seem to be constrained by physiographic province. Over the past five years researchers have identified greater than 20 earthquake swarms throughout the state. While not well understood, it appears that some regional stress is accommodated by these temporal and spatial clusters of small- to moderate-magnitude events.3
Another area of interest over the past year has been the Lake Mead region, which has experienced a heightened level of small magnitude seismic activity. Some of these small events are related to mining or quarry activity in the area, but certainly some strain release events are included in the group. Reservoir induced seismicity is a topic of interest in Arizona and although data near Lake Roosevelt have recently been examined, more research is required to elucidate the relationship, if any, between water level fluctuation and seismicity for reservoirs in Arizona.
Overall, seismic activity in 2011 represents a 160% increase from 2010 (Figure 5), a 100% increase from 2009, and a 31% increase from 2008. Despite the increase in total number of events for the year, the number of events > magnitude 3.4 has remained fairly consistent at 2-3 events per year over this same time period. The detection level for the group of networks within the National Earthquake Information Center is likely just above this level, therefore the detection of events smaller than this (events < magnitude 3.4 represent 99% of seismicity in 2011 and 96% of the total catalog) is highly dependent on local resources committed to their detection.
For Arizona’s seismic monitoring networks, catalog completeness is part of an ongoing effort to develop a database of seismicity which contributes to a reliable picture of all levels of activity within the state over a meaningful time period. It is through this effort that researchers will eventually have sufficient information for evaluating the real seismic risk in Arizona.
AZ Shakes – Earthquake Outreach Program 2011 (M. Conway)
With funding from the Federal Emergency Management Agency (FEMA) National Earthquake Hazard Reduction Program (NEHRP), AZGS operates the AZ Shakes earthquake outreach program. The two overarching goals of the AZ Shakes program: 1) increase public awareness by providing cogent information in non-technical, jargon-free language; and, 2) network with federal, state, county, municipal and educational institutions and entities to promote earthquake awareness in Arizona.
In 2011, the AZ Shakes program produced five earthquake related videos, three earthquake brochures, and participated in earthquake hazard training in Somerton, Arizona, and lectured on or exhibited information on earthquakes and related hazards in Arizona on numerous occasions. We attended and participated in the National Earthquake Managers meeting in Boise, Idaho, in April.
A new phase of AZ Shakes starts in 2012. AZGS research scientist Jeri Young will initiate risk assessment in the Flagstaff area using FEMA’s GIS-based HAZUS-MH (multi-hazards) software. This marks our first venture into the world of numerical risk assessment. We’ll continue with our earthquake outreach and education program as well.
AZ Shakes Videos - available at AZGS’s YouTube Channel.
- Lake Mary Fault, Flagstaff, Arizona – narrated by Dr. David Brumbaugh (Northern Arizona University, Arizona Earthquake Information Center)
- Little Chino Fault, Chino Valley, Yavapai County, Arizona – narrated by Brian Gootee (AZGS Research Scientist)
- Big Chino Fault, Chino Valley, Yavapai County, Arizona – narrated by Dr. Phil Pearthree (AZGS, Chief Environmental Geology section)
- Earthquake Monitoring in Arizona – narrated by Lisa Linville (Arizona Earthquake Information Center)
- 1887 Sonoran Earthquake – narrated by Dr. Phil Pearthree (AZGS, Chief Environmental Geology section)
AZ Shakes Brochures
- Yuma County has Earthquakes
- Coconino County has Earthquakes
- Yavapai County has Earthquakes (pending)
1Estimates vary slightly depending on which catalog is referenced. The USGS gives these events a 3.7 and 3.5 revised magnitude. Both the AEIC and the AZGS most often report duration magnitudes.
2Calculation from Lockridge, 2010 and based on the entire AEIC catalog which includes the work performed to complete a database of events for Arizona and dates back to 1830.
3See Lockridge report page 25 and 26 for reference and discussion.
Lockridge, J.S., 2011, Seismicity within Arizona During the Deployment of the EarthScope USArray Transportable Array, M.S. Thesis, Arizona State University, 102, p.