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Notable Arizona Seismicity -- Summer 2011

Article Author(s): 

Jeri Young

In June 2011, the Arizona Integrated Seismic Network (AISN) detected notable earthquakes in three areas: near Parks, near Tusayan, and just south of Flagstaff adjacent to the Lake Mary fault, AZ. There were two notable events near Parks this June and two similar events in April. The June events were M 2.37 and 2.4 in size. Tusayan experienced 9 small events that ranged from M 1.75 to 2.8. Their hypocenter depths were between 3.5 and 10.8 km and they were located 5 km southeast of the Bright Angel fault. . The events adjacent to Lake Mary, just south of the city of Flagstaff ranged in size from M 2.0 to 2.7 and had a depth range of 1.8 to 17 km (FIGURE: Lake Mary Swarm). Dr. Dave Brumbaugh at NAU stated that this swarm, and another similar swarm detected in 1979, indicates that the Lake Mary fault zone is still actively producing stress-related events. The maximum credible earthquake estimated for the Lake Mary fault is approximately M 6.8 from the Wells and Coppersmith regression, which correlates total active fault length with a maximum credible earthquake magnitude and yields a resultant moment Magnitude of 6.8 for the 45 km long Lake Mary fault.

Seismic Workshop at Palo Verde Nuclear Generating Station

In May of this year, members of the Arizona Integrated Seismic Network (AISN, which includes the AZGS and NAU) attended a workshop at Palo Verde Nuclear Generating Station (PVNGS) to discuss current earthquake monitoring efforts and the state of knowledge of seismicity and faulting.

The meeting was launched in response to the March 11th, 2011, M 9.0 earthquake that resulted in the damage of several nuclear reactors at the Fukushima Diachi Power Plant in northern Japan. PVNGS, located west of Phoenix, is the largest nuclear power plant in the United States and capable of servicing 4 million people. Workshop attendees also included several senior staff members and engineers from PVNGS, and a geologist with the United States Geological Survey (USGS). The workshop discussion began with a description of AISN operations, and how AZGS’s seven broadband stations work in tandem with NAU’s seven analogue stations, three USGS broadband stations, and stations positioned nearby in neighboring states. The AISN staff elaborated on historical seismicity and the potential for damaging earthquakes near several areas within and surrounding the state (FIGURE 1: Map of mapped faults, with PVNGS shown as yellow hexagon).

There are 4 known active faults north and southwest of the Phoenix area (FIGURE 1 and Table 1) that are capable of generating M 5.9 to 6.6 events. As part of the workshop, we discussed how intense ground shaking from these faults and California’s San Andreas Fault in California could impact PVNGS’s critical facilities. The plant’s safe shut-down mode is set for horizontal peak ground accelerations (PGA) of 0.25 g or 25% of the acceleration due to Earth’s gravity. Peak ground accelerations can vary within short distances, and they are greatly influenced by subsurface properties and earthquake focus depths; therefore, equating PGA to a specific earthquake magnitude is not accurate. In general, a PGA equal to 0.25 g is associated with very strong shaking. Site effects, such as how subsurface layers may amplify shaking and how the plant’s infrastructure would respond to sustained shaking has not yet been explored.

Table 1 showing known Quaternary faults near the Phoenix area.

Fault Name

Fault Length (km)

Max. Credible Earthquake

Horseshoe fault


M 6.6

Carefree fault


M 6.3



M 6.1

Sand Tank


M 5.9


Research Geologist
Arizona Geological Survey


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