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Arizona Earthquakes: Perceptions? Misconceptions?

The August 3, 2009 Gulf of California earthquake is a not-so-gentle reminder to Arizona decision-makers and public alike: we live on a dynamic planet prone to sudden release of pent-up energy.

At its closest, Arizona is a mere 70 miles – as the crow flies, or about 0.2 seconds as a seismic wave propagates – from the boundary between the Pacific and North American tectonic plates.  A geologic feature fully capable of generating M7.0 events with disturbing frequency, on the scale of decades to hundreds of years; a human scale!  

We at the Arizona Geological Survey, along with our research partners at Arizona State University, Northern Arizona University and the University of Arizona , are determined to meet this challenge head-on.  One of our chief goals is to support a fully-functional, integrated seismic network, to drive seismic hazard research and inform state-wide and county-by-county hazard assessment and risk management planning.  The AISN – Arizona Integrated Seismic Network – is now in place and data is rolling in.  But it is an expensive and time-consuming task to keep the seismometers running and to crunch the numbers.

Monday’s earthquake provides an opportunity to revisit old perceptions - or are they misconceptions? - of the earthquake risk potential in Arizona. 

“I was on the 5th floor of Phoenix City Hall standing near the elevators and I heard the cables vibrating for a few seconds…. Then I felt a gentle pulsing in the floor.”

One person’s observation of the 3 August 2009 earthquake, Phoenix, Arizona.

On August 3, 2009, at 10:59 a.m. MST, a magnitude 6.9 earthquake, originating in the Gulf of California (GCA) about 100 miles south of US-Mexico frontier, shook Baja and Sonora Mexico, southern California, and southern and central Arizona. 

Seismic/Geologic Summary:

  • Focus (location of earthquake) - Central Gulf of California (29.372° N., 112.814° W.)
  • Depth -- 10 km (6 miles)
  • Magnitude - M6.9  (major aftershock - M5.9; M5.8)
  • Rupture occurred on right-lateral strike-slip fault on North America-Pacific Plate Boundary
  • Travel-time of seismic waves to Phoenix (500 miles NE) - 1 minute, 2.9 seconds
  • Duration of anticipated smaller aftershocks - months
  • Electronic Data Capture in AZ - *AISN and AEIC Seismic Networks.

*AISN - Arizona Integrated Seismic Network operated by the Arizona Geological Survey with support from ASU, NAU and UA geoscientists.  AEIC - Arizona Earthquake Information Center at NAU in Flagstaff, AZ.  

Arizona Shaking.  From Yuma north to Prescott there were reports of ground shaking and related phenomena.  The reports are heavily concentrated along the Central Arizona corridor: Nogales, Tucson, Casa Grande, Phoenix Metro, and Prescott. In the Phoenix metro area there were more than 180 reports of a felt event; in Tucson there were 85 such reports.

Reports from the Phoenix metro area on the effects of ground shaking ranged broadly from: gently swaying window blinds; vigorous shaking and running for cover; to suffering from vertigo and motion sickness.  One building in downtown Phoenix was temporarily evacuated (Source: Channel 12 and the Arizona Republic , 2009).

On the Modified Mercalli scale of intensity – a measure of impact on human society – reports from Arizona ranged from I to IV, with most reports in the II to III range: felt by a few persons in taller buildings; to felt by a larger number of people and accompanied by rattling of windows, doors, or dishware. 

The graph on the left shows the timing of earthquakes in the Gulf of California on 3 Aug. On 5 August at 02:13 MST, a M5.6 aftershock occurred.

Primary Seismic Resources (State and National)

Arizona Earthquake Information Center (AEIC)

Arizona Integrated Seismic Network (AISN)

Arizona Geological Survey (AZGS)

Global Seismology and Tectonics (GSAT)

National Earthquake Information Center (NEIC)

Related Links

AEIC, NAU, 2009, Phoenix and Southern Arizona shaken by Mexican earthquake

GSAT, UA, 2009, August 3, 2009 Gulf of California Earthquake

NEICd, USGS, 2009, M6.9 Gulf of California Earthquake Summary

NEICc, USGS, 2009, M6.9 Gulf of California Earthquake Felt Reports

 

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