Background Information:

On Monday, August 03, 2009 at 10:59 a.m. there was an earthquake that occurred in the Gulf of California, between the Mexican states of Sonora and Baja California with a foreshock of magnitude 5.8 and mainshock of 6.9 magnitude with three aftershocks that did not happen right away. Two of the aftershocks were half anhour apart, and the third aftershock was actually two days after the earthquake had happened. The first aftershock happened at 11:33 a.m. with a magnitude of 5.0. The second aftershock happened at 11:40 a.m. The third aftershock happened on August 05, 2009 at 02:13 a.m. Figure 1, shows the location of the earthquake. The epicenter of this earthquake was located offshore, in the Gulf of California, between two islands. According to the MyFox News website, the Gulf of California was formed millions of years ago when the "tectonic forces shifted the Baja California Peninsula off the North American Plate." The quake was only 22 km away from Isla Ángel de la Guarda and 36 km from the Tiburón Island. It was 89 km northeast of Santa Isabel, Baja California, and 185 km west of Hermosillo, Sonora. The earthquake occurred along the Tiburón fault zone, which is part of the plate tectonic boundary between the North American plate and the Pacific plate. The plate boundary is beneath the Gulf of California, which are made up of transform faults (pull-apart basins) that are separated by small spreading centers. The earthquake did minor damage to some of the buildings in Puerto Libertad, El Desemboque, Punta Chueca, Bahía de Kino, Puerto Peñasco, and Hermosillo. Other cities like Ensenada, Tecate, Mexicali, and Tijuana also felt the earthquake. Amazingly, in the United States, some parts of Southern California, and Southern and Central Arizona also felt the earthquake.

Figure 1: Earthquake's Epicenter
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Figure 2: Mercalli Intensity Scale by the USGS
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In Figure 2, it shows the Mercalli Intensity Scale that encountered the earthquake. Even though it was a 6.9 magnitude earthquake, Figure 2 shows that the color indicated is a very moderate to strong shaking. It also shows how the damage level is too for the potential damage section. That indicates only to be very light to light. Some people reported from Arizona that they felt the earthquake range from I to IV, with a majority of the reports in the II to III range. Almost the entire Baja, California region in the Gulf of California is scattered with population. The population for Baja state has only 3 million people and a majority of them are on the border of California. The population for Sonora is even less, because there is only 2 million people living there. This earthquake did alert and shake up everyone. Many people were lucky, because there were no deaths and there was only minor damages.
It is earthquakes like this Gulf of Mexico quake that help to asses the risks of future hazards in Arizona. Arizona is a mere 70 miles from a major fault, in seismic wave terms that is less then a second travel time. The quake offers up parameters and adds to patterns of nearby quakes. This information is gathered and the hazard maps are updated to reflect where there is stress building up near the state, the 2008 hazard map is shown in figure 3.

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Figure 3 - Hazard map showing greatest hazard just outside the state in the SW corner
While it appears that most of the state's area is not susceptible to massive earthquake damage, a closer look at scale on the right side of the figure shows that increases start to ramp up at a considerable rate. This means that if an event is strong enough to do damage to the state of Arizona, the more powerful the earthquake is, the faster it will climb up the scale. The cities of Tucson, Lake Havasu, and Phoenix are not very far from regions in the yellow and red on the map, and can certainly suffer from an earthquake near the Gulf of California. (edit by Kevin Greenspon)


"AZGS | Shaking Arizona - Aug. 3, 2009 Earthquake." Shaking Arizona 3 August 2009 M6.9 Gulf Of California Earthquake. The Arizona Geological Survey, 2008. Web. http://www.azgs.state.az.us/earthquakes_shakingaz09.shtml

"Gulf of California Earthquake." Magnitude 6.9 - Gulf of California. USGS. Web. 27 October 2009. http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2009jwbh.php

"Earthscope ANF Website: Special Events: Magnitude 6.9." Magnitude 6.9 earthquake in the Gulf of California, Baja, Mexico. Earthscope ANF Website, 05 August 2009. Web. http://anf.ucsd.edu/spevents/2009/215/a/

“6.9 Magnitude Earthquake Hits Mexico’s Gulf of California.”MyFox. 04 Aug. 2009. 03 Dec. 2009. <http://www.myfoxdc.com/dpp/news/international/69_Magnitude_Earthquake_Hits_Mexicos_Gulf_of_California_08108549>.