On Monday 8 October 2012, slightly after 25 minutes past midnight local time (slightly after 6.25 am GMT) the United States Geological Survey recorded a Magnitude 6.0 Earthquake 9.9 km beneath the southern part of the Gulf of California in Mexico. This is quite a large quake, at quite a shallow depth, and onshore would no-doubt have caused considerable problems, but the location of the tremor was over 50 km offshore, and it does not appear to have caused any damage or casualties, though witnesses recorded feeling the quake on both sides of the Gulf.
Map showing the location of the 8 October 2012 quake, and the areas which felt the worst shaking. USGS.
The Gulf of California lies on the boundary between the Pacific and North American Plates, with Beja California lying on the Pacific Plate and the Mexican mainland on the North American. The Pacific Plate is moving northwest with regard to the North American Plate, while the North American Plate is moving southeast relative to the Pacific Plate. This creates a transform plate margin along the center of the Gulf of California, as the two plates slide past one-another, a margin that continues northward under California as the San Andreas Fault. The plates do not move past one-another smoothly, but continuously stick together then break apart as the pressure builds up, leading to regular Earthquakes beneath the Gulf of California.
Map showing the relative movement of the Pacific and North American Plates, and the fault system beneath the Gulf of California. Monterey Bay Aquarium.
See also Seismic activity beneath Apoyeque, Eruption on San Cristobal, Nicaragua, Substantial Earthquake in Costa Rica, Large Earthquake off the south coast of El Salvador and Earthquakes on Sciency Thoughts YouTube.
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