Jolting similarities between 2001 Nisqually quake and 8.1 Mexico earthquake

Mexico’s 8.1-magnitude earthquake was more akin to the 2001 Nisqually earthquake in Washington, where a break occurs within the oceanic plate being forced beneath the continent, according to a local scientist.

The powerful 8.1-magnitude quake that struck Mexico this morning occurred near an offshore region called a subduction zone, where a seafloor tectonic plate dives under a continent. A similar subduction zone, called Cascadia, lurks off the coast of the Pacific Northwest.

But after analyzing the seismic signals, scientists now say the Mexico quake was not actually on the subduction zone, but below it.

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In a subduction zone quake, tectonic plates jerk past each other. What happened in Mexico was more akin to the 2001 Nisqually earthquake in Washington, where a break occurs within the oceanic plate being forced beneath the continent, said John Vidale, former director of the Pacific Northwest Seismic Network at the University of Washington.

These so-called intraslab quakes are generally less powerful than subduction zone quakes, and their destructiveness can be blunted because they occur 30 miles or more below ground. When they strike offshore, intraslab quakes are also less likely than subduction zone quakes to trigger huge tsunamis.

In Washington, intraslab quakes are much more common than subduction zone quakes. Before the magnitude 6.8 Nisqually quake, the state was hit with a magnitude 6.5 in 1965 and a magnitude 7.1 in 1949.

The U.S. Geological Survey (USGS) says there’s a more than 70 percent chance the region will get another one in the next 50 years.

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The 1,700-mile long Cascadia Subduction Zone itself last ruptured in January 1700, triggering a magnitude 9 earthquake roughly 30 times more powerful than the Mexican quake. The resulting tsunami wiped out Native American villages along the ocean shore and was powerful enough to cross the Pacific Ocean and cause damage in samurai-era villages along the coast of Japan.

The U.S. Geological Survey estimates a 12-15 percent chance that the Cascadia fault will rupture again in the next 50 years, though other researchers put the odds as high as 30 percent.

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