Hi I’m Ali Sealander with UTD GeoNews and I’m here to tell you about a magnitude 7.4
earthquake that shook Mexico on the morning of June 23, 2020
The earthquake occurred off the coast of Oaxaca, in southern Mexico. Mexico has 32 states,
Oaxaca being the fifth largest in area and tenth most populous, including many indigenous
peoples, such as the Zapotec and the Mixtec. Oaxaca is a rugged land that is also one of
the most biodiverse areas in all of Mexico. The magnitude 7.4 earthquake epicenter was
in a rural area about 90 miles away from the capital city, Oaxaca. It shook the entire
area and was felt even 300 miles away in Mexico City. It damaged homes and buildings and unfortunately
as of this recording it has been reported that at least 7 people have been killed.
This earthquake was caused by the convergence of two plates, the oceanic Cocos plate and
the continental North American plate. Just offshore of Oaxaca the Cocos plate begins
to slide underneath the North American Plate. In a process called subduction. The interaction
between these two plates causes a zone of earthquakes where the two plates start to
grind against each other. Subduction zones are where the most powerful earthquakes occur,
with 9 of the 10 largest earthquakes over the last century occurring at subduction zones.
Earthquakes can occur as deep as 700 km in the Earth, but it’s the ones that occur
closer to the surface that are most dangerous. The rupture that caused the Oaxaca earthquake
occurred at just 26 km below the surface, not far from the subducting Cocos plate.
Earthquakes we’ve covered in previous GeoNews episodes have occurred because of faulting
due to stretching or strike-slip motion. The Oaxaca earthquake was different because it
was caused by compression associated with the subduction of the Cocos Plate beneath
the North American plate. One result of this compression is reverse faulting; this is where
the block above a fault plane moves upward relative to the block below. So, how do we
know these things? Basically, scientists use information from many seismic stations that
generate graphics called fault plane solutions that show the orientation of fault planes
and stresses. These are sometimes called “beach balls.” The different types of fault motions
produce different beach balls that geoscientists can readily recognize as strike-slip, normal,
or reverse faulting. The Oaxaca earthquake generated a compressional, or reverse faulting
beachball. This earthquake was bad, but it could have
been worse. Earthquakes in what geoscientists call the seismogenic zone, 20-40 km deep in
the subduction zone, are the most powerful on Earth. This is where the 2004 Sumatra earthquake
that killed a quarter of a million people occurred as well as the magnitude 9.0 earthquake
that hit Japan in 2011 that killed 16,000 people. Most deaths were the result of tsunamis
that these monster earthquakes also caused. It is these kinds of earthquakes that have
geoscientists nervous about what is possible to happen in the subduction zone of the Pacific
Northwest. Let’s hope nothing like that happens for a long, long time!
I’m Ali Sealander with UTD GeoNews and I’ll see you next time!