Earthquake Strikes Mexico

At 11:29 AM last Tuesday a strong earthquake struck the state of Oaxaca, Mexico. This quake was preceded by two milder earthquakes that stuck the area a 4.9 magnitude quake struck a 9:17 PM the evening before and a 4.3 magnitude quake struck at 5:07 AM. The big earthquake was followed by a series of aftershocks that measured at 4.6 magnitude earthquake at 12:31 PM and 5.0 magnitude two hours later, 4.6 magnitude a couple of hours after that. Tuesday night there was a 5.4 magnitude and 4.5 magnitude. Wednesday morning a 4.1 magnitude earthquake was recorded. Those are just the greater than 4 magnitude earthquakes that have been recorded an unknown number of unknown mild tremors have also occurred.

The severity of an earthquake is expressed in whole numbers and decimal fractions, and have evolved over time. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves, but does not effectively describe the destructive power of large earthquakes. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location, which is why the USGS collects comments and location from people who felt the earthquake.

The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is still the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a magnitude 7 earthquake is 10 times as large as a magnitude 6. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale. This earthquake falls in between major earthquake and great earthquake at 7.4 magnitude.

This is a very seismically active area. In 2017 an earthquake with a magnitude 8.2 struck the area. Historically, several significant earthquakes have occurred along the southern coast of Mexico. Located on top of three tectonic plates, Mexico is one of the world's most seismically active regions. Most of Mexico sits on the westward moving North American plate, but this area is also a subduction zone where two plates converge, and one plate is thrust beneath the other. This process results in earthquakes and volcanoes. The largest earthquakes on Earth occur in these areas.

The Pacific Ocean floor south of Mexico is being carried northeastward by the underlying Cocos plate at a geologically rapid pace of about 50 millimetres a year. Because oceanic crust is relatively dense, when the Pacific Ocean floor encounters the lighter continental crust of the Mexican landmass, the ocean floor is subducted beneath the North American plate creating the deep Middle American trench along Mexico's southern coast.