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Structural damage from the Magnitude 8.8 earthquake in Chile in 2016 (Source: Expansion - CNN)

What is the difference between an earthquake’s magnitude and intensity?

Think about sitting around a campfire. The fire emits a measurable level of heat, and the nearer you sit to it, the hotter the fire feels. If you are farther from the fire, the heat is less intense. This simple example can explain common earthquake measurements – magnitude and intensity – and what these earthquake scales mean.

Richter Scale

Consider, once again, the campfire. This temperature is measurable and absolute. When an earthquake occurs, the Richter scale measures the magnitude of the earthquake at its epicenter. The Richter scale was developed in 1935 as a way to quantify the strength of earthquakes. It is a logarithmic scale based on the amplitude of the waves recorded by seismographs. A logarithmic scale means a magnitude increase of 1 relates to an energy increase by a factor of 10. An earthquake measuring a 4.0 on the Richter scale is 10 times as strong as a 3.0!

Seismograph at Weston Observatory at Boston College, Weston, Massachusetts

Earthquake seismograph at Weston Observatory at Boston College, Weston, Massachusetts.

 

Modified Mercali Intensity Scale

Now, you know the closer to the campfire you sit, the hotter the flames feel on your skin. This generally holds true with earthquakes as well. Typically, the nearer the epicenter the stronger the ground shaking you would feel; however, there are other factors that affect the intensity of the earthquake you feel at your location. The type of earthquake, bedrock the shockwaves traveled through, and amplitude of the shockwaves from the earthquake are a few of these factors. The intensity you feel is measured on a scale called the Modified Mercali Intensity Scale (MMI). The MMI scale ranges from “Not Felt” and “Weak Shaking” up to “Violent” and “Extreme” with well-built structures suffering damage.

USGS map and intensity scale for 1971 San Fernando Earthquake (Magnitude - red-circled, epicenter - star, intensity - table)

USGS earthquake map and intensity scale for 1971 San Fernando Earthquake (Magnitude – red-circled, epicenter – star, Modified Mercali Intensity scale – table)

Other Scales Around the World

While the Richter scale is widely known and the MMI scale is used in the United States, there are other magnitude and intensity scales in use around the world. The Japanese Meteorological Agency uses a separate calculation for shallow earthquakes (depth < 60km) which has been shown to be reasonable when the magnitude is 4.5-7.5; however, this magnitude measurement has historically underestimated larger magnitude tremors. Additionally, Japan and Taiwan use the Shindo intensity scale which has significant correlation to the MMI scale. During the middle to late 20th century, the USSR, East Germany, and Czecholsovakia established and utilized the Medvedev-Sponheuer-Karnik scale (MSK) to evaluate shaking and effects from earthquakes. This scale was built upon in the 1990s by the European Seismological Commission as they shifted to implement the European Macroseismic Scale for European countries. The MSK scale continues to be employed in Russia, India, Israel, and the Commonwealth of Independent States.

You can read more about some of these other scales here:

JMA Shindo intensity scale: https://www.jma.go.jp/jma/en/Activities/inttable.html

MSK Scale: https://www.gktoday.in/gk/various-earthquake-scales/

 

Sources:

https://earthquake.usgs.gov/learn/topics/mercalli.php

https://www.japan-talk.com/jt/new/why-japan-doesnt-use-magnitude-for-earthquakes

Earthquake, History.

Earthquakes – An Unpredictable Force of Destruction

Earthquakes have caused massive devastation, and amounted to huge numbers of human casualties since the beginning of recorded history. The problem with these natural disasters has become compounded by our cities becoming developed more vertically in the form of taller buildings without the proper respect given to earthquakes during the engineering process. Along with the previously mentioned factor, the general population that doesn’t live in earthquake prone areas won’t know what to do in a situation like this. You can learn more about how to prepare yourself, and what to do during an earthquake event in RedZone’s blog. This blog will hopefully assist in understanding the geoscience that is occurring before, during, and after one of these events takes place.

The Earth’s Crust and Earthquakes

Of the inner Earths four internal layers, the crust and the upper most portion of the mantle play the most vital roles in the unseen processes that power earthquakes. The Earth’s crust is made up of 12 major plates that are very dynamic in nature.

Tectonic plates and Earthquakes

This map displays the 12 major tectonic plates throughout the world.

It is here at the tectonic plate boundaries that the earthquakes originate. As the plate boundaries come to a resting place due to its jagged edges, the remaining portion of the plate remains in constant movement. When the energy from the movement of the rest of the plate becomes too much force for an area of the plate boundary to hold, the edges of these plates shift and this is what causes an earthquake. The earthquake we feel on the ground stems from the seismic waves that are produces when the tectonic plates shift.

There are two primary wave types that are produced by this tectonic shift, the P wave (primary) and S wave (secondary). P waves have also been called the compressional waves due to the way these waves push and pull the matter they are travelling through. S waves are the waves we feel on the surface that create the movement on the earth’s surface. S waves are much slower to appear than the P waves for a seismologist to read.

Seismographic Readings and Determining the Epicenter

Scientists with their particular field of study in earthquakes, track these waves to give the public a rating on the Richter scale of how strong in magnitude an earthquake is. These experts also utilize the seismographs to locate where exactly the epicenter was. Triangulation is used to determine the precise location where the epicenter is. Three seismographs measure the difference in times that the P waves arrive at the seismographs and compare them with the time it take for the S waves to arrive at the same location. A circle is then created around the three selected seismograph locations with the radius being determined off the aforementioned time difference in seismic wave arrival. The point at which each of the three seismographs calculated circles meet is the epicenter.

Epicenter of Earthquakes

This diagram depicts a visual representation of how the epicenter of an earthquake is found from three seismographs.

 

Predicting Earthquakes

Unfortunately scientists have been unsuccessful so far in the prediction of when the next earthquake will occur. Earthquake prediction is more often defined as the probabilistic assessment of general earthquake hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades. Like many naturally-occurring phenomena, they are nearly impossible to accurately predict.  Prediction methods go back hundreds of years.j Methods generally involve precursors which among them include animal behavior, gas emissions, and even electromagnetic anomalies. Generally, Earthquake prediction is  thought of as an immature science with any claims of prediction found circumstantial and arguable.

Earthquake warning systems on the other hand have proven successful on a number of occasions especially in areas farther from an epicenter.  The effectiveness of the warning depends on the position of the receiver. After receiving a warning, a person may have a few seconds to a minute or more to take action. Areas near the epicenter may experience strong tremors before a warning is issued. Early warning systems have been prevalent in Japan, Mexico, Canada, and the United States for years.

Sources:

https://pubs.er.usgs.gov/publication/fs20163020

https://earthquake.usgs.gov/learn/kids/eqscience.php

https://earthquake.usgs.gov/learn/facts.php

https://www.nationalgeographic.com/environment/natural-disasters/earthquakes/

http://www.geo.mtu.edu/UPSeis/waves.html