After researching earthquakes a few weeks ago, I found a great deal of interesting information regarding energy release, frequency, and a multitude of other topics. Today I will write about the release of energy from earthquakes since the beginning of the 20th century and compare the earthquakes of the last decade to those of the past century.
The above figure shows the energy released by earthquakes from 1900 to 2001. A log plot was used to so that data across a large range of magnitudes could be viewed simultaneously. The energy released by an earthquake is given by the Gutenberg-Richter magnitude-energy relation. Here is the formula for energy in joules:
E = 10^(1.5M + 4.8)
This formula shows that for each point of increase in magnitude, an earthquake will release 32 times more energy (10^1.5 = 32). That is why even though earthquakes above magnitude 8, termed “great earthquakes”, are quite rare, the energy they release every year is approximately equal to or greater than the energy from all the earthquakes of magnitude 7 to 7.9 from the same year, which are called “major earthquakes”. This effect is noticeable in the figure below, which shows the total earthquake energy released every year for all magnitudes combined.
The greatest recorded release of energy by an earthquake was in 1960, when a magnitude 9.5 earthquake occurred in Chile. This is the largest earthquake on record, releasing 1.12 x 10^19 joules, which is the equivalent of 2.7 billion tons of TNT. In comparison, the Great East Japan earthquake of 2011, a magnitude 9.0 earthquake, released 2 x 10^18 joules, or the equivalent of 477 million tons of TNT. The figure below shows the amount of energy released by earthquakes each year on a linear scale, instead of the log scale in the previous figure. This scale allows one to appreciate just how large great earthquakes are in terms of energy released, when compared with earthquakes of smaller magnitudes.
According to the USGS centennial data, the five largest earthquakes from 1900 to 2001 were the aforementioned 9.5 in Chile in 1960, the 9.2 in Alaska in 1964, the 9.1 in Kamchatka in 1957, the 9 in Kamchatka in 1952, and the 8.8 off the coast of Ecuador in 1906. These earthquakes are responsible for the spikes in earthquake energy in those years, which can be seen in the figure above. If this graph were extended to 2011, there would be several more spikes in 2004, 2010, and 2011, which would represent the magnitude 9.1 earthquake in Indonesia, the 8.8 off the coast of Chile, and the 9.0 in Japan. The energy release from earthquakes in the last decade is actually higher than the energy released in the 1930’s, 1940’s, 1970’s, 1980’s, and 1990’s.
However, what does this mean in terms of earthquake predictions or apocalyptic scenarios? Not a whole lot. As the USGS wrote, “A temporal increase in earthquake activity does not mean that a large earthquake is about to happen. Similarly, quiescence, or the lack of seismicity, does not mean a large earthquake is going to happen.” The earthquakes that occurred in the last decade released a large amount of energy, comparable to the energy that was released from 1952-1964. However, we cannot predict whether these earthquakes will continue or whether there will be another relative drought for 30 years.
As far as these earthquakes being a sign of something larger, we should look at the history of earthquake measurement. Modern earthquake measurement methods are a fairly recent phenomenon, having existed for less than 140 years. The Rossi-Forel scale was invented in 1873, the Mercalli intensity scale was invented in 1902, and the Richter scale was invented in 1935. We have no accurate scientific measurements that extend further into the past, so we can only make comparisons with data within the past 140 years. This limited frame of reference makes any apocalyptic claim based on statistical information about earthquakes quite hard to support. Unlike sediments or ice core samples, we can only say that an earthquake is greater than what we have seen in the past 140 years, not over the past 1,000 or 1 million years. This limits how much we can say about how strong recent earthquakes are, historically. Earthquakes have undoubtedly been a part of earth’s history for millions of years and will continue into the foreseeable future. That doesn’t mean anything apocalyptic is happening.
While I was researching earthquakes, I found a lot of interesting information. Here are some of the more interesting links.
View Seismographs of Recent and Famous Earthquakes: http://rev.seis.sc.edu/earthquakes.html
Earthquake Prediction: http://earthquake.usgs.gov/learn/topics/?topicID=53&topic=Prediction
The Energy Release in Great Earthquakes:
Earthquake Density Maps: http://earthquake.usgs.gov/earthquakes/world/world_density.php
Earthquake Data from 1900 to 2002: http://earthquake.usgs.gov/research/data/centennial.php
Cool Earthquake Data Visualization: http://www.hivegroup.com/gallery/earthquakes/
Find Recent Earthquakes: http://earthquake.usgs.gov/earthquakes/recenteqsww/