Earthquakes are caused by the sudden release of energy through the earth’s crust, which creates seismic waves. The disruption caused by these waves creates the shaking that most people associate with earthquakes. The US Geological Survey (USGS) estimates there are ~500,000 seismic events every year. Most of them are too small for humans to notice and we only ‘feel’ ~100,000. Most earthquakes result from the movement of earth’s twenty tectonic plates, but they can also be caused by volcanic activity and meteors. Aftershocks from the largest events often last for months.
Measuring Earthquakes & Energy
Italian geologist (and priest) Giuseppe Mercalli created a system for categorizing quakes. The Mercalli scale assigned quakes a score ranging from I to XII based on their impact on people and structures. In addition to the oddity of using Roman numerals, the system had several obvious drawbacks. Impacts on people and buildings were highly subjective and it was nearly impossible to assign scores to quakes at sea or in unpopulated areas.
To correct these short-comings, Charles Richter and Beno Gutenberg developed a more scientific system. Using a seismograph, which had been invented more than 50 years earlier, and calibrating for the distance from the epicenter, they created a logarithmic scale. Thus, a quake with a score of 6 is ten times stronger than one with a 5 and a hundred times stronger than a 4. Strength in this case refers to the movement or shaking of the ground. The actual energy increase between numbers on the scale is 32 times, e.g., a magnitude 6 quake releases 32 times as much energy as a magnitude 5.
Since 1979 seismologists have used a slightly different logarithmic scale called the moment magnitude scale. The name was apparently a bit too technical for the news media, which continues to report the magnitudes as the Richter scale.
Map of 6.0 or greater earthquakes since 1900 courtesy of Phoenix7777 and openstreetmap.org.
Earthquake (Fun?) Facts
- The US Geological Survey estimates that since 1900 there have been 18 major quakes (7.0-7.9) and 1 great (8.0+) earthquake per year.
- Human activity can also cause earthquakes. According to the US Geological Survey, the leading causes of human induced quakes are wastewater injection, hydraulic fracturing, and enhanced oil recovery. Click here for more details.
- Four out of five earthquakes occur along the Pacific Ocean’s ‘Ring of Fire.’
- Side effects of quakes (avalanches, tsunamis, and landslides) and collapsing structures (buildings, bridges, etc.) claim the most lives.
- Alaska experiences the most natural earthquakes in the US, but California suffers the most damage.
- The 1556 Shaanxi earthquake is estimated to be the deadliest event causing more than 830,000 deaths.
- The 22 May 1960 Chilean earthquake, with a magnitude of 9.5, is the most powerful recorded seismic event.
- The 26 December 2004 Sumatra-Adaman earthquake in the Indian Ocean, which shook for 10 minutes, was the longest-duration quake ever recorded. It created a 100’ tidal wave and killed more than 225,000 people in eleven countries.
Predicting Earthquakes
Despite living most of my life in the eastern US, I’ve experienced two seismic events. The first was during my college days. An event in NY’s Adirondacks rattled windows more than 100 miles away. I only know that because someone told me – I slept through the whole thing. I was also sleeping for the second event. At 1:51 pm on 23 August 2011 a magnitude 5.8 quake struck 90 miles southwest of Washington DC. This time the shaking woke me. It also damaged the Washington monument, which remained closed until May 2014.
Probability of a damaging earthquake over the next 100 years courtesy of the US Geological Survey
As always thanks for reading.
Armen
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