The Longest Earthquake Observed

 2004 Indian Ocean Earthquake

For me, the 2004 Indian Ocean Earthquake was the first big earthquake of the 21st century. It occurred on December 25, 2004, about 100 miles (161 km) off the west coast of Sumatra, Indonesia. As an undersea megathrust earthquake, it had a magnitude 9.1-9.3 (depending on what source you refer to) and caused massive tsunamis that flooded coasts around the Indian Ocean.

Longest  Earthquake Observed

The quake has the distinction of being the third-largest ever recorded by a seismograph, and it was the longest ever observed, lasting from 8.3 to 10 minutes, depending on the station that was recording it.

While the earthquake itself didn’t cause much damage, the tsunami waves up to 100 feet (30 m) high killed an estimated 227,898 people in 14 countries.

The quake was also believed to have triggered a number of other earthquakes, including some in Alaska. 

Here's something I find fascinating. Some seismologists believe the 2002 Sumatra earthquake, which had a magnitude 7.3, may have been a foreshock (or pre-shock) even though it happened more than two years earlier.

The following image is a village left in ruins after the 2004 Indian Ocean earthquake caused a tsunami to sweep across coastal cities. 

(Image is public domain as it was taken by a member of the U.S. Navy during that person's normal official duties. Photo by Photographer's Mate 2nd Class Philip A. McDaniel. Retrieved from https://commons.wikimedia.org/wiki/File:US_Navy_050102-N-9593M-031_A_village_near_the_coast_of_Sumatra_lays_in_ruin_after_the_Tsunami_that_struck_South_East_Asia.jpg)

Energy Equivalence 

An earthquake of this magnitude releases as much surface energy as 26 megatons of TNT, or about the equivalent of 1,500 of the atomic bombs that where dropped on Hiroshima. While this is an enormous amount of energy, the Soviet Union tested a hydrogen bomb, nicknamed Tsar Bomba, on October 30, 1961, that had a yield of 50 megatons.

However, while the surface energy was estimated at 26 megatons, the total energy (most of it underground) was estimated to be 9,600 gigatons of TNT.

The tsunami was estimated to have an energy equivalent of 5 megatons of TNT.

Scientists calculated that the amount of energy released altered the Earth’s rotation, which may have shortened the day by 2.68 microseconds and caused a 1-inch (2.5 cm) wobble on the Earth’s axis. However, it should be noted that tidal effects of the moon cause the length of the day to increase by 15 microseconds each year, so the effect on the earth’s rotation is lost. Likewise, the naturally occurring Chandler wobble of the Earth, which can be up to 50 feet (15 m), quickly offsets any effect an earthquake may have on the Earth’s axis.

This next image is from the city of Banda Aceh, Indonesia. (Image is public domain, retrieved from https://www.usgs.gov/media/images/devastation-coastal-city-banda-aceh-indonesia-after-2004-indian-ocean-tsunami)

Big Quake, Increased Stress on Fault

A few months later, on March 28, 2005, a magnitude 8.7 rupture occurred at the fault’s southern end. It’s believed the December quake caused increased stress to build up, leading to the March rupture (Chile Earthquake Altered Earth Axis, Shortened Day), which was about 99 miles (60 km) southeast of the December event. Both quakes were on the same fault, just at different sections.

The linked article also mentions a study that suggests the massive magnitude 9.5 Chile earthquake of 1960 might have increased stress on the sections of the fault, resulting in the 2010 magnitude 8.8 earthquake in Chile.