4 Interesting Facts About Earthquakes

An earthquake occurs when the Earth's surface shakes or vibrates. This usually occurs near fault lines or plate boundaries. In the process from the first vibrations on the fault lines to the actual earthquake and to the aftershocks, many interesting things happen. Let’s us see the four interesting facts about earthquakes:

1) An Earthquake can Affect the Day's Length

When an earthquake occurs, changes in the rocks cause the earth to reshape itself, compressing to a smaller diameter. Once it becomes smaller in diameter, its speed of rotation changes and thus the length of the day becomes shorter.

A research scientist, Richard Gross of NASA's Jet Propulsion Laboratory in Pasadena, California, applied a complex model to perform a preliminary theoretical calculation of how the 2011 earthquake in Japan — the fifth largest since 1900 — affected Earth's rotation. His calculations indicate that by changing the distribution of Earth's mass, the Japanese earthquake should have caused Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds (a microsecond is one millionth of a second)¹.

Another example is the magnitude 8.8 earthquake that occurred in 2010 in Chile, for which Gross estimated that the earthquake shortened the length of the day by about 1.26 microseconds and shifted Earth's figure axis by about 8 centimeters¹.

Picture 1: Earthquakes affect the length of the day.

2) Mars is a Seismically Very Active Planet

NASA’s InSight lander reports that Mars is a seismically active planet. During its first year on Mars, the spacecraft recorded nearly 500 “marsquakes.” Although Mars shakes frequently, the majority of its quakes seem to be minor in size — less than magnitude 4.0. However, Mars doesn’t have active tectonic plates like Earth does. Instead, its earthquakes are triggered by the planet’s long-term cooling, an active process since its formation 4.6 billion years ago. As the Red Planet cools, it contracts, causing its brittle outer layers to fracture and the so-called marsquakes to occur².

3) Earth's Size Limits Earthquake Magnitude

An earthquake’s magnitude is decided by the tectonic fault it occurs on – the longer the fault, the larger the earthquake. The largest earthquake ever recorded on Earth was of a magnitude 9.5. According to the USGS, no magnitude 10.0 earthquake could ever occur since no fault is long enough to generate such an earthquake. As for the fault length needed to produce a magnitude 12.0 earthquake, it would need to be longer than the Earth itself — over 25,000 miles long³.

4) The Seismometer was Invented in China in AD 132

Chinese astronomer royal to the Han Dynasty (202 BC – AD 9, AD 25 – 220) Zhang Heng (AD 78 – 139) created a seismometer — an earthquake detector (known as didong yi 地动仪 in Chinese) in AD 132 — 1600 years before anyone in the West did. It was a large bronze urn with eight dragon heads gazing outward in eight directions. To indicate the direction of an earthquake, Zhang's device dropped a bronze ball from one of eight tubed projections shaped as dragon heads. The ball fell into the mouth of a corresponding metal object shaped as a toad, each representing the direction in which the seismic wave was traveling.

^{Picture 2: Earthquake detector by Zhang Heng.}

Sources:
1) Nasa. Japan Quake May Have Shortened Earth Days, Moved Axis. Accessed on 10-Aug-2022. Available at: https://www.nasa.gov/topics/earth/features/japanquake/earth20110314.html
2) Treehugger. 2021. 15 Groundbreaking Earthquake Facts. Accessed on 10-Aug-2022. Available at: https://www.treehugger.com/earthquake-facts-5116365
3) USGS. Can "MegaQuakes" Really Happen? Like a Magnitude 10 or Larger? Accessed on 10-Aug-2022. Available at: https://www.usgs.gov/faqs/can-megaquakes-really-happen-magnitude-10-or-larger