Japan's big earthquake: Why deeper means safer
Saturday's earthquake off the coast of Japan was big, magnitude 7.8, but it was also deep, at least 370 miles below the Earth's surface. Why that makes a difference.
Saturday's earthquake off the coast of Japan was big, magnitude 7.8, but it was also deep, at least 370 miles below the Earth's surface. Why that makes a difference.
A major earthquake 鈥� magnitude 8.5, says the Japan Meteorological Agency聽鈥� struck off the coast of Japan Saturday.
The temblor was felt throughout the country, knocking dish and a few people over, but聽there were no reports of deaths or major damage.
How could such a large magnitude quake cause so little damage and no serious injuries?
The answer lies far below the surface of the Earth.
Saturday's quake was a deep one,聽590 kilometers (370 miles) below the surface, according to the Japan Meteorological Agency. The US Geological Survey reported it as a magnitude 7.8 quake located聽678 kilometers (421 miles) below the surface.
The USGS categorizes earthquakes as shallow (up to 70 kilometers or 43 miles), intermediate (70-300 kilometers or 43-186 miles) or deep (186-434 miles).聽That puts Saturday's quake near the bottom of the Earth's crust.聽
Generally speaking, the deeper the quake, the less violent and less dangerous. That's because the聽strength of shaking - or energy released - felt from an earthquake diminishes with increasing distance from the quake's source.
When judging the potential impact on lives and property, both earthquake magnitude and depth must be taken into consideration.
For example, the April 2015 Nepal earthquake was just 9 miles below the ground, and killed an estimated 8,800 people.
The 2010 Haiti earthquake that killed more than 100,000 people had a magnitude of 7.0 (much less than Saturday's Japan quake) but it was very shallow, just 13 kilometers (8 miles) below the surface.聽
Also, the 1994聽Northridge, Calif., earthquake registered a magnitude of 6.7 but started at a depth of about 18 km (12 miles) and ruptured up to a depth of 5 km (3 miles). It caused damaged estimated to be between $20 billion and $40 billion and killed about 60 people, the USGS reports.
Moreover, there's evidence that suggests deep earthquakes are more efficient in dissipating stress than shallow ones, according to scientists who investigated a May 2013聽a magnitude-8.3 earthquake聽that struck beneath the Sea of Okhotsk, between Russia's Kamchatka Peninsula and Japan.
The researchers added, according LiveScience.com, that this 2013 "deep quake may have been good at releasing energy quickly because it ruptured within a very old rock. The northwest Pacific crust that is subducting in this area is some of the oldest, coldest oceanic crust subducting on Earth. Its age means it is brittle, enriched with concentrations of stress that enlarge the rupture zone and can easily be triggered, greatly shortening the delay between aftershocks."
While deep quakes may be less dangerous, they tend to be felt by more people, explains the USGS.
Deep quakes are more rare than shallow quakes. Only about 3 percent of total energy released by temblors comes from deep earthquakes, according to聽the Encyclopedia Britannica. More common are the shallow quakes which produce about 85 percent of the total energy released by the earth's shifting crusts.
How is earthquake depth determined?
Geologists rely on seismic measuring stations which聽measure the interval between the arrival of the聽primary (P) and聽secondary (S) waves. The difference between the P and S waves is used to measure the distance of the earthquake from the sensor.
Here's how the USGS describes the process for finding a quake's depth: