Four massive earthquakes struck Japan in less than three days this week. The strongest measured 7.3 on the Richter scale early Saturday. Massive damage to roads, bridges and buildings were reported. The death toll by Saturday’s end reached 41 as rescuers struggled to reach survivors. Thousands sustained serious injuries, with about 90,000 evacuated from the quake-hit southern regions and admitted to emergency shelters.

Warning calls have been issued to brace for more severe tremblors in coming days. The quakes also appear to have triggered an eruption at Mt. Aso, Japan’s largest active volcano. A tsunami warning was sounded but lifted a few hours later.

Why is Japan battered so often by earthquakes? Why hasn’t the Mt. Aso eruption triggered a tsunami? What are the relationships between Japan’s geographic location, earthquakes and tsunamis? The following report answers some of these questions.

This week’s chain of events began on Thursday when a 6.5 magnitude earthquake shook Japan killing nine. On Saturday, a “triple” quake struck three separate areas: Kumamoto city (Magnitude 7.3 at 1:25am), Aso district (M5.8 at 3:55am) and neighboring Ōita prefecture (M5.3 at 7:11am).

Japan and earthquakes

Baby rescued from Japan quake debris
Infant rescued from Japan quake debris

Geologists have long said that Japan is quake-prone. It is situated along the so-called Pacific “Ring of Fire” or the Subduction Zone. Call it the biggest crash scene on Earth. These boundaries mark the collision point between two of the planet’s tectonic plates. The plates are pieces of crust that slowly move across the planet’s surface over millions of years. This “ring” is actually an imaginary horseshoe-shaped zone that follows the rim of the Pacific Ocean, where many of the world’s earthquakes and volcanic eruptions occur.

Within the Ring of Fire, several tectonic plates — including the Pacific Plate beneath the Pacific Ocean and the Philippine Sea Plate — mash and collide. Scientists first identified subduction zones in the 1960s, by locating earthquakes in the descending crust. Now, new instruments can precisely track the shifting tectonic plates. Subduction zones occur all around the edges of the Pacific Ocean, off the coasts of Washington, Canada, Alaska, Russia, Japan and Indonesia. These subduction zones in the Ring of Fire are responsible for the world’s biggest earthquakes, the most terrible tsunamis and some of the worst volcanic eruptions, according to Live Science.

The Pacific Ring of Fire is the most active earthquake belt in the world. This week’s earthquakes in Japan appear to have been caused by the Philippines Sea Plate diving underneath the Eurasia Plate, according to Paul Caruso, a geophysicist with the US Geological Survey (USGS).

While Japan is no stranger to earthquakes, the 7.3-magnitude temblor is one of the largest ever recorded in this part of southern Japan, Caruso told Live Science. Since the start of this year, there have been numerous earthquakes across South Asia and the Pacific.

The tsunami factor

Not all earthquakes trigger tsunamis. There are certain conditions that could possibly lead to the deadly earthquake-tsunami combo: first, the earthquake must be at least a magnitude-7 tremblor; second, the quake’s epicenter has to be underneath the ocean and finally, the quake has to be shallow.

Saturday’s earthquake was shallow — about 6 miles (10 km) underground — but the epicenter was on land, meaning there aren’t likely to be any dangerous tsunamis as a result, according to Caruso. On the other hand, if the next at least 7.0 tremblor is shallow and centered under the ocean, expect a tsunami.

Why do quake numbers vary?

Why do scientific numbers measuring the power of the quake vary so much? Saturday’s jolt was reported by the US Geological Survey as a 7.0 magnitude quake. The Japan Meteorological Agency, on the other hand, pegged it at 6.5,with some media outlets reporting a 7.3 magnitude intensity. So which benchmark is right and which should we believe? The answer is that all are correct in their own way.

The oft-criticized Richter Scale basically uses an instrument called a seismograph to measure the shock waves created by an earthquake. Based on the readings of several of these devices and their distance from the quake, seismologists are able to triangulate the epicenter of the event and measure its strength. When an earthquake occurs, the public needs an all-encompassing number to put the event in perspective vs. previous quakes. The Richter number serves this purpose. But ultimately, all the different scales used by different agencies are dumbed-down scales derived from more complex scientific measurements.

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