Why Japan Has So Many Earthquakes—Atop the Ring of Fire

Did you know that Mt. Fuji, widely recognized as a symbol of the Japanese nation, is an active volcano that could still erupt? Did you know that 20% of the world’s major earthquakes with a rating of six or more on the Richter Scale happen in Japan?

If you pay attention to the news, you probably know the large effect earthquakes have on Japan. Minor earthquakes are a part of everyday life, and major destructive earthquakes, unfortunately, take their toll on the country regularly. To understand why Japan is subject to these natural disasters, you’ll have to learn a little about the island nation’s geology.

Read on to discover why earthquakes are a part of life for the Japanese and how this fact has shaped their country and culture.

Page Contents

Japan’s geology

The geology of Japan is some of the richest and most fascinating in the world. The country is well-known for its geological activity, including volcanoes, earthquakes, and tsunamis. These range from minor tremors to major destructive events like the 2011 Tohoku earthquake and tsunami which killed 15,899 people and caused $360-billion-worth of damages, making it the costliest natural disaster in human history.

Japan is an archipelago of islands that was itself formed by complicated processes over hundreds of millions of years. Of course, with such gradual and incremental processes, it’s impossible to pinpoint an exact moment when the landmass we know as Japan was born. Instead, we know that the land that forms Japan was once attached to the eastern part of Eurasia in what is present-day China.

Plate tectonics stretched out the Eurasian plate and pulled what is now Japan away from China and up to form islands. These islands slowly grew and merged into the much larger islands that make up Japan. In fact, the geological history of Japan is a complicated tale of tectonic plates.

Japan’s tectonic plates

Japan is an especially interesting case because its plate tectonics involve a junction of four different tectonic plates: the Pacific Plate, Phillippine Plate, Okhotsk Plate and Eurasian Plate. The four plates meet near Tokyo on Honshu, Japan’s main island.

Tectonic plates are massive pieces of Earth’s crust that float on top of the liquid mantle. Though they move slowly, just 3-5 centimeters per year, their enormous size gives them incredible force, momentum and power. As you might guess, crazy things happen when they run into each other.

While there are a number of things that can occur when plates collide, the main process at work beneath Japan is subduction. This happens when one plate, usually a denser sea plate, goes under another, often a less-dense continental plate.

Since four plates are involved in the formation of Japan, it makes a complex folded structure kind of like when you close a cardboard box without any tape. The Phillippine Plate subducts below the Eurasian plate, but the Pacific Plate subducts under the Phillippine Plate and the Eurasian Plate. Where one plate begins to dive down below the other, an oceanic trench forms. Where one plate is pushed up by the other plate moving below it, islands form.

Along with pushing one plate up to form islands, this process also stretches out the upper plate, in this case, the Eurasian Plate. The result is an arc shape where the plate dips down before lifting up. This depressed area filled with water over millions of years to form the Sea of Japan.

These same processes of plate tectonics are what cause the many geological phenomena in and around Japan. Subducting plates can cause liquid mantle to come closer to the surface. Eventually, it might burst through the crust forming a volcano. As the plates collide, they also shake and vibrate, causing earthquakes and in turn tsunamis.

The Ring of Fire

Specifically, Japan lies on the edge of an extremely active tectonic region called the Ring of Fire. This ring essentially surrounds the Pacific Ocean, going up the side of North and South American and coming down Japan, Indonesia, and New Zealand. The movement of the Pacific Plate and many smaller tectonic plates creates a lot of geological activity, especially in the northwestern region around Japan where there are several small plates.

The Ring of Fire extends in a horseshoe shape for 40,000 km (25,000 miles) and contains 75% of the world’s volcanoes and 90% of the world’s earthquakes.

How many earthquakes does Japan have?

Due to its position on the tectonic plates and within the Ring of Fire, Japan has a lot of earthquakes. In fact, it has roughly 1,500 earthquakes each year. This represents about 20% of the world’s earthquakes with a magnitude of 6.0 or higher.

How often is there an earthquake in Japan?

With 1,500 earthquakes a year, you can do the math and find that earthquakes are a pretty common occurrence. Of course, most of these are just minor tremors, but there’s still something noticeable nearly every day. In fact, it’s estimated a tremor occurs about every five minutes.

The US Geological Survey ranks earthquakes based on their “magnitude” using the Richter Scale. With this scale, earthquakes receive a number, each increase of one point representing 10 times more amplitude than the previous number. For example, an earthquake rated 5.0 is 10 times more intense than an earthquake rated 4.0.

To get an idea for the scale, a hand grenade rates about 0.5. Some large conventional bombs from World War II reached 2.5, the equivalent of 5.6 metric tons of TNT. At about 4.5 on the scale, earthquakes begin to become significant. This is also around the magnitude of the atomic bombs dropped by the United States in WWII.

The famous San Francisco Bay Area Earthquake of 1989 rated 6.9, the Tsar Bomba, the world’s largest nuclear bomb ever tested, rated 7.1, and the meteor impact that caused the extinction of the dinosaurs 65 million years ago is estimated at 13.0. Knowing this, you can get a sense of the incredible power released by the 2011 Tohoku Earthquake that rated a 9.0. In fact, an earthquake of 10 or higher has never been measured.

The frequency of earthquakes is inversely related to their magnitude. That means that more powerful earthquakes are less likely to happen while less powerful ones are more likely. In fact, the Tohoku earthquake is the only earthquake in Japan known to have surpassed 9.0. Many of the most serious earthquakes in Japan have topped 8.0, but these are still isolated to one or two a century. Serious earthquakes from 6.0-8.0 happen even more regularly, perhaps once a decade or so, and lesser quakes are quite common.

The Japanese earthquake season

Some people traveling or moving to Japan want to prepare for earthquakes as best they can. They often ask if there’s a particular earthquake season like there is for tornadoes and hurricanes. Unfortunately, these natural disasters are more predictable because they rely on atmospheric weather, which itself is regulated by the Earth’s regular seasons.

Earthquakes are caused by tectonic geological processes. Essentially, the tectonic plates are floating on top of a sea of liquid magma called the mantle, and they bump into each other causing earthquakes. This makes them random on a human time scale and mostly impossible to predict with our current technology.

Luckily, Japan invests a large amount of resources into earthquake prediction, alarm and safety, and the country is further aided by help from the US Geological Survey. The country has safety measures and regulations that make buildings as secure as possible, and warning systems exist to alert people if there’s a risk of tsunami. A lot of research is devoted to studying earthquakes and trying to figure out how to predict them.

Famous historical earthquakes

Since earthquakes are so common in Japan, they’ve naturally played a prominent role in the nation’s history. Here are some of the most famous that continue to affect Japanese culture to this day.

Great Kanto Earthquake of 1923

This is the deadliest earthquake in Japanese history. Beginning at 11:58 AM on September 1, 1923, it lasted between four and 10 minutes and destroyed much of Tokyo as well as Yokohama and much of the Kanto region. With a magnitude of 7.8, it even moved the 93-ton Great Buddha statue at Kamakura. Japan’s population had exploded with industrialism, but safety and structural engineering had not caught up. As a result, the quake killed approximately 142,800 people.

Hakuho Earthquake of 684

This is one of the earliest earthquakes recorded in Japanese history. According to the 8th-Century history book Nihon Shoki, it occurred in the 13th year of the reign of Emperor Tenmu. Current estimates put this in November of 684. The quake caused a tsunami that resulted in wide-spread destruction including the flooding of several square miles of rice fields and the sinking of many ships. The quake mostly affected the capital of Asuka and killed upwards of 1,000 people, a considerable death toll for the time period.

Genroku Earthquake of 1703

This is the only earthquake besides the Great Kanto Earthquake to kill over 100,000 people, and considering Japan’s population was less than half of what it was in 1923, this is all the more shocking. The 8.0-magnitude quake struck Sagami Bay southwest of Tokyo and created a tsunami that devastated the region with 108,000 deaths.

Kamakura Earthquake of 1293

With an estimated magnitude of 7.1-7.5 and a death toll of 23,000, this earthquake still ranks as one of Japan’s deadliest. It’s believed to have caused a tsunami that brought considerable destruction to Kamakura. This quake is also notable for its political implications. In the confusion, Kamakura Shogun Hojo Sadatoki attacked his rival Taira no Yoritsuna, killing him and 90 of his followers.

Tohoku Earthquake and Tsunami of 2011

This was Japan’s strongest earthquake in history with a magnitude of 9.0. That also makes it one of the most powerful ever recorded in the entire world. The quake caused a 55-foot tsunami that destroyed towns all along the northern coast of Honshu, Japan’s main island, and killed over 15,000 people. The earthquake was a major global event, and damage to the Fukushima Daiichi nuclear plant caused three nuclear meltdowns and release of radioactive contaminants that injured a number of workers and ultimately killed one via radiation exposure. This led to a global debate on the safety of nuclear power that continues to affect the world’s energy industry to this day.

Fun facts about earthquakes in Japan

Did you know…?

  • The intense geologic activity around Japan results in many hot springs heated by volcanic processes. Japan has a rich culture of using these springs for public baths known as onsen.
  • Earthquakes cause tsunamis when the movement of the seafloor is enough to move large amounts of water. These massive series of waves can reach up to 100 feet tall and move up to 500 miles an hour and cross an ocean in less than a day.
  • After the Tohoku Earthquake of 2011, Japan’s Honshu island moved a full 2.4 meters, nearly eight feet.
  • Earthquakes actually redistribute the mass of the Earth and therefore change the length of a day. The Tohoku Earthquake of 2011 shortened the day by 1.8 microseconds.
  • Because of Japan’s propensity for earthquakes and tsunamis, Tokyo was ranked as the most at-risk city for natural disasters of the world’s 30 “megacities.” A major earthquake centered in Tokyo would be a terrible, deadly disaster that would affect the entire global economy.

Back to Japan Junky