Japan is the world’s most seismically active nation, where the ground is almost constantly in motion. The frequency of earthquakes is remarkably high, with tremors occurring daily, though most are too minor to be felt by people. This pervasive seismic activity is an intrinsic part of the country’s geology, shaping its infrastructure, disaster planning, and daily life.
Japan’s Tectonic Setting
Japan’s location is the primary reason for its extraordinary level of seismic activity, as it sits directly atop a complex and highly active boundary between four major tectonic plates. The Pacific Plate and the Philippine Sea Plate are actively moving and diving beneath the continental plates: the North American Plate and the Eurasian Plate. This geological process of one plate sliding beneath another is known as subduction.
The subduction zones create immense friction and stress where the plates meet, particularly along the Japan Trench and the Nankai Trough. When the built-up strain exceeds the strength of the rock, the plates suddenly slip, releasing vast amounts of energy in the form of an earthquake.
Statistical Frequency of Earthquakes
Seismographs detect approximately 58,000 earthquakes annually across the region, though this number includes the smallest tremors registered only by instruments. When considering only the quakes strong enough to be felt by people, the average drops significantly but remains substantial, with roughly 1,500 tremors felt each year, translating to an average of four to five perceptible quakes every day.
Earthquakes of magnitude 5 or higher occur approximately 147 times per year, or about a dozen times each month. The more damaging events, those with a magnitude of 6 or greater, happen less frequently but still average around 10 to 11 times annually. Events of magnitude 7 or higher, which are capable of causing widespread damage, occur about one to two times each year.
For the most powerful mega-thrust earthquakes, those with a magnitude of 8 or higher, the frequency is much lower, occurring about once every 14 to 15 years on average. Despite the large number of daily tremors, the majority are minor, with about 3,500 quakes of magnitude 3 or higher occurring each year, which are generally not destructive but are felt by many people.
Understanding Japan’s Seismic Intensity Scale
Japan uses a unique system to measure the local impact of an earthquake called the Japan Meteorological Agency (JMA) Seismic Intensity Scale, or the Shindo scale. This scale is distinct from the Moment Magnitude Scale, which measures the total energy released at the earthquake’s source. Instead, Shindo quantifies the actual ground shaking and its effect on people, buildings, and infrastructure at a specific location.
The scale ranges from Shindo 0, which is undetectable by humans, up to Shindo 7, with levels 5 and 6 further divided into “lower” and “upper” categories, creating a total of ten intensity levels. A Shindo 4 earthquake is noticeable to most people, even those walking, and will cause hanging objects to swing significantly. At Shindo upper 5, people find it difficult to move, and dishes may fall from cupboards.
The highest level, Shindo 7, signifies catastrophic shaking where standing is impossible, and unsecured furniture may be thrown into the air, with widespread structural collapse possible. Because the Shindo scale measures local effects, a single earthquake can generate different Shindo values across the affected region, depending on the distance from the epicenter and the local soil conditions.
Notable Historical Seismic Events
The Great Kanto Earthquake of 1923 struck the Kanto Plain, including Tokyo and Yokohama, with an estimated magnitude of 7.9 to 8.2. The resulting devastation, largely from ensuing fires, claimed over 100,000 lives.
A more recent catastrophe was the Great Hanshin-Awaji Earthquake in 1995, which severely impacted the city of Kobe and was one of the few events to record a Shindo 7 intensity. This urban earthquake exposed weaknesses in older infrastructure and led to significant advancements in building codes and disaster preparedness.
The largest earthquake ever recorded in Japan was the 2011 Great East Japan Earthquake off the coast of Tohoku. This mega-thrust event registered a magnitude of 9.0 to 9.1 and generated a devastating tsunami that caused widespread destruction and the Fukushima Daiichi nuclear disaster. The event fundamentally altered the landscape and even shifted the main island of Honshu several meters eastward.