Japan is recognized as one of the most seismically active regions. The question of how many earthquakes occur there each month does not yield a simple, static answer because the definition of an “earthquake” can vary significantly. Seismologists track events that are only detected by sensitive instruments, while the public primarily focuses on those strong enough to be felt. To understand the true monthly frequency, it is necessary to differentiate between the thousands of minor tremors recorded and the smaller number of events that actually cause noticeable ground shaking.
Defining Seismic Event Frequency
The total number of seismic events recorded in and around Japan every month averages over 400 minor earthquakes. These events are micro-quakes, with a magnitude too small to be perceived by humans, only registering on the instruments of the Japan Meteorological Agency (JMA). This total number provides a picture of the constant, low-level tectonic activity occurring beneath the region.
The more relevant figure for the general public is the number of “felt” earthquakes, which are tremors strong enough to be noticed by people indoors. Data compiled by the JMA indicates that the country experiences an average of approximately 3,800 earthquakes with a magnitude between 3.0 and 3.9 annually. This translates to a monthly average of over 315 events strong enough to potentially cause noticeable shaking in the local area.
The monthly count of felt earthquakes is not consistent and can fluctuate based on geological factors. An average figure can be skewed by seismic swarms, which are clusters of earthquakes occurring over a short period. A single large event is often followed by a sequence of aftershocks that can continue for weeks or months, temporarily inflating the monthly totals. For example, the number of M4.0 or greater events can spike from the annual average of around 86 per month to several hundred following a major tremor.
Japan registers about 160 earthquakes each year with a magnitude of 5.0 or higher, which are strong enough to cause noticeable shaking. This means the country experiences more than a dozen significant quakes per month that can be felt by residents. The continuous monitoring of these different magnitude ranges is why the overall monthly count remains high, even when the number of damaging quakes is low.
The Tectonic Engine Driving Japan’s Activity
Japan’s high seismic frequency is caused by its location at the intersection of four major tectonic plates. The Japanese archipelago sits within the Pacific Ring of Fire, a horseshoe-shaped zone characterized by frequent earthquakes and volcanic eruptions. This geological setting creates intense and constant compressional stress as the plates grind against one another.
The country is wedged between the Pacific Plate, the Philippine Sea Plate, the Eurasian Plate, and the North American Plate (Okhotsk Plate). The two oceanic plates, the Pacific and the Philippine Sea, are constantly moving and diving beneath the continental plates in a process called subduction. This creates deep ocean trenches, such as the Japan Trench, which mark the boundaries where the plates descend into the Earth’s mantle.
The Pacific Plate moves at around 8 to 10 centimeters per year as it subducts beneath the North American/Okhotsk plate. This slow but powerful movement does not happen smoothly; instead, the plates lock together, building up strain over time. Earthquakes occur when the accumulated stress finally overcomes the friction between the plates, causing them to suddenly slip and release energy.
Having two oceanic plates subducting simultaneously beneath the island arc creates a volatile environment. This double subduction system is responsible for the frequent shallow and deep-focus earthquakes that affect the entire country. The constant geological motion is the fundamental reason why Japan experiences seismic activity on a near-daily basis.
Measuring and Classifying Earthquakes
The high number of seismic events reported in Japan is a result of a monitoring system that relies on two distinct measurement scales. These scales provide complementary information about any given earthquake. The first is the magnitude, which is a single measure of the total energy released at the earthquake’s source, regardless of location.
While the Moment Magnitude Scale is the modern standard for measuring energy, the Japan Meteorological Agency (JMA) also uses its own magnitude scale. This magnitude is a technical calculation that determines the overall size of the earthquake event. For example, a magnitude 3.0 quake releases a specific amount of energy at its hypocenter, the point underground where the rupture begins.
The second and more practical measure for the public is the JMA Seismic Intensity Scale, known as Shindo. This scale quantifies the local degree of ground shaking and its effects at various locations. A single earthquake has one magnitude value but can produce many different Shindo intensities across the affected area, depending on distance from the epicenter and local ground conditions.
The Shindo scale ranges from 0 to 7, with levels 5 and 6 each divided into lower and upper degrees, creating ten steps. An intensity of Shindo 0 is detected only by instruments, meaning the shaking is imperceptible to humans. Public tallies of “felt” earthquakes exclude these Shindo 0 events, defining the threshold for the monthly frequency figures tracked.