Are earthquakes becoming more frequent in Japan?
How to compare earthquake counts fairly—and why a few active days do not prove a lasting increase.
A JISHINDESU editorial guide based on the cited official and primary materials. It does not replace official safety information.
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First decide what you are counting
The question “are there more earthquakes?” has no clear meaning without a scope. You could count every detected event, only events above a magnitude threshold or only those that reached a chosen Shindo. Each produces a different number.
You also need one area and time range. A week in one prefecture should not be compared with a full year across Japan.
A short sequence can transform a chart
A large earthquake is often followed by many aftershocks. On a daily or weekly chart, this creates a sharp jump. It does not necessarily mean that activity across the country has permanently increased.
A swarm can also lift the total in one region. Always inspect location and time alongside the count.
Better observation can record more small events
Sensor networks, analysis methods and publication rules change over time. A denser or more sensitive network can detect more small earthquakes even when the Earth has not changed by the same amount.
Long-term comparisons need a consistent dataset and similar thresholds. Do not combine different systems without explaining the difference.
A simple example of a fair comparison
Suppose the map contains 40 events this week and 12 the week before. First see whether 25 of the 40 belong to one sequence in the same place. Then compare several months using the same magnitude or Shindo threshold.
That comparison can describe a change in the selected dataset. It still does not predict future earthquakes.
Scroll horizontally to view the complete table.
| Element | Useful comparison | Misleading comparison |
|---|---|---|
| Time | month to month | one week to a year |
| Area | same prefecture | one city to all Japan |
| Threshold | same M or Shindo | all events to strong events |
| Source | same dataset | different networks without adjustment |
Using JISHINDESU statistics
Choose the same time range and area, then compare the count and distribution of Shindo and magnitude. Open the map when you see a jump; it may reveal one local sequence.
Statistics organise the available history. They do not show whether a large earthquake will happen tomorrow and must not be presented as prediction.