1. Can Scientists Predict an Earthquake's Exact Time, Place, and Size?
Predicting an earthquake means limiting three factors with high accuracy: time, location, and magnitude. The Japan Meteorological Agency explains that such precise predictions are difficult with current scientific knowledge.[1] For the Nankai Trough earthquakes, they state there is no scientifically established method to predict the timing, location, or scale with high confidence at this time.[2]
The key point is that all three elements must be specific. Saying "a big earthquake will happen somewhere in Japan soon" is obvious for an earthquake-prone country and not useful information. It only counts as a prediction if it says, for example, "around 3 PM on [date] near [city]." The agency warns that claims specifying dates and locations are generally considered misinformation.[1] So, are there signs beforehand? Let's look at the idea that animals know.
2. Do Animal Behavior or Slow Earthquakes Warn Us Before a Quake?
Stories about animals acting strangely before earthquakes are well known. The United States Geological Survey (USGS) summarizes that no consistent, reliable behavior has been confirmed, nor a mechanism explaining how it would work.[3] In other words, behaviors useful for prediction have not been confirmed.
Another candidate is the "slow earthquake." This is when a fault moves so slowly that people do not feel shaking. According to JAMSTEC (Japan Agency for Marine-Earth Science and Technology), slow earthquakes occurred before both the foreshocks and the main shock of the 2011 Tohoku Earthquake.[4]
We only know this because we checked records after the earthquake happened. It was not identified as a precursor beforehand. Whether slow earthquakes trigger giant earthquakes is still unclear. While there are candidate signs, a path to actual prediction has not yet been found.
3. How Is an Earthquake Probability Like "X% Within 30 Years" Different from Prediction?
A concept similar to rain probability in weather forecasts exists for earthquakes. The Headquarters for Earthquake Research Promotion evaluates the likelihood of future earthquakes along active faults and subduction zones. They provide this as a long-term evaluation, often shown as the "probability of occurrence within the next 30 years."[5]
This is different from prediction. Prediction tries to narrow down "when" to a specific day. Long-term evaluation answers: "What is the likelihood of an earthquake of a certain size happening in a certain area over a long period?" Even if the probability is high, this evaluation cannot say if it will happen next week or in 25 years.
Regarding size, magnitude measures the size of the earthquake itself. Intensity measures the strength of shaking at a specific place. Think of magnitude as the brightness of a light bulb, and intensity as how bright it looks depending on your distance from it.[6] For the same earthquake, the intensity becomes smaller if you are far from the hypocenter.
Probability does not tell us "when," but it helps compare danger levels between regions to prioritize safety measures. What can we do just before shaking starts?
4. How Does the Earthquake Early Warning System Work, and What Are Its Limits?
The Earthquake Early Warning system is not a prediction; it uses the difference in speed between waves. When an earthquake occurs, shaking travels as two types of waves: fast "P-waves" at about 7 km/s (about 4.3 miles/s) and slow "S-waves" at about 4 km/s (about 2.5 miles/s). The S-wave causes the strong shaking.[7]
Seismometers near the epicenter detect P-waves. The Japan Meteorological Agency estimates the source and size, then sends alerts to areas before the S-waves arrive. Let's try a simple calculation. At 100 km (about 62 miles) from the epicenter, P-waves take about 14 seconds (100 ÷ 7), and S-waves take about 25 seconds (100 ÷ 4). The difference is about 11 seconds. However, this calculation assumes only speed differences. In reality, analysis and transmission take several seconds, so the usable time is shorter.
This delay creates limits. The agency states that for locations close to the epicenter, the warning may not arrive before strong shaking begins in principle.[8] Also, predicted intensity can have an error of about ±1 level. For shallow inland earthquakes, it is especially hard to warn people directly above the epicenter in time.
In short, the alert says: "An earthquake has just happened. Strong shaking will arrive soon in some places." Even a few seconds are meaningful for taking protective action. So how do we use these seconds and prepare daily?
5. How Can You Prepare at Home and Check Your Area's Earthquake Probability?
The Cabinet Office disaster prevention information page explains that 30 to 50 percent of earthquake injuries are caused by furniture falling or toppling.[9] It recommends assuming furniture will fall and securing it. Even if prediction fails or warnings are late, this is something you can do. Review furniture in your bedroom and items on high shelves with an adult. Moving heavy furniture alone is dangerous, so always ask an adult for help.
You can check the outlook for your area on the Headquarters for Earthquake Research Promotion's "List of Long-term Evaluation Results" page.[5] Look for active faults or subduction zone earthquakes near where you live and see if a probability within 30 years is listed. A high probability serves as a clue to compare danger levels between regions and prioritize safety measures.
Inability to predict exact times is not a reason for despair, but a reason to use what we know. Combining probabilities, early warnings, and home safety measures is how we manage the uncertainty of earthquakes today.