[Definitive Guide] Earthquake Basics and Essential Preparedness: From the Difference Between Seismic Intensity and Magnitude to Actions Before, During, and After an Earthquake

Introduction

You cannot choose exactly when an earthquake will occur.
However, what to prepare before it happens, how to move the moment it strikes, and what to prioritize after it occurs can be decided to some extent.

And in reality,
there is a significant gap between "knowing something about earthquakes" and
"being able to actually act during an earthquake."

In this article, we will first clarify the difference between seismic intensity and magnitude, and then summarize practical responses for before, during, and after an earthquake, as well as how to coordinate with family and housemates.

What is Seismic Intensity?

Seismic intensity represents how strongly the ground actually shook at a specific location.
Even with the same earthquake, the shaking differs between areas near the epicenter and those far away, so seismic intensity varies by region. The Japan Meteorological Agency also explains that seismic intensity represents the strength of the shaking caused by an earthquake at a specific location.

For example, even with the same earthquake, it is possible to have a seismic intensity of 5-lower in Prefecture A and 3 in Prefecture B.
In other words, it is easy to understand seismic intensity as a number that looks at the result of the shaking experienced at that specific point, rather than the magnitude of the earthquake itself.

What is Magnitude?

Magnitude represents the scale of the earthquake itself.
The Japan Meteorological Agency explains it as a value indicating the size of the earthquake, or the scale of the energy released from the epicenter.

Also, a difference of 1 in magnitude corresponds to a difference of about 32 times in energy.
In other words, even a small increase in the number represents a significant difference in the earthquake.

The Difference Between Seismic Intensity and Magnitude

In short,
magnitude is the cause, and seismic intensity is the result.

• Magnitude
  → The scale of the earthquake itself

• Seismic Intensity
  → The strength of the shaking at that location

The Japan Meteorological Agency explains this using the relationship between the "brightness of a light bulb" and the "brightness at a specific location." The brightness of the bulb itself corresponds to the magnitude, while how bright it appears at a distance corresponds to the seismic intensity.

Understanding this difference makes it much easier to process news reports such as
"The magnitude was large, but my area didn't shake much"
or
"The magnitude wasn't that large, but it shook strongly because it was nearby."

Things to Prepare Before an Earthquake

The most important part of earthquake preparedness is actually not after the earthquake occurs, but before it happens.
The government public relations office recommends keeping furniture out of bedrooms and children's rooms as much as possible, or using low-profile furniture and taking measures to prevent it from tipping over. Tipping furniture and falling objects are major causes of indoor injuries.

The first priority is to prevent the inside of your home from becoming a source of danger. Specifically, it is important to secure chests, bookshelves, cupboards, and televisions, and to arrange furniture so that it does not block exits. Ensuring safety in bedrooms and areas where you spend a lot of time is a high priority.

Next are emergency supply bags and stockpiles.
Government public relations advises that you should think about what you personally need when preparing an emergency bag and keep it in an easily accessible place, such as near the entrance or in the bedroom. Water, food, portable toilets, flashlights, mobile batteries, and regular medications are the basics.

Furthermore, something often overlooked is agreements with family members. Because phone lines can become difficult to connect during a disaster, disaster prevention manuals repeatedly emphasize the importance of deciding on meeting places and communication methods in advance, including for situations where family members are in different locations.

What to do the moment an earthquake occurs

The top priority when an earthquake occurs is to protect yourself first. Indoors, the basic action is to protect your head and body from falling or toppling objects. If you are in a situation where you can get under a sturdy desk, that is effective, but if moving would increase danger, you must make the judgment to protect your head right where you are.

Outdoors, you need to be aware of falling or collapsing roof tiles, glass, signs, and concrete block walls, and move away from buildings. The Fire and Disaster Management Agency calls for people in densely populated urban areas to be mindful of protecting their heads while keeping a distance from dangerous objects.

If you are near the coast, regardless of the intensity of the shaking, the basic rule is to assume a tsunami and evacuate immediately. Materials from the Fire and Disaster Management Agency also indicate that you should quickly flee to high ground when near the coast or rivers, and not return until warnings have been lifted.

Regarding movement during evacuation, the Fire and Disaster Management Agency advises evacuating on foot as a general rule. This is because using a car can cause traffic jams and potentially hinder rescue and firefighting activities.

What to do immediately after an earthquake

Even after the major shaking has subsided, the danger is not over.
Rather, aftershocks, fires, building damage, and information confusion begin from this point.

First, you want to check for injured people and fire sources.
In particular, regarding fire prevention after an earthquake, caution is needed with gas and electricity; materials from the Cabinet Office also highlight the importance of measures against electrical fires and the use of seismic circuit breakers. When leaving your home to evacuate, you should also be mindful of circuit breaker measures depending on the situation.

Also, even if the inside of your home looks safe, walls, ceilings, or furniture may have continued to tilt.
It is important not to force yourself back inside or stay near furniture that is about to topple.

Gathering information is also important, but misinformation spreads easily during disasters.
Therefore, the basic rule is to prioritize reliable sources of information, such as the Japan Meteorological Agency, local governments, NHK, and the Cabinet Office's disaster prevention portal.

Dealing with family and housemates

Dealing with an earthquake does not end with your own personal issues.
If you have family or housemates, the level of confusion will change significantly depending on whether you have shared rules in advance. Disaster prevention manuals recommend confirming meeting places and communication methods not only for when family members are together, but also for when they are apart.

It is especially important to decide in advance:
"What is the first thing to do when an earthquake occurs?"
"Where should we head if we cannot meet up?"
"Who will prioritize looking after children and the elderly?"
Just having this decided will reduce hesitation when an event occurs.

If there are small children or elderly people, the assumptions for evacuation speed and judgment differ.
Therefore, it is better not to think, "I can go right away," based on the perspective of a healthy adult. You also need to add items to your preparations beyond general ones, such as medicine, care supplies, and infant supplies, tailored to individual circumstances. Government public relations also advises that you should think about what you personally need when preparing emergency supplies.

Even if your housemates are not family, it is highly valuable to at least share:
"Where will we evacuate to in the event of a major earthquake?"
"How will we contact each other if we cannot return home?"
During a disaster, the presence of
a shared understanding in advance matters more than good intentions.

Things that are easy to overlook but important

When people think of earthquakes, they tend to focus only on the shaking, but in reality, secondary disasters are also quite significant.
Cabinet Office materials also emphasize fire prevention, ensuring outdoor safety, and measures against electrical fires.

Furthermore, if power outages, water supply disruptions, or communication failures occur, the foundations of daily life will collapse instantly.
Since there may be situations where electronic payments cannot be used, keeping some cash on hand is also a practical precaution. Emergency go-bags and home stockpiles are not just for peace of mind; they are directly linked to your ability to sustain life.

Moreover, the difficulty of an earthquake changes not only based on the location, but also the time of day.
At night, visibility is poor; in winter, the risks of hypothermia and fires from heating appliances increase; and during commuting hours, difficulty returning home and crowd chaos are likely to occur. That is why it is important to imagine scenarios beyond just 'what if it happens at home during the day,' such as 'at night,' 'while out,' and 'when family members are in different places.' This aligns with the Fire and Disaster Management Agency's approach to responding while away from home.

Summary

When trying to understand earthquakes, the first two things to grasp are the following.

Seismic intensity is the strength of the shaking at a specific location.
Magnitude is the scale of the earthquake itself.

Beyond that, what is truly important is not just knowledge, but preparation for action.

• Secure furniture

• Organize stockpiles and emergency go-bags

• Decide on meeting places and communication methods with family and housemates

• Immediately after an earthquake, protect yourself first

• Even after the shaking stops, be alert for fires, aftershocks, tsunamis, and misinformation

• Evacuate on foot as a rule, and prioritize tsunami safety when at the coast

Earthquakes cannot be prevented.
But damage can be reduced.

And in many cases, the difference is made not by your immediate reaction after it happens, but by how concretely you thought about it before it happened.