In order to check earthquake information, a minimum knowledge of earthquakes is a prerequisite. In this section, “seismic intensity“, “magnitude” and “long-period seismic motion class” are also briefly explained.
What is an earthquake?
An earthquake is technically a phenomenon in which the bedrock suddenly shifts. You can think of it as the movement of cracks in the ground.
Because earthquakes originate not only from the epicenter but also from the entire epicenter area (the area where the bedrock has shifted), the risk of a widespread tsunami is high if a major earthquake occurs in a shallow part of the ocean.
Since the media and other media refer to the location of the epicenter as the “epicenter,” the earthquake information we distribute also uses the term “epicenter” for simplicity, but in seismology, the term “epicenter” is generally used, and the term “epicenter” is not used.
What is seismic intensity?
Japanese TV and radio broadcasts say, “The seismic intensity of 6-lower was observed in…” and “The seismic intensity of 7 was observed in…”. The seismic intensity of 7 was observed in…” We often hear such phrases on TV and radio. In this way, the seismic intensity expresses the magnitude of earthquake tremors. There are as many seismic intensity levels as there are places to measure them.
The JMA seismic intensity scale used in Japan has 10 levels, which are rounded to the nearest whole number, called measured seismic intensity.
*The following table was prepared with reference to the JMA website and other sources.
Thus, the seismic intensity classes of the JMA exist up to seismic intensity 7. So, there is no class above seismic intensity 8 (*The reason will be explained later in a separate article).
What is Magnitude ?
On TV and radio, we hear, “Magnitude is 7.3…” Magnitude is 9.0…” etc. We often hear such phrases on TV and radio. Thus earthquake scale is called the magnitude. Denoted by the symbol M. There is only one magnitude for each earthquake. There are several types of magnitudes, but we will not discuss them here as they are technical and difficult to understand.
Magnitude takes the form of a logarithm. At first glance, there does not seem to be that much difference between an M8 mega earthquake and an M9 super quake, but a 0.2 increase in magnitude causes a jump of about 2 times and a 1 increase in magnitude causes a jump of 31.6 times(about 32 times), which means that an M9 super quake is equivalent to about 32 M8 mega quakes. A M9 super quake would be equivalent to about 32 M8 mega quakes. To put this in a slightly different perspective, let’s compare a major M7 earthquake to a M9 super quake. In this case, the M2 is different, and the formula is “31.6 x 31.6 = about 1000,” which means that an M9 super earthquake is equal to about 1000 major earthquakes of M7.
In the following section, we have compiled a rough estimate of the distance that the bedrock will shift according to the magnitude of the earthquake to help you get a better sense of the earthquake.
| Magnitude(M) | Distance of rock displacement movement (approximate) *All values in miles are rounded down. |
| M3 | About 500m |
| M6 | About 6 miles |
| M7 | About 18 miles |
| M8 | About 62 miles |
Typical major earthquakes that have occurred in japan (more details about each earthquake will be explained later).
| Earthquake name [Name of an earthquake disaster] | Magnitude(M) | explanation |
| The 2011 off the Pacific coast of Tohoku Earthquake [Great East Japan Earthquake] | M9.1 (M9.0 *) | The earthquake struck off the coast of Sanriku at 14:46 Japan time on March 11, 2011. Earthquake that caused the Great East Japan Earthquake. Japan’s worst postwar earthquake. Caused extensive damage mainly in eastern Japan. |
| The Great Kanto Earthquake of 1923 | M7.9～8.1 | The earthquake struck at 11:58 a.m. Japan time on September 1, 1923, with its epicenter in western Kanagawa Prefecture. The earthquake caused the Great Kanto Earthquake. It caused extensive damage mainly in the southern part of the Kanto region. |
| The 2016 Kumamoto Earthquake (main shock) | M7.3 | The earthquake struck the Kumamoto region of Kumamoto Prefecture at 1:25 a.m. Japan time on April 16, 2016, approximately 28 hours after the first M6.5 earthquake. Damage was mainly in Kumamoto Prefecture. |
| 1995 South Hyogo Prefecture Earthquake [Great Hanshin-Awaji Earthquake] | M7.3 | The earthquake struck Awaji Island (Osaka Bay) at 5:46 a.m. Japan time on January 17, 1995. The earthquake caused the Great Hanshin-Awaji Earthquake. It caused damage mainly in southern Hyogo Prefecture. |
*Magnitude used by the JMA.
What are long-period seismic motions and long-period seismic motion classes?
JMA has included long-period seismic tremors in its earthquake early warning announcements starting in 2023, as well as on TV and other media when an earthquake of a certain magnitude and magnitude occurs. Therefore, many members of the public may be hearing about long-period seismic motions for the first time. It is explained in detail here.
Long-period seismic tremors, briefly explained, are tremors in high-rise buildings that produce shaking with a natural period of 1-2 seconds to 7-8 seconds, and are more likely to occur after a certain large earthquake.
Many people may answer that the reason for the great difference in damage caused by buildings is that Japan used to be different from today in terms of earthquake resistance, but in fact, the cycle caused by earthquake shaking is also closely related. In particular, tall buildings such as skyscrapers are susceptible to damage from long-period seismic motion because they tend to resonate with long-period seismic waves. Note that the “seismic intensity” described above does not take any account of long-period seismic motion, and thus can not accurately reflect the magnitude of shaking of tall buildings.
