The indicators used to express the size of an earthquake are magnitude and seismic intensity. Let us accurately distinguish what each represents.
Basic Knowledge
Magnitude (M): A logarithmic scale representing the amount of energy released by an earthquake (its size).
・When M increases by 1, the energy increases by approximately 32 times; when M increases by 2, it increases by approximately 1000 times.
・Formula: log₁₀E = 4.8 + 1.5M (E is measured in ergs)
・Giant earthquake: M8 (8〜9), supergiant earthquake: M9 or higher.
・An earthquake has one magnitude value.
Seismic intensity: An indicator of the strength of shaking at a particular location.
・The Japan Meteorological Agency seismic intensity scale has 0, 1, 2, 3, 4, 5-lower, 5-upper, 6-lower, 6-upper, 7, totaling 10 levels.
・For the same earthquake, seismic intensity is greater closer to the focus and smaller farther away.
・It also varies according to building structure and ground type (soft ground amplifies shaking).
The Earthquake Early Warning system uses the difference in arrival times between P waves and S waves. It detects P waves near the focus and issues a warning before the S waves arrive. Electrical signals travel at the speed of light (approximately 300,000 km/s), so they are transmitted overwhelmingly faster than S waves (approximately 3〜4 km/s).
Magnitude（A logarithmic scale showing the size of an earthquake. An increase of M1 means approximately 32 times more energy）
Seismic intensity（The strength of shaking at each location. 10 levels）
Maximum seismic intensity（The value recorded at the location with the strongest shaking in an earthquake）
Earthquake Early Warning（A system that detects P waves first and issues a warning before S waves arrive）
Liquefaction（A phenomenon in which sandy ground becomes fluid-like because of earthquake vibrations）
In Depth
There are several types of magnitude: local magnitude (ML), body-wave magnitude (mb), surface-wave magnitude (Ms), and the most reliable moment magnitude (Mw). Mw is an excellent indicator because it does not saturate even for supergiant earthquakes of M9 or higher, and it is now the international standard. The 2011 Tohoku earthquake and tsunami had Mw9.0.
In the past, seismic intensity in Japan was determined from people's sensations and surrounding conditions, but since 1996 it has been automatically measured using instrumental seismic intensity meters. Instrumental seismic intensity is objective because it is calculated from time-history data of acceleration.
Liquefaction caused by earthquake motion is likely to occur in reclaimed land with sandy soil and a high groundwater level. During the 2011 Great East Japan Earthquake, extensive liquefaction occurred in reclaimed areas of Chiba Prefecture.
- Magnitude=the size of an earthquake (one value)
- An increase of M1 means approximately 32 times more energy; an increase of M2 means approximately 1000 times more
- Seismic intensity=the strength of shaking at each location (10 levels)
- For the same earthquake, seismic intensity is greater closer to the focus
- Earthquake Early Warning=application of detecting P waves first
- Liquefaction=likely in sandy ground with a high groundwater level
Points to Note
① Magnitude (size) and seismic intensity (strength of shaking) are different things. ② Seismic intensities 5 and 6 each have two levels, “lower” and “upper” (5-lower・5-upper・6-lower・6-upper). ③ Beware of the misconception that “an M7 earthquake had seismic intensity 7.” Seismic intensity varies with distance and ground conditions.
Practice
- How many times greater is the energy when magnitude increases by 2?
- How many levels are there on the Japan Meteorological Agency seismic intensity scale from 0 to 7?
- Using the difference in the speeds of P waves and S waves, explain how Earthquake Early Warning works.