1. What Does a Negative Earthquake Magnitude Mean?
Negative magnitudes do exist. The USGS describes earthquakes with negative magnitudes as "very small earthquakes that people cannot feel" [1]. Magnitude 0 does not mean "nothing." It is more like a mark on a ruler. If you extend the ruler to the small end, numbers like -1 and -2 appear past 0 [1].
In Japan, the National Institute of Advanced Industrial Science and Technology states that tiny earthquakes of magnitude 3 or less happen quite frequently and routinely in many places [11]. The ground is always moving slightly in places people do not notice. So, who decided where 0 sits on this ruler?
2. How Did Charles Richter Create the Magnitude Scale?
The idea of magnitude was first proposed by Charles Richter of the California Institute of Technology. He is said to have explained the first magnitude scale in a paper published in 1935 [7]. Richter used a specific seismometer called the Wood-Anderson seismometer. He defined that if the shaking recorded at a point 100 km (about 62 miles) from the epicenter had an amplitude of 1 mm, the magnitude was 3 [8]. The type of seismometer and the distance are fixed together in this standard.
Based on this, every time the amplitude becomes one-tenth smaller, the magnitude drops by 1 [1]. If we apply Richter's conditions (Wood-Anderson seismometer, 100 km from epicenter) to calculate, the pattern is: 1 mm → 0.1 mm → 0.01 mm → 0.001 mm → 0.0001 mm → -1. When the shaking becomes one ten-thousandth of 1 mm, the magnitude is -1. Negative numbers are not a calculation error; they are the continuation of the ruler extended to measure tiny shakes.
3. How Much More Energy Does Each Magnitude Step Release?
When magnitude increases by 1, the earthquake's energy becomes about 32 times greater. If it increases by 2, it is 32 × 32, or about 1,000 times [3]. USGS teaching materials note that a magnitude 8.7 earthquake has about 23,000 times the energy of a magnitude 5.8 earthquake [3]. The difference in numbers is only 2.9.
We can apply this same relationship to negative numbers. Using the author's calculation, if you multiply by about 32 four times for each step up, a magnitude 3 earthquake has about 1,000,000 times the energy of a magnitude -1 earthquake. Even if the numbers look small, a difference of 1 in magnitude means about 32 times the energy, and a difference of 2.9 means over 20,000 times. Compare the energy as multiples, not by adding the magnitudes.
4. Is There a Upper Limit to Earthquake Magnitude?
The magnitude scale itself has no set upper or lower limit. The USGS describes it as an "open-ended scale" [9]. However, earthquakes happen when rocks underground break. Even if the scale has no limit, the size of the fault created by breaking rocks is limited, so there is thought to be a limit to how big earthquakes can be.
The largest earthquake recorded so far was in Chile on May 22, 1960, with a magnitude of 9.5 [2]. To cause a magnitude 10 earthquake, a fault wrapping around most of the Earth would be needed. According to the USGS, no such fault is known [2]. Why do the numbers for big earthquakes sometimes change after they are announced?
5. Why Are the Magnitudes of Big Earthquakes Updated Later?
The Japan Meteorological Agency (JMA) primarily uses the "JMA magnitude (Mj)," which can be calculated consistently for over 100 years, including for small earthquakes. This allows comparison with past earthquakes [6]. However, this method has a weakness. It cannot correctly determine the size of giant earthquakes exceeding magnitude 8 because the number plateaus at a certain size [6].
For giant earthquakes, the "moment magnitude (Mw)" is used. This is proportional to the area of the fault multiplied by the amount of slip, giving it a clear physical meaning [6]. For the 2011 off the Pacific coast of Tohoku Earthquake, the initial report was Mj 7.9. It was later updated to Mj 8.4, then Mw 8.8, and finally Mw 9.0 on March 13 [6]. These changes reflect re-expressing the earthquake's size using a scale better suited for giant quakes.
6. How Do Intensity and Magnitude Differ When Finding Small Earthquakes?
Intensity (shindo) and magnitude are different. Intensity shows the strength of shaking at a specific location, while magnitude shows the size (scale) of the earthquake itself [5]. Think of a light bulb: magnitude is the brightness of the bulb itself, while intensity is the brightness reaching the wall of the room. The same bulb looks dimmer the farther away you are. However, shaking also changes due to ground conditions.
There are two things you can try nearby. First, make a table in a notebook. Set 1 mm of shaking as 3, and lower the number by 1 for every one-tenth decrease. You can verify with your own hand how -1 appears past 0. Second, look at the JMA earthquake information page. Choose earthquakes near your area from past lists and check their magnitudes. You can see visually how many small earthquakes occur. Behind the numbers are the limits of rock breaking lengths and the effort to describe earthquake size with different measurement methods.