What is the Richter scale?

A bit of history

Charles Richter and Beno Gutenberg developed the original scale in 1935 at Caltech, specifically to compare earthquakes in southern California. It was the first time anyone had a single number to attach to an earthquake instead of vague descriptors like "minor" or "major".

The original scale only worked for nearby quakes and only with one specific type of seismograph. Over the decades, several extensions were built (surface-wave magnitude, body-wave magnitude, local magnitude), all called variants of "Richter" in the press but technically different scales.

Why it is logarithmic

Earthquakes range from imperceptible micro-tremors to events that flatten cities. A linear scale would be unworkable - imagine if the smallest detectable quake was 1 and the largest known was 100,000,000. So the scale is logarithmic, base 10:

• A magnitude 5.0 earthquake produces ground motion 10x larger than a 4.0.
• It releases about 32x more energy.
• A 6.0 is 1000x more energy than a 4.0. A 7.0 is 32,000x.

This is why a "magnitude 5" can be locally damaging while a "magnitude 7" can level cities hundreds of kilometres apart. The numbers feel close; the actual physics is wildly different.

What each level feels like

Magnitude	Effects
1.0 - 2.9	Not felt by people. Detected only by instruments.
3.0 - 3.9	Felt indoors, similar to a passing truck. Suspended objects may swing.
4.0 - 4.9	Clearly felt. Dishes rattle. Cars rock noticeably.
5.0 - 5.9	Light to moderate damage. Cracked walls, fallen objects.
6.0 - 6.9	Strong. Significant damage in populated areas, especially with older buildings.
7.0 - 7.9	Major. Serious damage over a wide area. Loss of life common.
8.0+	Great. Catastrophic damage, regional disasters, tsunamis.

These are rough, because depth and distance matter a lot. A magnitude 6.5 directly under a city is very different from a magnitude 6.5 deep under the ocean 400 km away.

Magnitude vs intensity

This is the bit people mix up. Magnitude is one number describing the earthquake itself - how much energy it released. Intensity describes shaking at a particular place, and varies across the affected area.

The Modified Mercalli Intensity (MMI) scale runs from I (not felt) to XII (total destruction) and is based on what people felt and what got damaged. A magnitude 7 quake can produce MMI X near the epicenter and MMI III a few hundred kilometres away. Both numbers describe the same event from different angles.

Why moment magnitude replaced Richter

The original Richter scale saturates above about magnitude 7. Bigger earthquakes do not move the needle as much as you would expect, because the scale was tuned for smaller events.

Modern seismology uses moment magnitude (Mw), which is based on the actual physics: how much rock slipped, over how big an area, with how much force. It gives consistent numbers across the whole range, including the truly enormous events. The 2011 Tohoku earthquake (the one that caused the Fukushima tsunami) was Mw 9.0-9.1. The original Richter scale would have understated it.

For all practical purposes, when a news outlet says "magnitude 6.5", they almost certainly mean moment magnitude, and the number is calibrated to be comparable to the old Richter numbers. The "Richter" label has stuck around as a popular synonym even though the underlying math has changed.