Earthquakes and magnitude

A magnitude 7 earthquake shakes the seismograph needle 10 times as much as a magnitude 6, but releases about 32 times as much energy.

Magnitude and intensity

An earthquake is sudden movement on a fault. BGS describes it as the release of stored elastic strain energy as seismic waves, which travel through the Earth and shake the ground. There are two quite different ways to say how big one was. Magnitude measures the size of the earthquake at its source. It is one number, and it is the same wherever you are. Intensity measures how strongly the ground shook at a particular place, and it changes from place to place, depending on the distance from the earthquake, its depth and the ground underfoot. USGS compares the 1994 Northridge earthquake and the 2001 Nisqually earthquake: similar magnitudes, but Northridge, only 5 to 18 km deep, shook much harder than Nisqually, about 48 to 58 km deep.

Ten times the wiggle, 32 times the energy

Magnitude scales are logarithmic. USGS explains that each whole number step represents a tenfold increase in the amplitude measured on a seismogram, the size of the wiggle. Energy grows faster: each whole number means about 32 times more energy released. The exact factor is 10 to the power 1.5, which is 31.6. Two steps multiply: 32 × 32 is about 1,000, so a magnitude 7 releases about 1,000 times the energy of a magnitude 5. Small steps matter too. An increase of 0.1 means about 1.4 times the energy, so the difference between a 7.0 and a 7.1 is not trivial. Magnitudes can even be negative, for tiny earthquakes that only sensitive instruments detect. To give a sense of scale, HVO compares a magnitude 1.0 earthquake to the explosion of about 32 kg of TNT, the size of a mid-sized construction blast. Such blast comparisons concern similar seismic-wave amplitudes, not equal total energy.

From Richter to moment magnitude

The first magnitude scale was developed by Charles Richter for southern California. It used the largest wiggle on a seismogram. That works for moderate earthquakes, but for very large ones some amplitude-based scales underestimate the true size and some overestimate it. So USGS now reports large earthquakes using the moment magnitude scale, Mw. Moment magnitude is based on the physics of the source: how big an area of fault slipped, how far it slipped and how stiff the rock is. HVO notes that it is directly related to the size of the rupture zone and the total energy released. The scales are calibrated so that they agree on average where they overlap, which is why a news report of the Richter scale is usually close.

Where do earthquakes happen? Mostly at plate boundaries, as earlier units showed, and the largest at subduction zones. The two largest ever recorded, Chile in 1960 at magnitude 9.5 and Alaska in 1964 at 9.2, both happened where the Pacific seafloor sinks. Because energy rises so steeply with magnitude, the frequent small and moderate earthquakes around the world release far less energy than a single great one.