How We'll Measure the Big One

Earthquakes are difficult to measure quantitatively, due to the various forms of wave motion in play, and the differences in the way that this energy moves through the earth and is transferred to human structures.

Most generally, quakes are measured using the moment magnitude scale (MMS), which replaced the old Richter scale in most cases. Just like the Richter scale, the MMS is logarithmic and written with a single digit and one decimal place, so in media reports they are often conflated. The scales are similar, and the difference between them is quite technical, involving how to include seismic waves with very long periods: that is, longer, slower, bigger waves.

The Richter scale was developed for a particular type of seismograph, and used to measure Californian earthquakes that are typically strike-slip: when two edges of a fault rub against each other. In larger earthquakes, like a megathrust quake in which one tectonic plate slips underneath another along a much longer distance, very long period waves add destructive energy. The MMS takes these into account, and so the biggest quakes—like the biggest in known history, the 1960 Valdivia quake in Chile calculated at 9.5 Mw; and the recent 2011 Tōhoku earthquake off the coast of Japan measured at 9.0 Mw—are better described using that scale. A CSZ quake would likely be a megathrust quake along a long subduction zone, just like Valdivia and Tōhoku.

Magnitude measures the strength of the quake at its epicenter, or the point where the rupture begins. But as the CSZ Quake could potentially be a megathrust quake occurring along an entire fault line from Northern California to British Columbia, while it technically would have an epicenter, the shaking would be felt in a much wider area than a smaller quake. Destruction would radiate from a long line, rather than a single point.

One way of measuring the destructive force of a quake is its magnitude at its origin point, another is via its intensity at different locations, as the energy radiates outward. This results in a map of intensity zones, decreasing in intensity the greater the distance from the quake. In the United States, the Modified Mercalli Intensity scale is used for this kind of measure, defining twelve different levels using Roman numerals, from I to XII. Each level is described subjectively in terms of the damage a person would witness or the shaking they would feel, rather than a more technical measure of energy released.

In a CSZ quake, areas on the Pacific coast itself would feel an Modified Mercalli intensity of VIII to IX. IX is subjectively described as "Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations."

The intensity would be felt far less in the Portland area, which is 50-60 miles inland: most would feel V to VI. VI is described as "Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight."

In Eastern Oregon, several hundred miles inland and on the other side of the Cascades mountain range, the intensity would be felt around the range of IV: "Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing cars rocked noticeably."

Terraform editor Brian Merchant spoke to author Adam Rothstein about this series for Radio Motherboard, which is available on iTunes and all podcast apps.