Michael West

The Alaska Earthquake Center is revising the magnitude of the November 30, 2018 Anchorage earthquake from 7.0 to 7.1. In the three months since the earthquake, data have been reviewed carefully by numerous agency and academic groups. These evaluations are in fairly good agreement and all converge near 7.1.

There are multiple ways to calculate the magnitude of an earthquake. The modern standard for large earthquakes relies on the concept of seismic moment—a measure that accounts for the size of a rupture, how far it moved, and the friction. (Here is an excellent 5-minute explainer video from the IRIS Consortium). Seismic moment can be estimated by different combinations of seismic waves and analysis techniques. These approaches lead to slight variations in the magnitude estimate. There are now more estimates of the magnitude available than there were on or immediately after November 30th, and most of these tilt toward 7.1. If we wrote these estimates out to a few decimal places, the variations would be, say, 7.045, 7.093, 7.105, and so on. The seismology world has a long-standing convention to round magnitudes to the nearest tenth, however, because estimates of magnitude are neither consistent nor meaningful beyond about a tenth of a unit.

It is not uncommon to derive different magnitude estimates as more comprehensive analyses, or even new techniques, are applied. The 2002 Denali Fault earthquake is estimated by some scientists to be M7.8, and by others to be M7.9. At larger magnitudes, 8 and 9, the subtleties have an even greater impact. The 1964 earthquake was for many years considered to be magnitude 8.4-8.5. It wasn't until the seismic moment technique (described above) was developed that it was revised to 9.2, and even this continues to be debated in some circles.

The change in magnitude for the Anchorage earthquake has no impact on the events that actually unfolded on November 30th, nor on the numerous response activities. It does not impact our understanding of how or why the earthquake occurred. It is a minor adjustment that, as we move forward, helps put the earthquake in better perspective vis-a-vis other earthquakes in Alaska and globally.