I wasn't involved in this study, but I wrote the study that estimated the magnitude of this earthquake[0]. In case anyone is interested, usually the magnitudes of 'paleoearthquakes' (historic/prehistoric earthquakes discovered by finding evidence of old ground deformation) are estimated by relating the measured offset of the earth's surface or a rock/dirt layer across the fault line to the earthquake magnitude through empirical 'scaling relationships'; larger offsets are of course indicative of larger earthquakes. These are simply functions relating a measurable attribute of the earthquake to its magnitude. In the study I did, we combined the measurements of the offsets of a number of paleoearthquakes with estimates of the map length of the fault lines involved and used length-magnitude scaling relations to further refine the final magnitudes. There are some corrections for sampling bias that are included in there and it's all nice and Bayesian if anyone wants to nerd out on the stats.

When we did the study, it was speculated that two of the paleoearthquakes, one on the Seattle Fault and one on another fault on the Olympic Peninsula, could have actually occurred in a single event, but there wasn't much evidence to support this; we consider the magnitude of it on a paragraph at the top of page 1149 but not in the rest of the paper. The recent study (TFA) makes it highly likely that they were part of the same earthquake, but they could be separate earthquakes spaced a few minutes to a few months in time (think of the 7.8 and 7.7 earthquakes in Turkiye this spring, separated by a few hours).

A bit of context about the earthquakes in the Seattle region as well as Cascadia and other areas:

- The earthquakes in the Puget Lowlands and vicinity are relatively infrequent; there are about 15 known earthquakes over the past 17,000 years, and many of them are relatively small (M 6-7). However, they are spatiotemporally clustered[1]: There was a big cluster about 900 AD, and things have been mostly quiescent since then. It can be also shown from the geologic data that at the measurement sites ('paleoseismic trenches'), there haven't been any earthquakes since 17,000 years ago (when the Puget ice sheet retreated) on many of the faults, although the Seattle fault has had a number of earthquakes before.

- The big Cascadia subduction zone events are more frequent (perhaps every 500 years?) and larger, but they may not all be M 9 events, unlike what has been discussed in the famous New Yorker article. That article is based largely on the research of Chris Goldfinger, a scientist at Oregon State University, whose views are credible but on the high side of credible, in the eyes of many other scientists in the region. Many of the earthquakes suggested by the geologic data could be smaller earthquakes (M 7.5-8.5) which won't cause as much ground shaking over such a wide region.

- Earthquakes cause seismic waves at the fault surface, and these attenuate as they travel through the earth towards the surface. The initial magnitude of the waves as the earthquake occurs can be different for subduction zone earthquakes than for shallow earthquakes in the crust, and the attenuation is different for these as well. But importantly, not only are subduction zone earthquakes far off shore, but much of the seismic energy is released deeper in the earth as well, which means more attenuation of ground shaking by the time the waves make it to Seattle.

- A Cascadia earthquake will cause widespread but perhaps moderate damage across the PNW with perhaps, but a strong Seattle fault earthquake will absolutely destroy central Seattle, particularly Pioneer Square and Sodo. The fault comes ashore at Alki Point, for reference. However areas farther away (Edmonds, Tacoma, etc.) will not see nearly as much damage.

- SF and LA both have higher seismic hazard than Seattle[2], considering all earthquake sources, the frequency and magnitudes of earthquakes from the sources, and the seismic ground motions emanating from all of these earthquakes to a site within any of the cities, according to the most recent USGS national seismic hazard model. (See Figure 12 for hazard curves for major US cities).

[0]: https://rocksandwater.net/pdfs/styron_sherrod_bssa_puget_eq_...

[1]: https://pubs.geoscienceworld.org/gsa/geosphere/article/10/4/...

[2]: https://journals.sagepub.com/doi/10.1177/8755293019878199