Not every CSZ megathrust quake is a magnitude 9+. Magnitude is basically a function of rupture length. The longer the ‘unzipping’, the larger the magnitude. Based on core samples, scientists have discovered 4 primary CSZ rupture patterns shown in the DOGAMI image below. *Mw = moment magnitude. Wichita State University provides the energy released scale shown here.

It’s worth noting that the above image was published before the number of known CSZ earthquakes jumped from 41 to 43. Those two extra earthquakes changed the recurrence intervals for segments A & B above. You can see the details about the change on Surviving Cascadia’s 17% in 50 Years page.

Events where the entire fault ruptures (as shown in figure A below), also known as full-rips, create ∼M9 earthquakes. As shown above, when a partial rupture occurs, the result is an earthquake in the ∼M8 or high M7 range. To learn more about how magnitude is a function of rupture length, check out this KVAL interview with Chris Goldfinger. Below is another figure showing the 4 primary rupture scenarios plus three others. The image is discussed in the video. The earthquake ‘names’ (white lettering) can be cross-referenced to their ages here.