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.
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.
Media outlets often report about the inevitable next big one, but just how big will the next one be?
Scientists don’t know for sure, but GPS offers some clues. Check out this article by the American Association for the Advancement of Science which states:
“At the Cascadia subduction zone off the Pacific Northwest, for example, GPS stations on land suggest that enough strain has accumulated to drive a magnitude-9 earthquake when the fault finally ruptures. But the land-based measurements also hint that strain along the fault’s midsection, off the coast of Oregon, is being relieved by a type of harmless slip called creep. That suggests the fault could rupture in pieces, in a series of independent, smaller quakes. But without offshore measurements, scientists only see half the picture, says Harold Tobin, a geophysicist at the University of Washington in Seattle.”
The average number of years between CSZ megathrust earthquakes is 223. We have currently gone 322 years without one. So let’s move to a follow-up question. Do all long intervals (of say 322 years or more) between major CSZ earthquakes result in full-rips? Since pressure in the fault builds up over time, theoretically, the more time that passes without a rupture, the greater the stress on the fault. Page 119 of the USGS paper from which the above image comes says,
“Our data suggest that the minimum time between full-rupture great earthquakes is ~300 years”.
History shows us only 8 of the past 46 intervals between major CSZ earthquakes were longer than 322 years. On two occasions those longer intervals preceded partial-rip earthquakes (8.0-8.9). Here are the two (additional data available).
T10c (Section C): Preceded by a 344-year interval
T15a (Section D): Preceded by a 447-year interval
The other six intervals were followed by full-rips. So…
3/4th of the time, intervals longer than 322 years resulted in FULL-RIPS. Ouch! Regardless of whether it will be an M8 or M9, the effects will devastate the region. Take this opportunity to take the City of Salem’s 2-Week-Challenge so you’ll be ready when it comes.
(Repeat disclaimer: I don’t have a science-related college degree; however, the data analyzed on the pages of this website come from legitimate scientists and scientific organizations. My background is in accounting and analytics. As a member of the local Community Emergency Response Team, I have asked questions that led to the work laid out on these pages. If you see an error, please let me know! If you have information you think I should add, same goes.)