How Big Will it Be?

Question: How big will the next Cascadia Subduction Zone earthquake be? Short answer, science doesn’t know for sure, but GPS is giving us 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.”

In a KVAL video interview with Dr. Chris Goldfinger, he discusses the paleoseismic data from our fault, explaining how earthquake magnitude is a function of rupture length. The various segments of the CSZ fault from the video are shown below. I highly recommend watching the video!

As Dr. Goldfinger explains, sometimes the entire fault breaks (Segment A) generating a 9.0. Sometimes portions of the fault rupture creating smaller quakes.

So let’s move to a follow-up question. Do all long intervals between major CSZ earthquakes (of say 321 years or more) result in 9.0 earthquakes? 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.

Does history back up the theory? Well, maybe. History shows us only 8 of the past 46 intervals between major CSZ earthquakes were longer than 321 years. On two occasions those longer intervals preceded partial-rip earthquakes (8.0-8.9). 

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…

  • 1/4 of the time, intervals greater than 321 years produced partial ruptures
  • 3/4th of the time, they resulted in FULL-RIPS. That’s 75% of the time!

Does that mean the next earthquake will be a 9.0? No, but in addition to the GPS and paleoseismic data above, it’s interesting to look at the historical data showing a correlation between long intervals and large earthquakes. Things to ponder.

(Repeat disclaimer: I don’t have a science-related college degree; however, the data analyzed in 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 which 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.)