10/03/2021 Thoughts

It appears as though a slow-slip began in the southern region on 09/17/2021. The amount of tremor, duration (2+ weeks) and migration activity all align with specifications for PNSN-verified events in the north.

This current event comes just 5 months after the last. In a 2007 paper, researchers Michael R. Brudzinski and Richard M. Allen wrote: “There are three broad (300–500 km), coherent zones with different recurrence intervals (north 14 ± 2, central 19 ± 4, south 10 ± 2 months)”. Looking at the southern region, the average recurrence interval up until 2017 was right at 10 months. Since then, the average recurrence interval has dropped nearly in half to 4.57 months.

Funny enough, the northern region began acting up on 09/26/2021. Migration appears to be moving north, and the daily epicenter counts look ETS-ish, but the tremor has only been going for a week. It may die out or continue on to become an official ETS. (I’m curious if the PNSN will blog about it soon.) If this does develop into an ETS (slow-slip), then the north is behaving much like the south (and we have two simultaneous ETS events happening…again). The average recurrence interval in the north, up until 2017, was 13 months (in line with the 2007 paper). Since then, the average recurrence interval has dropped—again nearly in half—to 5.75 months.

So, what does this mean? The answer to that question is well out of my purview! I will say, these slow-slip events have only been recorded for the past 20 or so years. It’s been nearly 322 years since the last major CSZ earthquake occurred on 01/26/1700. Science is a long way from understanding how the slow-slip intervals playout through the much longer megathrust earthquake intervals.

It is thought that slow-slips transfer stress to the locked zone, bringing it closer to failure7, so some worry more frequent slips could bring that failure sooner. Who knows. Chances are the intensity of an event is going to impact stress levels too. With that line of thought, here is a look at the number of slow-slip epicenters per year.

There is one problem with this type of analysis though. The equipment which records these events on the PNSN website was recently updated twice. The organization writes, “The most significant result of these changes is a lower detection threshold – that is, we can detect more tremor than before. This change creates a break in the catalog (on 18-Feb-2018), such that comparing activity levels before and after the change must take this into account.”

So how much of the increase in epicenters, particularly in 2018 & 2019 is actual change vs equipment sensitivity? Who knows. The slow-slips do appear to be happening about twice as often over the past four years. Does that mean anything significant? Again, who knows. Time will tell.

I’ll leave you with just one final thought—if slow-slips do bring our fault closer to failure, each slow-slip has the potential to be the straw that breaks the camel’s back. An Oregon State University paper writes, “Slow slip directly triggers seismic slip – we can see that”6 Take this as an opportunity to review your reunification plan or take inventory of your supplies. Do one thing today that will help you be more prepared.

*Note: I’m a CERT member located in Salem, Oregon. My background is in accounting and analytics, which means I’m not an expert on this subject matter (seismologist, geologist, etc.), and unfortunately, there isn’t a publicly available list of southern or central slow slips from the experts to verify or view. Dr. Noel Bartlow with the University of Kansas is currently working on a list for publication. When it is available, I’ll review my data for accuracy & will absolutely post her list for reference. Until then, comparisons with and analyses of verified activity in the north is where I start: https://pnsn.org/tremor/overview