*Interchangable words: Slow-Slip Event (SSE) = Episodic Tremor & Slip (ETS) = Tremor*
I was wrong. Those are humbling words to write. They are also important words. Learning rarely occurs without missteps and I’d like to share a few of mine here.
In early 2020, I thought the PNW slow-slip intervals were shrinking. This thought led to concern, based on research out of NASA’s Jet Propulsion Lab, that the fault might be edging toward failure. Spoiler alert—they aren’t shrinking.
Sometimes, it’s wonderful to be wrong. The earthquake will come—someday, but the more time we have, the more the region can prepare.
In the space that follows, I will do my best to walk through the reasons I believed the intervals between slow-slips were shrinking, as well as the lessons I’ve learned that contradict that reasoning.
Part 1: March of 2019
Tremor activity in March of 2019 was not initially considered an ETS in the PNSN Tremor Log. Later discussions within the log and Seismo Blog stated:
“several stations have very obvious, fairly large recent displacements (THUN, CPXX, P420, and P421). Thus, at least the south end of the Puget Sound zone seems to be having a significant ETS while to the north it doesn’t seem to really be much, yet.”
The quote, as well as activity on the Tremor Map, led me to believe the tremor was a northern slow-slip, yet the dates were still not listed on the Tremor Log as a northern-region event. Look at the comparison images below to see how including March 2019 changes the purple bars in the chart.
- Purple bars = 2019 activity.
- March 2019 included in data = left image.
- March not included = right image.
Truth be told, I didn’t have much luck getting an answer as to whether March was or wasn’t a northern event until Dr. Zhen Liu with the Jet Propulsion Lab explained that the GPS data showed stronger displacement in the Central region than the northern during that timeframe. He kindly provided these two images.
Lesson 1 Conclusion: I had assumed, based on the tremor activity and the log/blog, that March was a northern event. GPS data doesn’t agree, so March has been removed from my interval list.
Part 2: Segmentation
In 2017 and 2018, the slow-slips did something a bit strange. Each event took a small two-week-long “breather” in the middle of completing its journey—like a pause. Below, each slide comparison shows a single ETS, with the first image occurring before the pause and the second image occurring after. Note how the tremor in the second image picks up close to where it left off in the first and then continues north.
Left slide comparison (02/18/17 – 03/05/17) vs (03/18/17 – 04/06/17).
Right slide comparison (05/09/18 – 05/23/18) vs (06/18/18 – 07/15/18)
After 2018, the ETS events got even stranger, taking “pauses” that were months in duration. I believed these were separate events. It turns out they aren’t. According to the Pacific Northwest Seismic Network, just like the 2017 and 2018 situations, the northern slow-slips in 2019 and 2020 simply took longer breaks. Below, look at how, despite those longer pauses, the tremor flows over different areas of the northern region during the two phases. Again, the second phase travels north of the first.
Left slide comparison (08/11/19 – 08/25/19) vs (11/07/19 – 11/27/19)
Right slide comparison (10/04/20 – 11/16/20) vs (01/26/21 – 02/11/21)
Lesson 2 Conclusion: Rather than the region experiencing a change in intervals, the slow-slip events have segmented so that the area a single ETS used to cover in one fell swoop now happens in stages. In other words, longer intervals do not equal separate events.
* Another northern ETS occurred from 09/22/21 – 10/11/21. Because of the pattern seen over the past few years, the PNSN cautiously expects the second half of the northern area to see tremor activity soon.
With this latest correction, the slider below compares tremor bursts as separate slow-slip events (left) versus combined events (right). With the 2.5-month intervals gone, the interval irregularity is much less jarring … and there’s still one more step (or misstep) to go over.
Part 3: Defining Intervals
According to Merriam-Webster Dictionary, an interval is defined as “a space of time between events or states.” Oxford Dictionaries defines it as “a pause or break in activity.” This is how the charts above were made—measuring the time from the end of one slow-slip to the start of the next.
I have recently learned the slow-slip intervals are instead measured from the start of one ETS to the start of the next. Here is a slider to show how that correction changes the chart, yet again.
To review, here are my three areas of missteps. (Part of this humbling experience is knowing there may be others I haven’t even uncovered yet… but I hope not.)
1) I had included March of 2019 in the data.
2) I had not combined the tremor segments that make up a single slow-slip event.
3) I had not measured intervals from the start of one slow-slip to the start of the next.
To put this into perspective (and further humble me, I suppose), below is a comparison of my original chart (left) and the 3-misstep-corrected version (right).
Up until quite recently, I hadn’t felt as though there was a thorough discussion on this topic flowing well between the experts and non-experts. That’s changed a bit and thankfully I’ve become aware of all of these missteps through those conversations. I hope to continue to learn all I can.
If you’d like to read thoughts about this topic, scroll down to the Oct 4th entry on this PNSN Tremor Log for explanations from field expert Steve Malone, PNSN Professor Emeritus. I am grateful to Steve and his colleagues who help facilitate the Facebook Pacific Northwest Earthquake Discussion Group. They have helped straighten me out on this and many other topics. If you’d like to join the group, I highly recommend it!
So, in the end, I was wrong. I hope that means there is more time before the earthquake will hit but who knows. While I’m probably more prepared because of the worry these missteps initially caused, I’ll take all the time I can get to get more prepared. I hope these explanations have made sense. As always, thoughts or comments are welcome through the contact page.
Cascadia Subduction Zone 