SSE Activity

Slow-Slip Events (SSE), also known as Episodic Tremor and Slips (ETS) normally occur in the Pacific Northwest like clockwork. However, their pattern in our northern region has changed. Why do these events matter? Many in the science community believe these SSEs are key to better understanding the complex system of subduction zone tectonics.

Recent research has found:

  • “A recent major study from their seismologists correlated every large earthquake to a slow-slip event that was happening at the time — including the 2011 Tohoku earthquake.”1
  • “Researchers have shown that the magnitude 7.3 quake that struck Papanoa on 18 April 2014 was caused by a slow slip event that had begun two months earlier.”2
  • “Slow slip directly triggers seismic slip – we can see that”3
  • “Every slow slip event adds stress to the adjacent locked megathrust zone bringing it closer to failure.”4

Now take a look at the following two lines from 2019 research out of NASA/Caltech (If you would like a copy of this paper, I will gladly email it to you. Just let me know):

  • “Capturing the intrinsic pattern change would require an extensive observation period at least over several or more SSE cycles. However, if a sequence of SSE is observed with increasingly shortening of recurrence intervals, then it will be a clear indication that the fault is close to a megathrust earthquake, which seems to be the case of Boso SSEs before the 2011 Tohoku-Oki earthquake (Supplementary Figure S1, Hirose et al. 2012, Kato et al. 2014). Similar SSE shortening has also been observed before some other subduction earthquakes, for instance, the Guerrero SSEs before the 2014 Papanoa earthquake (Radiguet et al. 2016).”5
  • “We find that due to the interaction of the SSE region and the megathrust earthquake region, SSE become more frequent before megathrust earthquake, in line with observations.”5

New 2020 research out of Berkeley states:

  • “It is possible that over time the episodic tremor and slip events will migrate closer to the locked zone over time. If this were to occur, it may indicate that the next big earthquake is on the horizon. It is also possible that slow-slip events will become larger or more frequent when a large earthquake is imminent. It is therefore important to monitor episodic tremor and slip in Cascadia over time.”7

The 2019 & 2020 research is important, because we have recently had some unusual intervals in our northern region. Now, the region experiences tremor/slip activity throughout the year. Watch this video to see some of that activity.

Because there is frequent slip movement, knowing what is and isn’t an actual ETS event requires scientific analysis beyond my abilities. Therefore, the dates I am providing below come directly from the Pacific Northwest Seismic Network. I couldn’t get the Excel cells to copy correctly in this website, so page four lists the PNSN links from which I found the dates.

As you can see, the interval preceding the March 2019 ETS was the start of a strong change in pattern. I’m a visual learner, so I plugged these intervals into Excel to produce the following graphs. Each point on the graph represents the number of months between SSE events (intervals). They are chronological with the most recent interval on the right.

The red line represents the 20-year average interval length for the northern region. 6

In the next chart, we are looking at only the 5 most-recent intervals (13-17 from above). Rather than viewing them in months, this chart shows the time in days. For perspective, 350-500 days is the average range for normal intervals. As you can see, the 4 most recent intervals have fallen way below pattern, with the most recent lasting only 60 days.

To expand on this a little more, we said above that 350-500 is the normal average number of days. The past 4 intervals have come at 239, 139, 75 and now 60 days. To put it in perspective, those intervals fall at 68%, 40%, 21% and now only 17% of the LOW-END (350) of the ‘normal average’, respectively.

After seeing the recent decrease in intervals, I corresponded via email with both Dr. Liu & Luo concerning their 2019 paper. Here are my takeaways from those correspondences:

  • They confirmed that the change in fault behavior was observed during 2019 in the Cascadia Subduction Zone by him and his team.
  • They confirmed the Cascadia Subduction Zone could experience a shorter ‘transition period’ before a major event, compared to other subduction zones due to the frequency of SSEs in our region.
  • Dr. Liu stated, “If intrinsic change dominates, we would expect the trend of continuing shortening of slip interval.” -Intrinsic change would indicate the fault is close to a megathrust earthquake, according to his research.

Okay, so how does this recent activity compare to other intervals which broke pattern in the north? Check out this reaction to a 10.5-month interval in 2011.

https://assets.pnsn.org/oldets/summer2011/

If a 10.5-month interval garnered that level of attention, it is easy to see how intervals of only 7.5, 4.5, 2.5 and now 2-month durations… get attention!

In other words, they have fallen well off the beaten path.

Seeing this behavior and comparing it to the NASA/Caltech findings is pretty shocking. With the help of Greg Walsh, City of Salem’s Emergency Manager, we reached out to the following institutions:

  • NASA’s Jet Propultion Lab
  • California Institute of Technology
  • Pacific Northwest Seismic Network at the University of Washington
  • Southern California Earthquake Center
  • University of Oregon
  • Oregon State University
  • Stanford University

Our outreach held two objectives:

  • We requested any global data which is comparable to the past year’s SSE behavior in our northern region. In particular, we were interested in those patters which did not result in a megathrust earthquake. (since patterns like ours DID precede the Japan 2011 9.1 and the Mexico 2014 7.8).
  • Assessments from those institutions.

As for the request for data:

  • So far, no institution has been able to provide data showing slip behavior like ours occurring without preceding a megathrust earthquake. That doesn’t mean it is not possible! Our equipment/timeline just haven’t witnessed it.

Here is the breakdown of the assessments we received:

  • The Pacific Northwest Seismic Network (PNSN) is one of the leading organizations monitoring this activity. They agree the activity is unusual and interesting. They also caution that we do not panic, because the research correlating slips to megathrust earthquakes is still new.
  • Researchers only learned of slow-slips about 25 years ago, tracking them since the 90’s. In geologic terms, that is very short. Many scientists from the other institutions agreed with the PNSN. Because our data has been collected over such a short time frame, we just don’t know enough to know if the change we are seeing is cause for worry. Another argument against worry is that modeling is more simplistic than that of real life. We are grateful for the thoughtful responses we received.

It is entirely possible the system will reset and that these strange behaviors are not indicators of the CSZ entering into the final phase of the Super-Cycle. There is more to be learned!

The last SSE ended on 2/8/2020. The interval preceding it was 60 days. If we look out 60 days from 2/8/2020 that gets us to 4/8/2020. If the pattern we have been seeing since March of 2019 continues with increasing shortening of intervals, we’d see another SSE prior to that date. However, it is also possible that instead of an SSE we could see a megathrust earthquake instead. Basically, if we are nearing out end of the Super-Cycle, we may or may not get more SSEs before the rupture.

The fact is, our situation comes down to whether on not the CSZ behaves as the NASA/Caltech research findings suggest. If it does, our intervals are already incredibly close together & our risk of an earthquake occurring in month or so, is high.

However, if the CSZ goes until April 8th without a sip or rupture, than the world learns it IS possible to get this kind of slip behavior without preceding a major event. Only time will tell.

In conclusion, science supports the possibility that we could be entering the final phase of our Super-Cycle. The history certainly shows we are due for a major event. Looking at both the interval history of major earthquakes and this chaotic behavior of SSE intervals, it is not irrational to at least consider the possibility.

Perhaps it is a clue. Think of how lucky that would make us. Not the ‘being close to destruction’ part, obviously, but the ‘clue’ part. Research just happens to come out in early 2019 saying a certain type of behavior has, in two locations around the globe and in computer models, been shown to precede major events. Two months later, we start a year that resembles the behavior observed in that research.

Earthquakes give no warning. They are all but ghosts, yet perhaps this activity has our ghost showing a little skin ;). Use the information as just one more piece of inspiration. Get prepared.

Monitoring (updated daily at 6:00pm) of these slips can be viewed at: https://pnsn.org/tremor

Citations are listed on page 3