The 7/12/2019 Three Lakes Earthquake

At 2:51 AM on Friday, July 12, citizens across Western Washington and beyond were awoken by an earthquake centered approximately 1 mile northwest of Monroe. With a magnitude of only 4.6, this was only a moderate quake and no damage was reported, but I’m sure more than a few glasses were shattered across the Puget Sound region. According to the official Pacific Northwest Seismic Network (PNSN) event page for this earthquake (link), this quake was relatively deep at approximately 18 miles. Because of its depth and location, it couldn’t be definitively traced to any particular surface-mapped faults, though it was close to three larger fault zones: the Southern Whidbey Island Fault Zone (SWIFZ), Rattlesnake Mt. FZ, and an unnamed NE-trending fault zone.

In the interest of avoiding a single, dissertation-length blog post, I’ve divided this blog into two parts. This blog focuses on the 7/12/2019 Three Lakes earthquake. “Part 2” discusses the three different types of faults and the four different types of earthquakes found in the Pacific Northwest. If you don’t know too much about earthquakes, it may make sense to start with part 2 to get some background information. You can find part 2 here (link).

The July 12, 2019 Three Lakes Earthquake

I remember the Nisqually Earthquake of February 28, 2001, and if you lived in the Pacific Northwest at the time, I’m sure you do too. I was in second grade at McGilvra Elementary School in Seattle, and we had actually just completed a fire and subsequent earthquake drill. No more than 10 minutes after our earthquake drill, our teacher yelled, “Earthquake! For real!” and we all bolted under our desks for a second time, thrilled that we were able to do this twice in a time period shorter than our morning snack break. I still remember the slow, rolling rumbles that reverberated throughout the room, and how I felt like I was on the bow of my dad’s boat on the open ocean, heading straight-on into a heavy sea. Second grade was a blast in general, but those 30-60 seconds were definitely one of the highlights of the year.

Friday’s quake was closer to the surface and the Seattle metropolitan area than the Nisqually quake, but with a magnitude of 4.6, the 6.8 Nisqually earthquake was an incredible 2,000 times stronger! This is because the moment magnitude scale that we use to measure earthquakes is a logarithmic scale, meaning each whole number represents a ten-fold increase in wave amplitude on a seismograph and a 32-fold increase in energy released. As such, Friday’s quake was more of a wake-up call (literally, since it occurred in the middle of the night) than anything else, and a very healthy one at that, since the Pacific Northwest is woefully unprepared for the inevitable, far stronger earthquakes that will hit us in the future.

My parents live on Southern Whidbey Island, and they were two of many thousands who woke up Friday morning when the quake hit. They told me that the shaking was much more “rattlely” compared to the “rolly and wavy” Nisqually quake, and that they heard it more than they felt it. There were lots of “clinking” sounds – perhaps due to the silverware in the cupboard, or perhaps due to the fact that their house has a metal roof (or both!). My mom was scared and couldn’t go back to sleep for an hour, while my dad dozed off immediately after the shaking stopped. My mom estimates that the shaking lasted around 10 seconds, but given that the earthquake woke her up, it could have been 10.1 or so (she’s a very light sleeper!).

Most reports I saw on social media also mentioned the “rattlely” quality of this quake compared to the Nisqually earthquake. This was likely because the people who made these reports (most reports were from Seattle north) were much closer to the epicenter than they were during the Nisqually quake, and the Nisqually quake was twice as deep. Shorter wavelengths dissipate more quickly when they pass through a medium, so by the time the waves from the Nisqually quake got to these folks, the shorter wavelengths had attenuated and the “slow rollers” were more dominant. This phenomenon holds true for any wave that passes through some medium – it’s why you can hear loud bass from far away, or why the sky is red when the sun is low on the horizon (and sunlight has to pass through more atmosphere), as red light has a longer wavelength than blue light.

Here are a couple more graphics showing the intensity (using the Modified Mercalli Intensity scale) and the peak velocities of this earthquake. There are a bunch of other awesome graphics here. All credit goes to the Pacific Northwest Seismic Network.

Compare the intensity of the Three Lakes quake to that of the Nisqually quake, and you can see how much weaker this quake was.

As I mentioned above, it’s tough to trace this quake to any particular fault zone. Compare the location of the quake (and subsequent smaller aftershocks) to this awesome map of faults from Jessica Czajkowski and Jeffrey Bowman (full map is here) and you’ll see that there are no significant fault lines over the earthquake region.

Instead, the earthquakes occurred between the South Whidbey Fault Zone to the west, the Rattlesnake Mountain Fault Zone to the south, and an unnamed NE-trending fault zone to the east.

When the location of the earthquake doesn’t line up with any fault zone – we have another tool we can use – beachball diagrams! Beachball diagrams are used to find the focal point solution of an earthquake – in other words, the faulting process that caused it. The beachball diagram revealed that the Three Lakes quake occurred along a reverse fault, but unfortunately, most of our faults here are reverse faults, so that doesn’t provide us much guidance either. The exact fault that the Three Lakes quake occurred on may forever remain a mystery.

One thing we do know, however, is that the Three Lakes quake occurred on a crustal fault. Crustal faults are relatively shallow faults on the North American plate, and many of them are scattered throughout Western Washington. Even though the Cascadia Subduction Zone gets all the attention, a massive, shallow crustal fault earthquake in Western Washington could cause far worse damage locally. One of these crustal faults lies directly under Seattle, and it’s only a matter of time before it ruptures again.

Security cam videos

Of the many advances in technology we’ve seen since 2001, one is the advent of the home security camera! Cameras from around the sound recorded the earthquake from start to finish. Here are two of the better ones I found online.

And I thought it was a bear on my deck lol #earthquakeseattle https://t.co/PFOV3JJrvM

— Jessi Bloom (@jessibloom) July 12, 2019

Congrats on making it to the end of this blog, I hope you enjoyed the journey! For part deux focusing on the different types of faults and the Cascadia earthquake sources, click here.

References:

Pacific Northwest Seismic Network. (2019, July 12). Earthquake Report – Event 61535372. Retrieved July 22, 2019, from https://pnsn.org/event/61535372

Pacific Northwest Seismic Network. (n.d.). PNW Earthquake Sources Overview. Retrieved July 25, 2019, from https://pnsn.org/outreach/earthquakesources

ShakeMap. (n.d.). Retrieved July 15, 2019, from https://pnsn.org/shakemaps/61535372

Smith, G. (2017, December 24). Understanding the mystery of earthquake Beach Balls. Retrieved July 15, 2019, from https://earthquake-report.com/2014/05/17/understanding-the-mystery-of-earthquake-beach-balls/