Post mortem - Oregon Earthquakes

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Good initial write-up on the quake swarm a couple days ago.  From the Pacific Northwest Seismic Network:

Blanco Fracture Zone swarm: Active, unusual, interesting... but not concerning
As of the writing of this blog (Dec 9, noon), 88 earthquakes have popped off on the Western extent of the Blanco Fracture Zone. Fifteen have been magnitude 5.0 and greater with the two largest events being M5.8. The smallest (reported) earthquake is M3.2 though there surely are many smaller ones, but the seismic stations used to detect and locate the events are 300 miles away on land. At those distances, the ground motions from smaller earthquakes will fall below background noise levels and thus go undetected. The magnitude scale is logarithmic, such that a M5.0 earthquake releases 32 times the amount of energy as a M4.0, so the vast majority of action has been properly detected, characterized and cataloged. Note that while the USGS lists all of the event depths as 10 km, their actual depths are best described as “shallow”. Constraining depths with data from stations far away and for shallow events is very difficult, so 10 km is just a default number given for shallow events with source-station geometries like we have here.

And some history:

How unusual is this? In the history of well-recorded M4.5 and greater earthquakes in this region, which is about five decades long, there have been many other swarms, including in the summer of 2019 when the largest event was a M6.3. However, there has not been as active a swarm as this one in terms of number of moderate sized events (see the figure below).

What defines a “swarm”? It’s a nebulous term that generally means a group of earthquakes close in space and time, depending on the background seismicity and how long a time window you are considering. Those can vary greatly, but in general we talk about swarms as being within a distance of a few rupture lengths of the largest event(s) and lasting a few days in time. What are the rupture lengths of these events? The two M5.8s probably had rupture lengths of a few miles long, which may have seen slip of maybe a foot or so. If these events were on land and you were nearby, 24 magnitude 4.5-5.8 earthquakes within a day would be a lot of shaking! And maybe some cracked streets and foundations and a few broken plates, but probably not collapsed buildings.

More at the site.  There is a lot more number-crunching that needs to be done but this is a great preliminary report.

My personal hope was that this was the Cascadia Subduction Zone letting off some steam.  Not to be:

The slip on these earthquakes are unlikely to have caused significant changes in stress along the Cascadia Subduction Zone 200 km closer to shore, which in the past has produced M9 earthquakes as recently as January 26, 1700. This is true for a few reasons:

So we are still primed and ready and overdue for a big one.  The last time it let loose was in 1700.  These quakes happen about every 300 years.  That would make the next one due in 2000.  Oh. Wait.

A very well researched and fascinating telling of the 1700 quake can be found here:

The orphan tsunami of 1700—Japanese clues to a parent earthquake in North America

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This page contains a single entry by DaveH published on December 11, 2021 6:30 PM.

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