So how does this swarm compare to others? The figure above clearly shows many more events, but simply counting earthquakes doesn’t tell the whole story. What seismologists usually care about is total energy release - OK fine we actually care about moment release, but energy is good enough here (check out this USGS seismo-lingo cheat sheet
). Remember that 32x difference in energy release for every magnitude unit? Well if you add up all the energy released from these 88 events, it would be about as much as from that one M6.3 in 2019, which saw no follow up increase in seismicity on the Cascadia Subduction Zone (CSZ). Even a M7.0, which may have a rupture length of about 20 miles, on this western segment of the BFZ would be unlikely to significantly load the CSZ, though that’s based on observations of earthquakes that didn’t
happen following other earthquakes far away.
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:
1) Changes in static (background) stress following earthquakes don’t reach very far, maybe only a few rupture lengths; remember the largest rupture in this swarm was likely, at most, only a few miles long. There can also be dynamic changes in stress as the shaking from seismic waves pass a loaded fault ready to go, but this usually only follows truly massive M8+ earthquakes and the triggered earthquakes are usually small.
2) This region is characterized by young crust freshly born (geologically speaking) at the nearby Juan de Fuca Ridge. Young crust will “absorb” changes in stress more than brittle old crust - think of applying stress to a gooey cookie fresh out of the oven that easily breaks into many small pieces rather than a week-old brittle cookie that breaks in half. So basically, the CSZ sitting 200 km away across relatively gooey crust, would unlikely notice the equivalent of a M6.3ish earthquake (all 88 swarm events combined).
By the way, 1) is much more important than 2) with the distances involved. It’s as if a baby was screeching right next to you at the dinner table, in what seismologists call the “near field”. Your ears would hurt and there would be significant stress changes (ha!). But if you’re a few houses away, in the “far field” like the CSZ is to these events, the only way you’d know something happened was if seismo-dad gave you a link
to a recording of the screeching which would be far less stressful and maybe even enjoyable.
Got tsunami? Tsunamis are generated when a not-small chunk of the seafloor moves up or down in an earthquake or underwater landslide. The BFZ is a transform fault boundary, like the San Andreas Fault, where the relative motion across the fault is horizontal. So no vertical movement, and no tsunami.
In summary, there was a pretty interesting swarm of 88 earthquakes way out offshore, but the hazard along the Cascadia Subduction Zone is not measurably higher today than it was at the beginning of the week. The Pacific Northwest is still earthquake country so stay prepared - check out this great earthquake kit
. And if you feel shaking or get a ShakeAlert Message: Drop, Cover, and Hold On.