doucheluftwaffle t1_j7ozgpr wrote on February 8, 2023 at 11:20 AM

OP- the small little quakes that we have in the PNW don’t occur on/in the cascadia subduction zone.

The little earthquakes that we have are a result of us being pushed north by the San Andreas Fault. Assuming you’re in WA state, the further north you go towards Bellingham, the geology up there is mostly granite. So when we’re pushed northward, there’s no where for us to go except into the granite and lala you get the occasional low magnitude earthquake.

On land are major faults are strike slip and thrust faults and not subduction. Those faults aren’t going to help relieve Cascadia nor are they foreshocks to “The Big One.” The quakes on these faults are from normal movement and occasionally they get stuck.

As for being overdue- it’s nearly impossible to predict when Cascadia will rupture. However, geologists can study the sediment layers on the coast along with the ghost forests. Look up WA coast ghost forest; it’s really fascinating. They can also look at Native American Legends along with the written records in Japan and deduce that every X amount of years the Cascadia Subduction Zone ruptures with some regularity.

Typically scientists cant say with certainty whether or not an earthquake is a foreshock. Its only after a big one can they say that the previous one was likely a foreshock. For example; in 2002 Sumatra had 7.3 quake and then in 2004 they had a 9.1. It was only after the 9.1 did they say that the 2002 7.3 was a foreshock; separated by 2 years.

If you look at the Tohoku Japan quake (Fukushima) on they had 2 foreshocks; a 7.3 on 3/9/2011 and a 6.4 on 3/10/2011. Then on 3/11/2011 they had a 9.1.

CrustalTrudger t1_j7p4jnd wrote on February 8, 2023 at 12:19 PM

> and deduce that every X amount of years the Cascadia Subduction Zone ruptures with some regularity.

I guess this depends on your definition of "regular." If you look a the intervals between events reconstructed from the turbidite record (Table 12 on page 115 of Goldfinger et al., 2012), you'll see that these aren't exactly evenly spaced. E.g., the spacing in years between events is 232, 316, 446, 311, 982, 492, 415, 665, 661, 1189, 508, 715, 443, 548, 733, 195, 117, 577. From this you can calculate an average and it tells you that generally you'd expect an event every few hundred years, but after a given event, there's not necessarily anything to indicate whether the next one is going to be in ~100 years or ~1000 years. I would not describe that as having a particularly "regular" pattern of strain release.