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Originally posted by lasertaglover
reply to post by caitlinfae
Im glad to see aftershocks so quickly, id be really worried if the pressure stayed locked.
This area really has been in my thoughts for days now. Hope this just released some pressure....
Peace
bkaust
[snippied out the other quote to save space]
Although I just saw in the quakewatch thread the biggest was a 6.0 - and they just had a 5.9. Wouldn't that imply that that 6.0 is not really the main shock since there is a .1 difference between? Doesn't it need to be a whole 1 down for them to usually categorise it as the biggest in a sequence? It's a lot of steady 4's & 5's too making me a bit nervous!
JustMike
bkaust
[snippied out the other quote to save space]
Although I just saw in the quakewatch thread the biggest was a 6.0 - and they just had a 5.9. Wouldn't that imply that that 6.0 is not really the main shock since there is a .1 difference between? Doesn't it need to be a whole 1 down for them to usually categorise it as the biggest in a sequence? It's a lot of steady 4's & 5's too making me a bit nervous!
Hi BK,
first off, congrats on the pregnancy! Hope it all goes great!
Now to your query. I guess you're referring to main shocks versus aftershocks. Generally speaking, with larger events (like 7.0-plus MWs and especially when they're over 8.0), the rule of thumb is that aftershocks can be up to around one full mag less than the main shock. True, if there's a mag 5 you can often expect a few mag 4s to follow, but as they're relatively small events they're not a huge concern. It's the powerful aftershocks that follow big events that are the worry, as they can be quite damaging on their own.
To put it another way, if following shocks are one mag or less than the main event and are within a reasonable distance of the main shock's epicenter (based on rupture length of the main shock event), then they're normally classified as aftershocks.
However, with quakes like the Canadian ones in the past few days, that doesn't have to apply. We could say that the 5.9 was not actually an aftershock of the 6.0, but was still consequential to it. However, some might still call it an aftershock, even though it doesn't really fit the technical definition.
Now, let's say a mag 7-something (or bigger) happens within that same region in the near future. In that case, the mag 6.0 and the 5.9 were foreshocks, in the same way that the mag 7-range quake off Japan a few days before their big one in March 2011 was identified as a foreshock -- but only after the huge quake happened, of course. Until that terrible moment, the mag 7 event was thought to be a main shock and in fact, most scientists were surprised by the bigger one that followed.
In short, it's generally impossible to say if a quake is a foreshock or a main shock right after it occurs. It's what happens or doesn't happen after that allows us to identify them that way. The only exception is with very large quakes in the high mag 8-plus range (ie, including mag nines). If we get eg a mag 8.8, we had better hope it's a main shock and not a foreshock of something much bigger!
About all the 4s and 5s. These are pretty normal following such a shallow event in the mag 6 range. I'd actually be more worried if they didn't happen, because in that scenario, we'd have indications of a stuck fault that jarred loose, then stuck fast again. But as they're popping off like they are, it means things moved and now they're slowly settling again.