When is an earthquake not an earthquake?

[Cuddles]

When it's an aftershock.

Seriously though, when does an earthquake count as an aftershock and when is it counted as a separate event? I ask because of this story on the BBC. Apparently a 6.0 earthquake 3 months after a previous earthquake still counts as an aftershock. I'm no earthquakeologist, but I always had the impression that aftershocks generally happened hours, maybe a few days, after the original, and that anything so long afterwards would be a separate event. Sure, the big one in May will have had some influence on the recent one, but the same could be said for every earthquake that has ever happened in the area.

So, does anyone know if there is a standard way of determining what is and isn't an aftershock? Or is it just a case of thinking that since they're in the same place, one was probably caused by the other?

 

[ernon]

When it's an aftershock.

Seriously though, when does an earthquake count as an aftershock and when is it counted as a separate event? I ask because of this story on the BBC. Apparently a 6.0 earthquake 3 months after a previous earthquake still counts as an aftershock. I'm no earthquakeologist, but I always had the impression that aftershocks generally happened hours, maybe a few days, after the original, and that anything so long afterwards would be a separate event. Sure, the big one in May will have had some influence on the recent one, but the same could be said for every earthquake that has ever happened in the area.

So, does anyone know if there is a standard way of determining what is and isn't an aftershock? Or is it just a case of thinking that since they're in the same place, one was probably caused by the other?

The term aftershock is misleading. They are additional earthquakes of smaller magnitude than the original shock, that occur at the same epicenter. If the same section of fault slips a month later but is smaller in intensity it's an aftershock, if it's bigger, it's an earthquake.

 

[bokonon]

What Ernon said.

In addition, if the latter earthquake is bigger, the earlier earthquake becomes a foreshock.

 

[lenny]

The term aftershock is misleading. They are additional earthquakes of smaller magnitude than the original shock, that occur at the same epicenter.

regardless of the time delay? really?

how does the insurance industry define "an" earthquake then? presumably it groups damage caused by some aftershocks as the same event, but those months later, after repairs may have been made, as different events...

how delayed can an "aftershock" be before you have to pay a second deductible?

 

[CapelDodger]

how delayed can an "aftershock" be before you have to pay a second deductible?

I guess that's linked to how much re-construction has been completed. That in turn would be linked to getting the initial insurance pay-out, arranging the contracts, and having some actual work done. Three months is unlikely to cover that period.

 

[Cuddles]

The term aftershock is misleading. They are additional earthquakes of smaller magnitude than the original shock, that occur at the same epicenter. If the same section of fault slips a month later but is smaller in intensity it's an aftershock, if it's bigger, it's an earthquake.

But given how long the Earth has been around, that would make pretty much every earthquake an aftershock. Surely there must be some kind of limit on when you'd still call it an aftershock? Or is it just an arbitrary choice that doesn't really mean anything?

 

[shadron]

Yes, it's pretty much arbitrary. Even in places where there are not usually earthquakes, the earth can be restless. I managed the seismic facility at Regis College in Denver many years ago, and we used to record "earthquakes" when sonic booms hit the side of our building (yeah, that long ago) and shook the foundations. The San Andreas has a more or less continual series of earthquakes in the 1-3 range happening all the time. Aftershocks? Did we have one yesterday?. Preshocks? Dunno - ask me again tomorrow.

That is to say, they cannot be classified until after the event is well in the past.

 

[lenny]

But given how long the Earth has been around, that would make pretty much every earthquake an aftershock.

the key may be "the same epicenter"; presumably most "shocks" well separated in time have empirically distinct epicenters, and so not all earthquakes since the biggest one (yet) count as aftershocks (at the moment)...???