When is an aftershock an earthquake?

January 22, 2010 9:27 PM

Why is a 6.1 "aftershock" called just that- an aftershock, and not another 6.1 earthquake?
posted by xmutex to Science & Nature (10 answers total)

I believe (and I am not a seismologist) that it's because an aftershock is a smaller quake where the causation links directly back to the recent, and larger quake. If the subsequent quake is of higher magnitude than the previous quake, then it IS considered an earthquake.
posted by roomthreeseventeen at 9:35 PM on January 22, 2010

if the subsequent quake is of a higher magnitude, it is re-designated as the earthquake, where the previous "earthquake" is now classified as a foreshock.
posted by nadawi at 9:38 PM on January 22, 2010

Anything happening within the same time period, to my understanding, is seen as part of the same seismic event. As nadawi said, the terminology is based on the severity.
posted by mikeh at 9:50 PM on January 22, 2010

All aftershocks are earthquakes. Calling it an aftershock just means it is tied to a larger seismic event, occurring at roughly the same location and depth as the original quake.
posted by cecic at 9:59 PM on January 22, 2010 [2 favorites]

The Wikipedia article on aftershocks has some very good information, including about Omori's Law. It's Omori's Law that explains why, for instance, there are still occasional aftershocks from the 1812 New Madrid earthquake in Missouri.
posted by cerebus19 at 10:08 PM on January 22, 2010 [2 favorites]

It's important to realize that aftershocks are very common after major quakes as the rumbling subsides, like a piano string vibrating away into silence. It isn't like aftershocks are a surprising and notable event; there are at the least scores or even hundreds of measurable aftershocks. Technically -- and certainly to a seismologist -- every single aftershock is an earthquake; it's simply the fact that they were preceded by a larger one that lends them this label of convenience.

That said, by rule of thumb, a quake of 6ish intensity (depending on the scale used) is going to be about 1/10 as damaging as a quake of 7ish intensity. It's very serious, and would be if it were the major quake, but it just isn't going to be as important when following a hugely damaging or fatal quake.

In a cluster, the earthquake with the largest magnitude is called the main shock; anything before it is a foreshock and anything after it is an aftershock. A main shock will be redefined as a foreshock if a subsequent event has a larger magnitude....

An earthquake large enough to cause damage will probably be followed by several felt aftershocks within the first hour. The rate of aftershocks decreases quickly - the decrease is proportional to the inverse of time since the main shock. This means the second day has about 1/2 the number of aftershocks of the first day and the tenth has about 1/10 the number of the first day....

We call an earthquake an aftershock as long as the rate at which earthquakes occur in that region is greater than the rate before the main shock. -- USGS
posted by dhartung at 10:27 PM on January 22, 2010

That said, by rule of thumb, a quake of 6ish intensity (depending on the scale used) is going to be about 1/10 as damaging as a quake of 7ish intensity.

Jesus...talk about muddling your math...this is not a rule of thumb!! This is how logarithmic scales are exactly defined.

Okay, so "6ish" Richter is "about" 1/10 as intense as "7ish".

But 6.000 Richter is exactly 1.000/10.000 as intense 7.000.
posted by randomstriker at 10:52 PM on January 22, 2010 [1 favorite]

He did say "damaging", not "intense"— I doubt damage scales linearly with energy release.
posted by hattifattener at 11:32 PM on January 22, 2010 [1 favorite]

hattifattener: He did say "damaging", not "intense"— I doubt damage scales linearly with energy release.

Wikipedia says that if "intensity" means "amplitude", then randomstriker is right: 6 is 1/10 as intense as 7. But energy release (and damage) scales as the 3/2 power, so a 6 causes approximately 1/30 the damage of a 7.
posted by jweed at 11:57 PM on January 22, 2010 [5 favorites]

Just a tangential aside, but earthquake frequency also follows the same kind of power law (if I'm remembering my seismology right) - a 6.0 is ten times as common as a 7.0, too.
posted by wanderingmind at 8:08 AM on January 23, 2010