Distinguishing earthquake foreshocks and aftershocks

Taking a break from the political chicanery and going back one of the topics which drew me here many years ago, I ran across this very interesting article at PhysOrg:

Up to now, there was no way of predicting whether a powerful earthquake was likely to be followed by one of even greater magnitude. But the results of a study recently published in Nature by Laura Gulia and Stefan Wiemer from the Swiss Seismological Service (SED) at ETH Zurich awaken hopes that we will soon be able to do just that, in real time.

Any time there is a sizable earthquake, there are usually posts along the lines of, "I hope this isn't a precursor to a larger quake." Until this new study, we really had no way of knowing whether or not a quake was a fore shock unless a certain period of time had elapsed and an aftershock sequence establishes itself.

What has been found is that a parameter called the, "b-value," can give an indication as to whether or not any particular quake is a fore shock or a main shock in a sequence:

In seismically active regions, the b-value is usually close to one, meaning that there are about 10 times as many magnitude three earthquakes than quakes with a magnitude of four or higher.

The way it works is that after an earthquake this b-value goes to a particular value and that value can show whether or not an earthquake will be followed by a stronger quake or not. The scientists who wrote the study have devised a, "traffic light," type system to show whether or not a stroger quake might follow:

When the b-value drops by 10 percent or more, the traffic light turns red, suggesting acute danger of an even more powerful quake. In most cases, though, the b-value rises by 10 percent or more and the traffic light turns green, giving the all-clear by predicting a typical sequence that will gradually fade away. This happened in 80 percent of the sequences captured in datasets examined by the researchers. The traffic light shows amber when the b-value rises or falls by less than 10 percent, meaning it is unclear what will happen next.

Granted, this research is still very much in the earliest stages of development, and there are still many hurdles to overcome, but if additional investigations bear his out, we may see warning systems based on this in seismically active areas.

One question that may occur to many is just how accurate is thos system, surprisingly (or not), the accuracy is much more accurate than nearly any weather forecast:

The traffic-light system devised by the researchers turned out to be accurate in 95 percent of the cases they examined.

As with any system, there are things which would need to be implemented in order for this to be rolled out:

The system's successful deployment would also require a dense seismic network and corresponding data processing capacity. By no means all regions that could benefit from such a traffic-light system currently have such resources.

As with any new system, it may take some time for widespread adoption, but imagine what this could give areas in which earthquakes are the predominant natural hazard.

Source

Please, if you will, discuss.

Hi, jANDc!

Really interesting!
I was wondering if it rang true for the Ridgecrest, Calif. quakes?
Found this!

"At 8:19 p.m. PDT on July 5, a larger, 7.1 Mw  earthquake occurred in the Ridgecrest area, revealing the 6.4 Mw  earthquake to have been a foreshock. Both earthquakes have been described by the United States Geological Survey as occurring via shallow strike-slip mechanisms. Subsequent seismic activity occurred along two intersecting faults in the Little Lake Fault Zone.

Aftershocks
A significant series of additional earthquakes followed the foreshock, with the majority of magnitudes ranging approximately 2–4 Mw . By the evening of July 5, more than 1,400 earthquakes occurred."

SOURCE: Bing

Now that raises more questions!
What is the magnitude of quakes that that applies to?
What time frame is usual between foreshock & main quake?
How many small quakes usually follow the foreshock?
In Ridgecrest it was more than 1,400 in one day!

And 95% accuracy! In hindsight anyway. Awesome!
WOQ

a reply to: wasobservingquietly

Hey there WOQ,

Those are all really good questions! Not sure if the paper itself answered any of them and even though it is linked in the PhysOrg article, it is behind a paywall so we are unable to read the full paper.

I made this thread from my phone, but when I get a chance I will see if I can find a free version the next time I'm on a PC.

Thanks for stopping by!