Earthquake detection

[stratik]

I'm curious if anyone can direct me towards information regarding earthquake detection. What I primarily wish to know is if an earthquake occurs on one side of the globe, does it register on the opposite side? And if so, how?

[fafalone]

The seismic waves travel through the Earth... so when theres an Earthquake on one side of the world, the other side feels it a little while later.

[Skye]

And the timing and strength of the waves at different points can give a bit of an idea of what's down there.

[YT2095]

It registers all over the globe

that`s how we can triangulate to find the epicenter.

amazingly enough, not just earthquakes can be detected! train crashes also, and few years ago when the Kursk submarine sank, it was possible even to detect the 2 lots of explosions that happened onboard also! pretty amazing stuff!

[stratik]

Thanks for all the responses. Can anyone site a specific example of an earthquake on one side of the globe that led to a rictar scale measured quke on the opposite side? The 8 point quake that happened recently in Japan for instance....was there a reactionary aftershock in Europe?

[YT2095]

well I don`t know of any such occurance myself of what you`ve described.

All Shockwaves as mentioned in the above posts have SOME effect all over the world, and the Richter scale will handle those too, although they will be in the regions of 0.0001 on the scale, so a direct answer would be a YES.

as for an 8 in Japan and something similar in Europe a few hours later, no, but that isn`t a surprising answer, when you consider the nature of our liquid core, and that the quakes are essentialy just "stress releivers" for rock. that is NOT to say that the quake in Japan wouldn`t/couldn`t have caused OTHER stresses in another place on the globe that may release itself in Europe EVENTUALY (could be a 100 years?).

but the safe money would say no, as the fault line that Japan sits over is in no way connected to earopean continents, it`s more likely to effect Hawai and other contries along the same fault line.

[stratik]

Very interesting. So what I'm getting is that quakes don't necessarily produce an exact opposing effect per se. They are simply registered across the globe. Now if seismic waves travel through the Earth, does that mean they register strongest or weakest on the opposite side? And if they register weakest, how do we know that waves have indeed passed through the planet, and not simply traveled around the circumference?

[VendingMenace]

the register weaker the farther away they are from the epicenter

 

how do we know that waves have indeed passed through the planet, and not simply traveled around the circumference?

 

An excellent question!

 

Well, mostly it has to do with timing. We can measure how fast the waves from an earthquake travel through the crust. And we know the distance around the earth. Given this, we can calculate how long it would take earthquake waves to travel around the earth to the point you are measureing them at. Thus, if the waves take a different amount of time, then they must have taken a different path.

 

As it turns out. The waves from an earthquake travel both through the earth as well as around its circumfrence. The waves that travel through the center have a shorter path to travel. IN addition, the material in the inside of the earth is much more dense and, as such, the waves travel faster through this medium. Hence, waves that travel through the earth arrive at the other side much more quickly than do those that travel around the perimiter.

 

THere is more though.

 

Earthquakes produce two different types of waves, p and s types. The p types are caused by compression of the earth. They pressure waves (hence, they are called p-waves ) much the same way that sound is pressure waves in the air. The other type of waves, s waves, are more akin to the waves that we observe when you pluck a gutiar string. That is these waves cause the earth to "vibrate" up and down, in a sigmodail (hence, "s" waves ) fasion.

 

Now, it turns out that p-waves are able to travel in solid and liquids, while s-waves are able to travel in only solids. Thus, as earthquake waves travel through the liquid matle of the earth, the s-waves dissapear, and only the p-waves continue and are measured on the other side of the earth. However, for the waves that travle around the earth, both the s and p types are observed. This is how we know which sets of waves travel through the center of the earth and wich do not. Those that travel around the earth have both s and p type waves, while those that travel through the earth only have p type waves.

 

Many things are learned about the interiour of the earth by measureing this waves. Many (if not all!) of the assertions made about the interior of the earth, find their basis in the measurments of earthquake (real or artificial) waves. It pretty cool stuff really.

 

Anyways, i hope that was a decently clear explination, however, feel free to ask more questions, yours have been most excellent so far!

[stratik]

Sir, what an excellent response. I'm sure I will have follow up questions, but I have much to chew on with your reply first