When an earthquake happens, doesn't every seismograms record it just about the same time depending on the time zone?
It is like if a gun goes off in a large empty building, wouldn't every person in the place hear it about the same time?
A small quake that happens locally wouldn’t show up on stations that were farther away. So if South Ca had a 4.0 or something, that’s not going to show on stations in Europe or Asia - it gets lost in the background noise.
A large quake (like the Sumatra quake) does show up on stations worldwide. But - not at the same time. It takes time for the wave to get from where it happens to far away places. Remember, the Earth is 8000 miles in diameter.
Also, as far as I understand it, there are two basic flavors of waves that come from quakes. One is like a pressure wave - the kind of wave you get like if you use a plunger in your toilet.
The other is an orthogonal (sideways) wave. The kind of wave you would get if you tied a rope to a doorknob, stood back and shook the rope up and down.
The difference being that the pressure type wave can travel through solid rock or liquid no problem. The sideways type of wave can travel through solid rock, but gets dampened and lost traveling through liquid.
And I think it was measuring these types of waves from distant quakes around the world that has led to the current understanding of the structure of the Earth.
Fragile crust, dense and liquid mantle, with a solid core.
No. There are no people in an empty building.
:-)
That's not the way it works. There is a measurable travel time involved, which is a function of distance, the type of rocks and path being traversed, type of wave, etc. (The greater the distance, the longer the delay -- just like the sound from lightning....)
Not only is travel time a function of distance, but, generally speaking, the time difference between the arrival of the "P" (compressive) and "S" (shear) waves increases with distance.
Knowledge of the arrival times of both the P & S waves -- and reference to a "travel time" diagram, table, or program allows seismologists to estimate the distance to an event with good accuracy.
So, no, seismic waves definitilely do not arrive everywhere at the same time. (Just as thunder from a single lightning strike is not heard everywhere at the same tme...)
I’m with you Bendy!