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Okay, #SeismoTwitter, it's (almost) #SeismogramSaturday! I'll be offline for most of Saturday, so I'm posting a little early. Here's today's seismogram! 
Today, we have show shaking caused by the M9.0 Tohoku earthquake on March 11, 2011. This figure shows shaking at a station in North Dakota (caused by a quake in Japan!).
The three wiggles here show how the ground shook in an east-west manner (top panel), north-south (middle) and vertical (bottom).
Seismometers like this one are called "3-component" instruments, because they show all three directions of motion. More simple seismometers usually only show vertical shaking.
The fact that we can record shaking in 3 directions is really cool, as it helps us identify the type of wave and the direction from which it has come.
This seismogram shows the P and S waves that we discussed last week, but this time we see that they shook the ground in all three directions. And because we're *really* far from the quake, 10 minutes passes between the arrival of the P and S waves.
Unlike last week, this seismogram shows two additional phases, called surface waves. There are two types of surface waves: Love, and Rayleigh waves (shown here).
Love waves shake the ground just like a snake slithers--moving side-to-side, but not vertically. You can see that here, as the vertical component (the bottom panel) registers no motion, while the horizontals are recording the Love wave.
A little later, the Rayleigh wave shows up. This one moves kind of like an ocean wave--up and down, and moving in the direction between the quake and the seismic station.
Surface waves, as you can tell here, are the largest of the seismic waves. But at distances like this one (nearly a quarter of the way around the world!) they would not be felt by anything other than incredibly sensitive instruments.
This seismometer was located 80 degrees, or 8800 (!) km from the quake. This is crazy far away, but this was also a *really* big earthquake--the 4th largest ever recorded instrumentally. Humans wouldn't feel this shaking, but seismometers can.
The names on each panel provide information about the type of instrument or the type of channel: The "B" in BHZ, BHE, and BHN tell us that this is a "broadband" station, recording a wide range of frequencies (high notes and low notes).
The "H" in the channel name tells us that this is a high gain seismometer--a very sensitive instrument, recording with very high fidelity.
The third letter is the orientation: E-W, N-S or vertical (labeled as Z). There are also stations for which the channels are described as component 1 or 2--this means that they are not necessarily oriented purely E-W or N-S.
If the instrument is oriented at some weird angle, it turns out that we can do pretty simple math to see what the data would look like in a pure E-W or N-S direction. Linear algebra, for the win!
There's lots more info in this image, including other seismic phases but those will be saved for another #SeismogramSaturday. This one was just meant to show horizontal and vertical components, plus surface waves.
As before, post questions here! And a happy #SeismogramSaturday to you all!
D'oh! I forgot an important piece of information. Surface waves are so called because they move on Earth's surface only. This is in contrast to body waves (P and S waves), which move through the body, or interior of the planet. eqseis.geosc.psu.edu/~cammon/HTML/C…
In really large quakes, surface waves can go around and around the planet before they lose energy and taper out. But they go in both directions, passing one another at the antipode.
You can see this on this IRIS figure--the waves arrive again and again, and at seismic stations on the far side of the world (180 degrees away), the arrive at the same time. iris.edu/gallery3/var/r…
Does one of my more knowledgeable colleagues want to tackle normal modes for this audience?
