Thread by @geophysichick

Hey #SeismoTwitter! I propose that we do #SeismogramSaturday, to teach about the world of our wiggles. Feel free to pick your favorite (or least favorite) seismogram to tell us about. We'll do it next week too.

I'll start. This is a normal little quake. Info follows:

First, a seismogram literally shows us how the ground shook when seismic waves reached the seismometer. The vertical axis is a proxy for this (I'll explain the units later).

This image shows a magnitude 2.0 earthquake that took place near Monroe, WA, a few days ago. No one felt it, since it was tiny and took place about 17 miles underground.

You can see two places where the amplitude is largest--the first represents when the P wave arrived. P waves are what we call longitudinal waves, shaking the ground in the direction that they travel. iris.edu/hq/inclass/upl…

The second, less clear amplitude increase, is the S wave. This "secondary" or "shear" wave shakes in a direction perpendicular to the direction it's moving. iris.edu/hq/inclass/upl…

P waves move faster than S waves, so they always arrive first. It's like lightning and thunder, and in fact you can use the time difference between them to estimate the distance between the seismometer and the quake.

Since P waves arrive first, the ground is often still shaking a bit when the S wave arrives. This makes it much harder to tell exactly when the S showed up, so there's more uncertainty in its arrival time. This is one cause of uncertainty in the earthquake location.

(the biggest source of uncertainty in earthquake locations, by the way, is the speed at which the waves were moving).

You can see that the wiggle goes down when the P-wave arrives. That means that the ground literally dropped down first (to a tiny, tiny degree that no one would be able to see or feel). We use this information to tell us how the fault moved.

A common misconception is that S waves are always larger than P waves. As you can see here, this isn't true...it depends on where you are relative to the fault plane (and how the quake moved). Surface waves are the largest amplitude waves (we'll talk about them another time).

About the vertical axis: seismometers usually record the velocity of ground motion (or acceleration), but data are often shown in "counts", which is just a generic digital unit. We have to go through a bit of contortion to convert it into meaningful values such as micrometers/s.

Ask your questions here! If you're a seismologist, post your own #SeismogramSaturday event. Maybe show us your cool phase arrivals (surface waves or PKIKP) or your volcanic earthquakes or your unknown events.