November 6, 2011 in Earthquakes
One of the few things that can travel faster than seismic waves is news about seismic waves. As a result, last night’s earthquake in Oklahoma is already ancient history. However, Ralph Waldo Emerson said “We learn geology the morning after the earthquake”, so let’s see if it’s true.
Above, you’ll see our seismograph from Keystone College (a member station of the Lamont-Doherty Seismic Network). This is a broadband readout, which means that it is recording all frequencies of waves. Note the first arrival at ~3:57 GMT, although the bulk of the energy arrives after 4:00 GMT.
This readout is only showing the short period waves and looks a lot different. First, the first arrival is more visible, because waves that travel through the Earth (p and s) are dominated by these frequencies. Second, look at all those aftershocks! I count at least 8, but there have been many more that can’t be seen here.
We live in a three-dimensional world and that means that a seismometer moves in three directions simultaneously. This readout from Lamont’s seismometer (PAL) shows all of this motion.
Where are the P and S wave? Beats me. I would expect to be able to pick them out, but I can’t seem to find them. They’re there, but are not easily visible in this seismograph.
Why is the shaking so much more in the North-South direction? The bulk of the shaking is from the waves that travel along the surface (Love and Rayleigh). They say that you can only tell how far away an earthquake is using a single seismic station, but that’s not entirely true. The waves are coming from the West, which means that the horizontal Love waves are more pronounced in the North-South direction. That at least tells you the direction (either from the east or west). Well, sort of.
So there you have it. The largest quake in Oklahoma’s history (assuming the magnitude doesn’t get downgraded by the USGS). If you have any other questions, let me know and I’ll do my best to answer them.