Phil Plait showed the spectacular animation of seismic waves propagating across the US from the 5.8 Virginia earthquake last week, but left out part of the story. A commenter, davenquinn, picked up some details. If you look at the video:
you see an enormous number of sensors in the Midwest and Great Plains. These are areas not known for seismic activity, so what are they doing with so many sensors?
They are part of a travelling array (the 'transportable array') of seismometers, part of the Earthscope initative from the National Science Foundation.
The idea is to have a substantial number of seismometers moving stepwise across the US every few years. Then, having a dense array of seismometers, particularly to have them in places that we don't normally, will show us things that we don't normally see. That spectacular wave propagating across the country is one of those things. Get in to the data and you start seeing that the seismic waves didn't travel the same speed in all directions. And that tells us something about what the earth is made of. No idea what is up, but take a look at the Texas-Louisiana area. Early in the propagation of the waves, they follow along with the rest of the US. But later in the animation, that area shows much larger amplitude variations. Why?
In other words, more fun science to be done!
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The first waves to arrive are The P waves (regular acoustic waves) that follow the quickest but not the shortest route through the crust and mantle. The biggest amplitude arrivals are the PcP waves that are reflected off the liquid core of the Earth.
The waves that arrive at about 10 minutes are probably Rayleigh (surface waves) that travel at about 3 km/s. They are subject to scattering, reflections and so on in the shallow geology, so they appear noisy compared to the other waves. They can be high amplitude because they spread out in only two dimensions, instead of three for the body waves.
Why specifically they are relatively higher amplitude in Texas relative to further north is unknown to me but I'd guess it has something to do with the near surface geology of the Eastern US.
you see an enormous number of sensors in the Midwest ... These are areas not known for seismic activity
Is joke, yes? New Madrid?
:-) Yes, New Madrid -- last significantly active over 100 years ago. Of course one day it will be active again. But aside from that, the midwest doesn't see much over 4.5, which was too weak for me to feel even from close range. See http://earthquake.usgs.gov/earthquakes/states/historical_state.php for a state by state listing, in which New Madrid gets a listing to itself.
Andy: Thanks for the elaborations. I'll hope a Texas geologist shows up to shed light on their peculiar quake response.
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