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!
Showing posts with label earthquake. Show all posts
Showing posts with label earthquake. Show all posts
30 August 2011
23 August 2011
My first earthquake
Yes, I was one of the many who felt the 5.8 to 5.9 earthquake centered in Virginia. The final scoring and location will come from the US Geological Survey -- http://www.usgs.gov/ This is the first time I've noticed an earthquake in my life. Growing up around Chicago and then living around here doesn't give you a lot of exposure. The strongest here that I've been present for was 4.3 to 4.4, as I recall it. I didn't notice it at all, not even in the sense of 'oh, so that's what was happening'.
This time, it was noticeable. The initial stage was some fairly minor, fairly high frequency vibration -- similar to having some heavy trucks drive past, or some kinds of construction drilling. Since there is construction going on in my area, I figured this was it. For the first few seconds. Then there was a rapid increase in the size of the vibrations, and a decrease in their frequency. Rather than having some vibrations passing through the building, we moved to having the building itself swaying back and forth. Earthquake!
By my count, I felt the quake for about 30 seconds. (If that's too long compared to the USGS figures, blame my inner ears.) Call it 20-30. One friend estimated the swaying as being 4 cycles per second in the real quake phase. My guess is closer to 2. I'll have to see what the USGS has. There are probably confounders from the engineering of my building. So far, all seems ok at work and home. Some of the more precariously perched items fell off book cases at home, but that's about it. Fingers crossed that this is true for everybody.
A question popular in our parking lot at work (we'd been evacuated post-quake) was to wonder what fault line caused the quake. My early answer, which was supported by a snippet on the radio (i.e., don't place a lot of confidence here) is that there was no fault line involved. This is simply a much stronger version of the usual earthquake for this region -- adjusting to the fact that the Laurentide ice sheet is gone. In areas that the ice sheet occupied -- down to southern Illinois or in to Pennsylvania -- the land sank under the weight of the ice sheet. This squeezed some of the more fluid parts of the mantle out from there, and over to the areas in front (south) of the ice sheet. That elevated the areas around, say, Virginia. Once the ice sheet was removed, the fluid started oozing back to its original location. As it does so, the crust creaks its way back in to position. Creaking = earthquake.
This time, it was noticeable. The initial stage was some fairly minor, fairly high frequency vibration -- similar to having some heavy trucks drive past, or some kinds of construction drilling. Since there is construction going on in my area, I figured this was it. For the first few seconds. Then there was a rapid increase in the size of the vibrations, and a decrease in their frequency. Rather than having some vibrations passing through the building, we moved to having the building itself swaying back and forth. Earthquake!
By my count, I felt the quake for about 30 seconds. (If that's too long compared to the USGS figures, blame my inner ears.) Call it 20-30. One friend estimated the swaying as being 4 cycles per second in the real quake phase. My guess is closer to 2. I'll have to see what the USGS has. There are probably confounders from the engineering of my building. So far, all seems ok at work and home. Some of the more precariously perched items fell off book cases at home, but that's about it. Fingers crossed that this is true for everybody.
A question popular in our parking lot at work (we'd been evacuated post-quake) was to wonder what fault line caused the quake. My early answer, which was supported by a snippet on the radio (i.e., don't place a lot of confidence here) is that there was no fault line involved. This is simply a much stronger version of the usual earthquake for this region -- adjusting to the fact that the Laurentide ice sheet is gone. In areas that the ice sheet occupied -- down to southern Illinois or in to Pennsylvania -- the land sank under the weight of the ice sheet. This squeezed some of the more fluid parts of the mantle out from there, and over to the areas in front (south) of the ice sheet. That elevated the areas around, say, Virginia. Once the ice sheet was removed, the fluid started oozing back to its original location. As it does so, the crust creaks its way back in to position. Creaking = earthquake.
Subscribe to:
Posts (Atom)
Posts
