Earthquake Experts???

[Mr. Bloom]

As many of you know, we've had a rash of small earthquakes in the last two weeks. 27 tremors...but all were in nearly the same place as noted by the star on the attached map:

http://neic.usgs.gov/neis/bulletin/neic_rmar_z.html

We live under the "E" in Evansville.

They've stopped calling these aftershocks...but rather are now calling them PRE-shocks.

Is this hype or is there a scientific distinction?

We've confirmed we have earthquake coverage on the house...with a $42,000 deductible!

[Zen]

The short answer is, nobody knows.

The long answer, as explained here, is nobody knows.



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[Mr. Bloom]

Tell me some good news!

[Melalvai]

I heard the seismologist in Bloomington slept through the big earthquake a couple weeks ago.
That would suck.

[SouthernBelle]

That's the zone that created Reelfoot Lake back a couple of centuries ago via the New Madrid Fault. It is speculated that it was one of the strongest quakes ever.

http://www.reelfoot.com/new_madrid_earthquake.htm

[Red Rock]

Silver- I do not know if anything I say here will help but I minored in Geology(which is not much since there are Ph.D's on this site).

But when the earth shakes, the plates are moving and releaving stress. Since there seem to be many happening in the same place. The plate or fault is kind of skipping/shifting along releasing the stress on the fault. This can be a good thing because it is happening in small movements and not as one large one. How long has it been since this fault had a major shift/quake? 10 years?, 50 or 100 years? Are they(the experts)expecting a large quake soon? Unfortunatley, as with all things in Nature, there is no "offical warning" of "the big one". I would just be prepared as much as you can be under the circumstances.

Ellen

[trickytiger]

Well, I WAS a professional geologist, before ditching that to follow the physical therapy profession instead. I still have an unabating love for geology, just not the typical jobs available these days!

Have a look at this page, it may explain things a little more clearly:

http://pasadena.wr.usgs.gov/step/aftershocks.html

[DrBadger]

OK, one of the PhDs piping in here... (while I am supposed to be working on revisions to my paper on earthquake triggering )

What Zen and Ellen said is pretty much true... we really don't know what causes earthquakes in the middle of tectonic plates like the midwest is. Out in California and places like Japan, Alaska, Chile, and Sumatra we know what causes the earthquakes. The crust of the earth is broken up into large plates that move on top of the molten mantle, in these locations they are either pushing past each other (California), or one plate is sinking under the other (Japan, Chile, Alaska, and Sumatra). The middle of the plates like the midwest is a different story.

It is generally thought that these intra-plate earthquakes occur along a pre-existing zone of weakness in the crust. The New Madrid and Wabash fault zones in the Indiana-Illinois-Misouri area is thought to be a failed rift system. A rift is what forms when the mantle begins to rise up through the crust and force the plates apart. This is happening currently in east Africa (you can do a search for the East African Rift Zone), if the rift succeeds then you end up getting something like the Mid-Atlantic spreading ridge where new material is coming up from the mantle forming new crust and forcing the plates away from each other. But many of the rifts fail, this is thought to be one of those places. But since there is a difference in the crust in this zone due to the failed rift there is now a zone that is weaker than the rest of the crust, maybe from the higher heat flow due to the mantle having risen up in this area (but this hasn't been proven).

OK, so now you have a zone that is weaker than the surrounding crust, but you still have to stress the area in order to create an earthquake. There are a couple of ways that could happen in the New Madrid and Wabash zones. The North American plate is moving generally towards the west (but really rotating counter-clockwise around a pole down near South America). Some people think that the plate doesn't behave completely rigidly and it is accommodating some of this deformation through intra-plate earthquakes like the ones you have been experiencing. There however, isn't a lot of evidence from GPS that there is the right kind or magnitude of deformation across this area to explain the earthquakes this way. A more accepted theory (and the one I like better) is that the earthquakes are in response to the crust rebounding after the last ice sheets melted in Canada and the northern US. Basically the crust of the earth is floating like icebergs or wine corks on the molten mantle. If you put more weight on the top of the crust through things like thick ice sheets you push the crust down deeper into the mantle. Once the ice melts the crust has to pop back up. As the area around Hudson Bay and the Great Lakes pops back up it places a lot of stress on the area to the south of it, right around the New Madrid and Wabash fault zones. Since there is already a zone of weakness there this stress causes the faults to slip and create earthquakes.

Now, the million dollar question... what does this mean for future earthquakes and the ones you are having now. Well, unfortunately we really don't know...part of the problem is that we really just have hypotheses about how these earthquakes are even being triggered. We do know that there were a series of big earthquakes in 1811-1812, but if these were triggered by the crust bouncing back up after the last ice age, we don't know if there will be any more. The rates of uplift have slowed down, but not stopped. So the question remains, is there enough stress built up in the crust to cause big earthquakes like there were in the past? Or, since the rates have slowed are you just going to get swarms of little earthquakes like you are experiencing now? So, at this point there is no way to say whether the earthquakes you are having now are aftershocks, pre-shocks, or just some sort of swarm that will end sometime soon. Also, there is really no difference between a pre-shock, a main shock, and an aftershock....they all look the same scientifically. The labels are just arbitrary. We generally will call the first "big" even the main shock and everything after that the aftershock... but what if one of those events is bigger than the first event we called a main shock? Was that now a pre-shock and the new one the main shock? Or is the bigger one just a really big aftershock? Like I said, arbitrary labels. In essence, every earthquake is a pre-shock to the next one in the region.

In terms of what you should be doing, the biggest thing is making sure your family has an emergency plan. Know what to do if there is a big earthquake, you want to get down on the ground, ideally under something heavy and strong like a table or a doorway and cover your head. Wait for the shaking to stop and then carefully make your way out doors. If outside, stay away from buildings and things that can fall on you. In your house, make sure that things like bookshelves are braced to the wall and that heavy items are secured or moved to low shelves/areas. Have an emergency kit with flashlight, shoes, glasses, prescriptions, water and canned/dried food. Generally, under your bed is a good place to keep it. Decide on one family member who lives in a different place that you will call in the event of an earthquake (or really any emergency) and tell everyone else to call them to get updates on you. There is a good chance that cell phone circuits will be full and people won't be able to get through to you, so it is better to keep the calls out of the region of the emergency. If you are really curious the USGS has a really good pamphlet put together mostly for people living in places like California, but the tips are good for anywhere. it is called "putting down roots in earthquake country" You can download it here: http://www.earthquakecountry.info/roots/index.php

OK, that was probably WAY more information that you ever wanted, but you did ask! Let me know if you have any more questions. And from some one who studied these things, and was sad that in the 6 years I lived in CA I only felt 2 earthquakes, enjoy and be amazed at the power of our earth!

Ellen (the other geo one!)

[cyclinnewbie]

I heard the seismologist in Bloomington slept through the big earthquake a couple weeks ago.
That would suck.

How big was the big quake? If it was small enough to sleep through, I wouldn't worry about it!

Anyhoo, we've been having kind of the same type of thing here in the PNW....lots of very very low level tremblers. Can't even feel 'em. Apparently the plates are sliding against each other or something....I don't even know...

[mimitabby]

How big was the big quake? If it was small enough to sleep through, I wouldn't worry about it!

Anyhoo, we've been having kind of the same type of thing here in the PNW....lots of very very low level tremblers. Can't even feel 'em. Apparently the plates are sliding against each other or something....I don't even know...

yeah, that's what i was thinking. You'd have to sleep like a log (a really sleepy log) to miss some of the quakes I've experienced in Seattle. As a matter of fact, the Nisqually quake destroyed the building i was in... You'd have to be dead to sleep through that.

anyway, I don't waste a lot of energy worrying about earthquakes, I don't live in a stone house.

[Mr. Bloom]

How big was the big quake? If it was small enough to sleep through, I wouldn't worry about it!

Anyhoo, we've been having kind of the same type of thing here in the PNW....lots of very very low level tremblers. Can't even feel 'em. Apparently the plates are sliding against each other or something....I don't even know...

It woke me up in Bloomington and this was 120 miles from epicenter. A lamp was rattling and then it was like a high wind was hitting the condo with creaking and cracking.

THEN, Silver called me to say they had just had a pretty big quake...but this was 5.6

[shootingstar]

Well Dr.Badger, wish that Vancouver, BC area would be better prepared for the big shake that geologists /seismologists have been predicting for past 2 decades.. No one is sure when.

2 weeks ago I was chatting with one of the civil engineers where I work..already they have noticed soil settlement which isn't necessarily just due to softer soil. He did say a suburban area which is delta/floodplain could collapse or get partially underwater when we get the big shake.

The organization I'm with now, is building a 1 km. long road bridge that 4 lanes wide with bike path. They have to install seismic drains...which allows the soil to slide somewhere around the structural foundations when vibration/settlement occurs...but still...that's just such a minor structural detail for baby tremors...

Right there is an all-time high price real estate boom in Vancouver...but you know, one day,...this might go down ..literally...with the shake.

[Mr. Bloom]

Thanks for the great info! This is not too much...but it just confirms that we have no absolute knowledge or guarantees in life!