Earthquakes actually aftershocks of 19th century quakes

Nov 04, 2009
An aerial view of the San Andreas fault in the Carrizo Plain, Central California. Image: USGS, Wikimedia Commons.

(PhysOrg.com) -- When small earthquakes shake the central U.S., citizens often fear the rumbles are signs a big earthquake is coming. Fortunately, new research instead shows that most of these earthquakes are aftershocks of big earthquakes (magnitude 7) in the New Madrid seismic zone that struck the Midwest almost 200 years ago.

The study, conducted by researchers from Northwestern University and the University of Missouri-Columbia, will be published in the Nov. 5 issue of the journal Nature.

"This sounds strange at first," said the study's lead author, Seth Stein, the William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences at Northwestern. "On the San Andreas fault in California, aftershocks only continue for about 10 years. But in the middle of a continent, they go on much longer."

There is a good reason, explains co-investigator Mian Liu, professor of geological sciences at Missouri. "Aftershocks happen after a big because the movement on the fault changed the forces in the earth that act on the fault itself and nearby. Aftershocks go on until the fault recovers, which takes much longer in the middle of a continent."

The difference, Stein explains, is that the two sides of the move past each other at a speed of about one and a half inches in a year -- which is fast on a geologic time scale. This motion "reloads" the fault by swamping the small changes caused by the last big earthquake, so aftershocks are suppressed after about 10 years. The New Madrid faults, however, move more than 100 times more slowly, so it takes hundreds of years to swamp the effects of a big earthquake.

"A number of us had suspected this," Liu said, "because many of the earthquakes we see today in the Midwest have patterns that look like aftershocks. They happen on the faults we think caused the big earthquakes in 1811 and 1812, and they've been getting smaller with time."

To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults. They then looked at data from faults around the world and found the expected pattern. For example, aftershocks continue today from the magnitude 7.2 Hebgen Lake earthquake that shook Montana, Idaho and Wyoming 50 years ago.

"This makes sense because the Hebgen Lake fault moves faster than the New Madrid faults but slower than the San Andreas," Stein noted. "The observations and theory came together the way we like but don't always get."

Aftershocks go on for long times in other places inside continents, Stein said. It even looks like we see small earthquakes today in the area along Canada's Saint Lawrence valley where a large earthquake occurred in 1663.

The new results will help investigators in both understanding earthquakes in continents and trying to assess earthquake hazards there. "Until now," Liu observed, "we've mostly tried to tell where large earthquakes will happen by looking at where small ones do." That's why many scientists were surprised by the disastrous May 2008 magnitude 7.9 earthquake in Sichuan, China -- a place where there hadn't been many earthquakes in the past few hundred years.

"Predicting big quakes based on small quakes is like the 'Whack-a-mole' game -- you wait for the mole to come up where it went down," Stein said. "But we now know the big earthquakes can pop up somewhere else. Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake. We don't see that in the Midwest today, but we want to keep looking."

More information: The Nature paper is titled "Long Aftershock Sequences within Continents and Implications for Earthquake Hazard Assessment."

Source: Northwestern University (news : web)

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User comments : 13

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Gravity2
2 / 5 (4) Nov 04, 2009
Earthquakes occur along expansion joints. All rivers are expansion fault lines. The Mississippi and the Colorado Rivers quake constantly. The San Andreas is constantly quaking from the mouth of the Colorado up through southern California to Arcarta where it dives into the Pacific and emerges again in Alaska. Alaska also experiences intense earthquake activity contineously. Then there is the Aleutian archipeligo which never stops quaking as Russia separates from Alaska. See my Web page at www.widemargin2000.com to read more about what is happening "as the world quakes"
Gravity2
Nov 04, 2009
This comment has been removed by a moderator.
Gravity2
Nov 04, 2009
This comment has been removed by a moderator.
Gravity2
2 / 5 (4) Nov 04, 2009
There is a lot of Media Hypertension about rising sea levels ostensibly as aresult of melting icecaps. Lowlying states like Bangledash will be inundated by twenty feet of rising sea. Nothing of the sort is going to happen. It is all figmental.
If all the Icecaps melt, and they certainly will not, sea level will not rise. The perennial decline in sea level due to expansion of our planet will effectively negate any rise.
vidar_lund
3.7 / 5 (3) Nov 04, 2009
There is a lot of Media Hypertension about rising sea levels ostensibly as aresult of melting icecaps. Lowlying states like Bangledash will be inundated by twenty feet of rising sea. Nothing of the sort is going to happen. It is all figmental.
If all the Icecaps melt, and they certainly will not, sea level will not rise. The perennial decline in sea level due to expansion of our planet will effectively negate any rise.

Excuse me, but how exactly does the earth expand, is it the yeast?
StillWind
2.3 / 5 (3) Nov 05, 2009
The expanding Earth Theory isn't new. In fact it is older than the bogus Plate Tectonic Theory that is currently in vogue.
One has to wonder how such a ridiculous theory was ever taken seriously, except that many scientists are extremely ignorant of anything but their specific area of study.
All our current theories are being proven wrong, one by one.
How can anyone take seriously our cosmology when it relies on a fudge factor of greater than 90%, which is currently being covered up by silly ideas of Dark Matter and Dark energy.
Anything else that needed such an enormous "fix" would be easily forgotten.
Science is simply another religion that is protected by priests that hope to preserve their places of power.
StillWind
1 / 5 (2) Nov 05, 2009
BTW, Neal Adams explains the mechanism behind the expanding Earth.
frajo
3.7 / 5 (3) Nov 05, 2009
Expanding planet lifts land but not sea?
Fascinating. None of my 300 SF books told me that story.
vidar_lund
3.7 / 5 (3) Nov 05, 2009
The expanding Earth Theory isn't new. In fact it is older than the bogus Plate Tectonic Theory that is currently in vogue.

It hasn't occurred to you that any significant expansion of the earths diameter would have been discovered long time ago by the precise measurements done by satellites orbiting the earth? Have you heard about GPS?

Plate tectonics is not a theory anymore, it is a proven fact that is observed all over the world and the measurement of the movement of the continents is becoming very precise. Again I suggest you google GPS. But I guess the guys that designed the GPS system have conspired with the plate techtonic guys to produce fake measurements.
out7x
Nov 05, 2009
This comment has been removed by a moderator.
StillWind
1 / 5 (1) Nov 07, 2009
Plate Tectonics is completely false and easily proven to be so. In fact, I can't think of a more ridiculous idea, but then again I'm just basing my position on physics, paleontology, and observation.
GPS is irrelevant, as it has only been in use for a short time, and isn't sensitive enough to measurer the small amount of expansion that we've seen since it was put into service..
LuckyBrandon
2 / 5 (1) Nov 07, 2009
plate tectonics makes perfect sense. if you wre to take a super hot ass ball of iron or something, then put a nice thick layer of dense mud around it...what would happen...
the mud would dry up, then begin to fracture, and as more drying occurred, those fractures would move...
..just a simple example there, as to make it more realistic, you would have to have something essentially with currents running through it to simulate magma...but the concept seems sound...
Gravity2
1 / 5 (1) Nov 21, 2009
The observations of the aftershock patterns vis a vie the San Andreas and the Mew Madrid faults are interesting. The New Madrid fault is along the Mississippi. The Mississippi is a major fault line in the middle of North America. It is a natural expansion point for a continent that is breaking in two. The San Andreas is not in mid-continental North America. Both fault lines, however, quake from time to time and from place to place in an unpredictable pattern. We have to realize that the quakes are a manifestation of an expanding planet. We have to learn also that there are more insidious elements of this expansion. e.g. we engineers build bridges and dams across river valleys and they are all fault lines and they are all expanding.
Gravity2
1 / 5 (1) Nov 21, 2009
Many bridges that span the Mississippi have failed because the River is expanding. Earthquakes are manifestations of Earth Expansion. The sooner we learn that fact and starte looking beyond the better for all the sciences E.G. Isostatic Rebound is false. It does great disservice to geology. Sea Level Datum is false; the sea level does not rise and will not rise. Sea level falls and has been falling for a very long time. Its not going to stop now. The entire history of civilizations will have to be rewritten because of the expansion of our earth. see www.widemargin2000.com or email richard_guy72@yahoo.com or tel; 347-275-5616,
Gravity2
1 / 5 (1) Nov 21, 2009
GPS is accurate in one respect and totally inaccurate in the other. "Stillwind" you are right. GPS is new and incapable of measuring the slight expansion of the earth. It will take millions of years to come up with a noticable factor of expansion. By that time GPS should have ironed out all its kinks. In the last 4000 millions years the earth has expanded to almost twice its original size. We really dont have to wait that long. Let us look at sea levels. Sea levels have been receding for millions of years as the earth expanded. We dont have to go back million of years to check if sea level is receding just look at recent recorded history. Babylon in Mesopotamia was once a seaport but today it is 400 miles from the Persian Gulf. Ur of the Chaldees after Babylon was a port but is today 200 miles form the gulf and 135 feet above sea level, Baghdad once was on the Gulf but today is 350 miles from thhe sea: see: www.widemargin2000.com or email: richard_guy72@yahoo.com or Tel; 347-275-5616.
Gravity2
1 / 5 (1) Nov 21, 2009
At the time of the Exodus the Jews crossed the Red Sea at Pithom because Pithom was then a harbor on the Red Sea. Today the Red Sea has receded 50 miles from Pithom. Archaeologists have missed this point because we dont know that the seas have receded because of the expansion of our earth. It is important that we do research into the phenomenon of the Receding seas so that we will understand why all ancient civilizations started in high regions around thew world. It will alos lead us to the realization that our earth has expanded. See www.widemargin2000.com or email richard_guy72@yahoo.com or tel 347-275-5616