MessageToEagle.com

High Seismic Velocity Anomaly Around 100 Kilometers Beneath Southern California - One Of The Most Seismically Active Areas In The World

19 March, 2013

Share this story:
Follow us:

MessageToEagle.com - Southern California is seismically one of the most active areas in the world and has been studied by many seismologists so far.

It's a very sensitive area because of the well-studied and carefully observed - Isabella Anomaly, which is the best known example of a high seismic velocity anomaly beneath the southern Central Valley of California.

It's a piece of stalled oceanic lithosphere still attached to the unsubducted Monterey microplate.


Schematic of the hypothesized remnant slab, attached to the Monterey microplate (and possibly its neighbor to the North), and translating with Pacific plate motion underneath the North American plate. Light gray, dark gray, and beige denote lithosphere of Pacific, Farallon, and North American origin respectively. Copyright © 2012 Elsevier B.V


The Pacific-North American plate boundary forms a broad zone of deformation extending across this area. Isabella feature, which is in fact a surviving slab of the Farallon oceanic plate, extends down to about 300 km depth, and its main part extends to deeper than 200 km depth.

Most of the Farallon plate was driven deep into the Earth’s mantle as the Pacific and North American plates began converging about 100 million years ago, eventually coming together to form the San Andreas fault.


The creeping section of the San Andreas Fault lies directly between the Monterey microplate and the Isabella anomaly, i.e. above the proposed remnant slab. The presence of a slab could potentially help to explain the aseismic nature of this section of the fault. Copyright © 2012 Elsevier B.V


Large chunks of an ancient tectonic plate that slid under North America millions of years ago are still present under parts of central California and Mexico, according to new research led by Brown University geophysicists.

Around 100 million years ago, the Farallon oceanic plate lay between the converging Pacific and North American plates, which eventually came together to form the San Andreas fault.

As those plates converged, much of the Farallon was subducted underneath North America and eventually sank deep into the mantle.

Off the west coast of North America, the Farallon plate fragmented, leaving a few small remnants at the surface that stopped subducting and became part of the Pacific plate.

But this new research suggests that large slabs from Farallon remain attached to these unsubducted fragments.
The researchers used seismic tomography and other data to show that part of the Baja region and part of central California near the Sierra Nevada mountains sit atop “fossil” slabs of the Farallon plate.


Blue swath shows approximate position of Isabella Anomaly


“Many had assumed that these pieces would have broken off quite close to the surface,” said Brown geophysicist Donald Forsyth, who led the research with Yun Wang, a former Brown graduate student now at the University of Alaska.

“We’re suggesting that they actually broke off fairly deep, leaving these large slabs behind.”

Geologists had known for years about a “high velocity anomaly” in seismic tomography data near the Sierra Nevada mountains in California. Seismic tomography measures the velocity of seismic waves deep underground. The speed of the waves provides information about the composition and temperature of the subsurface. Generally, slower waves mean softer and hotter material; faster waves mean stiffer and cooler material.


The Isabella anomaly (IA, above), is at the same depth (ca. 100 km) as other fragments of the Farallon plate under Oregon and Washington, is on a line with fragments off the California coast, and has a similar seismic tomography signature. Credit: Forsyth lab/Brown University


The anomaly in California, known as the Isabella anomaly, indicates that a large mass of relatively cool and dehydrated material is present below the surface.

Just what that mass was wasn’t known, but there were a few theories. It was often explained by a process called delamination.

A few years ago, scientists detected a new anomaly under the Mexico’s Baja Peninsula, due east of one of the known coastal remains of the Farallon plate. Because of its proximity to the Farallon fragment, Forsyth and Wang thought it was very likely that the anomaly represented an underground extension of the fragment.

Carefull studies of the area suggest that the Isabella anomaly in California might also represent a slab still connected to an unsubducted fragment of the Farallon plate, so scientists compared the Baja and Isabella anomalies to anomalies associated with known Farallon slabs underneath Washington and Oregon.

The study found that all of the anomalies are strongest at the same depth — right around 100 kilometers. And all of them line up nearly due east of known fragments from Farallon.

“The geometry was the kicker,” Forsyth said. “The way they line up just makes sense.”

The findings could force scientists to re-examine the tectonic history of western North America, Forsyth said. In particular, it forces a rethinking of the delamination of the Sierra Nevada, which had been used to explain the Isabella anomaly.

“However the Sierra Nevada was delaminated,” Forsyth said, “it’s probably not in the way that many people had been thinking.”

The findings are published in the Proceedings of the National Academy of Sciences.

For more: http://www.sciencedirect.com/science/article/pii/S0012821X12001501

© MessageToEagle.com

See also:
Geological Rift That Baffles Scientists Is Still Expanding

Follow MessageToEagle.com for the latest news on Facebook and Twitter !

Don't Miss Our Stories! Get Our Daily Email Newsletter

Enter your email address:


Once you have confirmed your email address, you will be subscribed to the newsletter.

Recommend this article:


Amazing Journey To The World's Largest Cave That Has A Jungle And Waterfalls Hidden Deep Inside!

We continue to look for amazing places on our planet and this is one of them. Son Doong is the name of the world's largest cave in the world. It is not only huge, but also a very beautiful place filled with wonders. It is almost like an alien world on Earth. What is truly fascinating is the jungle inside the cave...

Antarctic Ice Shelves Breaking Up Fast

Subscribe To Our Space, Astronomy, Astrophysics, Earth and Xenology News!

Grab the latest RSS feeds right to your reader, desktop or mobile phone.

Subscribe to RSS headline updates from:
Powered by FeedBurner

Go to - MAIN PAGE

Copyright © MessageToEagle.com All rights reserved.
Go to - MAIN PAGE


Get our top stories
Follow MessageToEagle.com

 Subscribe in a reader

Join Us On Facebook!

Other Popular Articles

Dangerous Fast and Furious - Birth Of Africa's New Ocean
The only places where mid-ocean ridges appear above sea level are Ethiopia and Iceland. Two new studies into the so-called “plumbing systems” that lie under volcanoes could bring scientists closer to predicting large eruptions and reveal new information about where magma is stored and how it moves through the geological plumbing network.


Incredible Images Show Giant Sinkhole In Sweden Keeps Expanding!

It looks like something taken straight from a horror movie. An enormous hole leading to hell, some would say. But this is not a movie. This is a real and dangerous phenomenon. New shocking images clearly show the enormous pit in Sweden is expanding...


6 Surprising Things Falling From The Sky

Stories of strange things falling from the sky date back a long time ago. Over the years, there have been many bizarre instances recorded when things have fallen from the skies that simply do not belong there. We have put together a list of some surprising things that fell from the heavens. These are things you certainly don't expect to fall down on you...

New Shocking Discovery Challenges The Origin Of Earth
Where did our planet come from? We already know that our Sun does not originate from M 67. A new surprising discovery is now forcing scientists to re-evaluate the origin of Earth.


Exploding Lakes - Horrifying Natural Phenomena

Exploding lakes are a terrible natural phenomena and very serious environmental problem.
These lakes are capable of killing thousands, even millions of people and animals living in the region...
Lake Nyos is an active crater lake that formed by an eruption about 5 centuries ago...

Antarctica's Mysterious Lake Untersee And Its Otherworldly Ecosystem
A research team has now returned to Lake Untersee to begin the first real studies of this incredible environment. This new high-quality image details the conical stromatolites that have formed on top of a boulder at a depth of about 20 meters, other cones can be seen populating the sloping bottom...

Solar Tornadoes As Wide As Five Earths Discovered
Solar tornadoes several times as wide as the Earth can be generated in the solar atmosphere, say researchers in the UK. "This is perhaps the first time that such a huge solar tornado is filmed by an imager. Previously much smaller solar tornadoes were found my SOHO satellite. But they were not filmed," says Dr. Xing Li, of Aberystwyth University.

Extreme Weather: Rare Supercell Thunderstorm Produces Monster Hailstone
Just recently scientists announced they found new evidence linking Earth's extreme weather to human-caused global warming. Now, the National Weather Service office in Honoulu in coordination with the State Climate Extremes Committee has confirmed that a rare supercell thunderstorm has produced new state hail size record.


Who Installed "This Object" At The Bottom Of The Ocean?

Our oceans are full of secrets, and this is one of them. Deep beneath the waters we find ruins of ancient lost civilizations, wreckage of old ships and planes, and sometimes we come across very strange, unidentified objects. Looking at this odd construction, one has to ask - Who installed this object at the bottom of the ocean?

Extreme Life In Tough Environment Of Hot Earth Without Oxygen
Once upon a time, Earth was still a hot, violent place at this time, with volcanic activity dominating the early Earth. The sky was cloudy and grey, keeping the heat in despite the sun being weaker than today. The water temperature of the oceans was much higher at 40-50 degrees (the temperature of a hot bath) and circulating currents were very strong.

Living Earth Simulator - Supercomputer Predicting The Future
In Douglas Adams book the Hitchhikers Guide to the Galaxy we encounter a machine called Deep Thought. It is the most powerful computer ever built. Deep Thought is capable of answering questions concerning life, the Universe, and simply everything. Now scientists are planning to create a similar machine. It is called the Living Earth Simulator (LES).

New Evidence Links Earth's Extreme Weather To Human-Caused Global Warming
The past decade has been one of unprecedented weather extremes. Scientists of the Potsdam Institute for Climate Impact Research (PIK) in Germany argue that the high incidence of extremes is not merely accidental. From the many single events a pattern emerges. At least for extreme rainfall and heat waves the link with human-caused global warming is clear...

Growing Stones An Incredible Geological Phenomena
Earth is an amazing planet and our nature is full of wonders. We have previously written about incredible singing plants. This time we would like to focus our readers' attention on another amazing geological phenomena, namely so-called growing stones. It is difficult to image that stones can really grow, but these stones seem to be alive!

Amazing Phenomenon Of Singing Plants
Plants are very much alive. Not only do they dislike human noise but they also posses the capacity to learn and communicate. Perhaps even more astonishing is that plants can also make music. Have you ever heard the incredible music of the plants? Plants can actually sing and compose music and listening to it is truly beautiful and relaxing!


Easter Island's Statues Reveal Bodies Covered With Unknown Ancient Petroglyphs

Standing some 2,000 miles west of Chile, on the Easter Island, 887 mysterious giant statues have intrigued scientists and the public for years. For a long time it was believed that the massive statutes consisted of just the heads. Now, scientists can reveal remarkable photos showing that the bodies of the statues go far deeper underground than just about anyone had imagined...


Extreme Earth: Where Is The Hottest Place On Earth?

There are many extreme places on our wonderful Earth. Among them is the hottest place on Earth. But where exactly is it? To answer that question is not as easy as it might seem. This question is in fact still widely debated because, how the temperature is measured plays an important role when scientists attempt to decide what place is hotter than the other.


"Something Is Seriously Wrong: Too Many Strong Earthquakes", Scientist Says

Does a supernatural seismic activity threaten our planet? Apparently, something unknown has triggered a "terrible and global domino", according to scientists who observe recent earthquake reports. Seismologists recognize that what happens to the seismic activity goes beyond the level generally considered as normal...

Amazing Alien Landscape On Earth - Dallol Volcano
If you want to experience an alien landscape it is not necessary to travel to another planet. In fact, there are places on Earth where the landscape is truly bizarre, almost extraterrestrial many would say. Looking at this amazing scenery, one can easily ask - Are we still on Earth?

Giant Underwater Canyon Zhemchug Can Only Be Seen Entirely From Space
We continue to explore amazing places on our beautiful planet. This time we take a journey deep beneath the oceans. Hidden beneath all that water are some of the biggest natural formations on earth. Deep underwater we find the longest mountain ranges, the tallest volcanoes and the deepest canyons. We might think that Grand Canyon is big, but it is nothing compared to Zhemchug that is so huge it can only be seen entirely from space!

W3Counter