How was western North America formed? Geologists have long held that as North America broke away from Pangea and headed west, it ran into the Farallon plate. Subducted under the North American plate, the The Farallon acted almost like a conveyer belt, creating the extensive area of elevated topography that dominates the West—the coastal ranges, the Rocky Mountains and the high plateaus in between. This mountainous area consists of dozens of crustal blocks of varying age and origin, welded onto the American continent over the past 200 million years.

But something was missing in this explanation, says Karin Sigloch, of Ludwig Maximilian University in Munich. “How these blocks arrived in North America has long been a puzzle.”

So with colleague Mitchell Mihalynuk of the British Columbia Geological Survey, Sigloch went to work on the puzzle. The scientists used a technique called seismic tomography. Seismic tomography makes it possible to probe the geophysical structure of Earth’s interior down to the depth of the lower mantle by analyzing the propagation velocities of seismic waves. The method can image the remnants of ancient tectonic plates at great depths, revealing ocean floor that subducted a long time ago, disappearing from the surface and sinking back into the mantle.

The new data from their study suggest that the Farallon Plate was far smaller than had been assumed, and underwent subduction well to the west of what was then the continental margin of North America. The researchers also determined that there was likely another, previously unrecognized oceanic plate involved in the formation of the West.

As the North American plate moved westward, the initially met and consumed the previously unknown oceanic plate, now detected seismologically beneath east coast of modern North America. Only then did the continent begin to encounter the Farallon plate. On its westward journey, the scientists conclude that North America overrode one intervening island arc after another—annexing ever more of them for the construction of its wild, wide mountains of the West.

The study is published in this week’s Nature.

Image: Karin Sigloch

UC Berkeley’s Roland Burgmann is describing two large earthquakes that occurred in April of this year, in the Indian Ocean, off the coast of Sumatra. While the two ‘quakes caused little damage, the first, measuring magnitude 8.7, was the largest strike-slip temblor ever recorded, and the events seem to be triggering much more than shaking.

Three papers this week in Nature analyze the before, during, and after of the two earthquakes, and all three seem to arrive at the same conclusion: the Indo-Australian tectonic plate is breaking into two separate plates.

If you’ve visited the Academy’s Earthquake exhibit and planetarium show (and you can now do so virtually, through an iTunes course), you know that earthquakes result when continents break apart and plates grind against each other. Scientists say that’s exactly what is happening in this Indo-Australian region right now. Not surprisingly, very slowly.

Thorne Lay, of UC Santa Cruz and co-author on one of the Nature papers, says that the process of forming a new plate boundary will take millions of years and is likely to require hundreds if not thousands of earthquakes like the larger one in April. “This was a huge earthquake, but it’s going to happen again and again to make a through-going fracture that separates the plates.”

Lay and his colleagues’ paper analyzes what happened during the 8.7 quake. It appears that it ruptured over a complex network of at least four faults lying at right angles to one another. According to Lay, the energy released on each fault individually was about magnitude 8, adding up to a total event magnitude of 8.7 (a revised estimate higher than the 8.6 value initially reported). The initial shock was followed two hours later by a magnitude 8.2 aftershock on yet another fault to the south.

The paper by Burgmann and his colleagues from the USGS in Menlo Park, takes it from there. The study shows that these two quakes triggered other, distant earthquakes hours and days later. In fact, the seismologists’ analysis found five times the expected number of quakes during the six days following the April 11 quake and aftershock!

“We found a lot of big events around the world, including a 7.0 quake in Baja California and quakes in Indonesia and Japan, that created significant local shaking,” Burgmann says. “If those quakes had been in an urban area, it could potentially have been disastrous.

“Until now, we seismologists have always said, ‘Don’t worry about distant earthquakes triggering local quakes.’ This study now says that, while it is very rare—it may only happen every few decades—it is a real possibility if the right kind of earthquake happens.”

Image: Thorne Lay/UCSC

Besides the 1960 and 2010 earthquakes there have been several other large quakes. Science News provides the reason:

Off the western coast of South America, the Nazca plate of Earth’s crust dives beneath the South American plate, pushing up the Andes and building up stress that gets relieved occasionally in powerful earthquakes.

In 1835, Charles Darwin was in the area and experienced a large magnitude-8.5 earthquake. Though there have been five other great earthquakes since then, none have ruptured in the same area that shook in 1835, a locality now known as the Darwin gap. Scientists, aware that the Darwin gap has been accumulating pressure for over 100 years, had been expecting a large quake in that gap.

But the 2010 quake was north of the Darwin gap. According to New Scientist, researchers studying the area after the recent quake

used tsunami, GPS and radar data to assess the amount of land movement during last year’s quake. By feeding this into a model, they were able to estimate the amount of slippage on the fault and the variation in the release and accumulation of stress along it.

And, the results were striking (from Scientific American):

When pressures build up enough, they snap and cause a quake. Some areas, deep below ground to the north of Concepcion, slipped almost 20 meters in the 2010 earthquake but the area of the “Darwin gap” barely moved.

So rather than relieving and reducing the possibility of another large earthquake in the area, this most recent quake may have increased the possibility of another magnitude-7 or -8 earthquake occurring in the near future. The researchers published their findings in this week’s Nature Geoscience:

…increased stress on the unbroken patch may in turn have increased the probability of another major to great earthquake there in the near future.

Image credit: R. Stein, Lorito et al/Nature Geoscience 2011

A study published online today in the Journal of Zoology reports that common toads may be able to detect impending seismic activity and alter their behavior from breeding to evacuation mode.

Researchers from The Open University studying toads about 46 miles from the epicenter of the 2009 L’Aquila earthquake in Italy found that 96% percent of the males abandoned their breeding grounds five days before the earthquake hit. As soon as the earthquake was over, they returned. This behavior was viewed as unusual because breeding sites are male-dominated and the toads would normally remain from the point that breeding activity begins, to the completion of spawning.

This shift in the toads’ behavior coincided with disruptions in the uppermost layer of the Earth’s atmosphere, called the ionosphere, which was detected using very low frequency radio sounding.

In this case the cause of the ionosphere disruptions was not confirmed, but the release of radon gas or gravity waves prior to an earthquake have both been attributed to changes in atmospheric electric fields and currents.

While there have been many suspicions of animals sensing earthquakes, it’s been difficult to demonstrate. In this case, however, the researcher happened to be in the right place at the right time. Biologist and lead author of the study, Rachel Grant, was studying toads in that area for four years when the earthquake hit. Now she’s published ground-breaking research.

“Our study is one of the first to document animal behavior before, during, and after an earthquake. Our findings suggest that toads are able to detect pre-seismic cues such as the release of gases and charged particles, and use these as a form of an earthquake early warning system.”

Creative Commons image by WWalas