2006 Plate Motion Reversal Unlikely To Have Eased Seismic Strain, Earthquake Anticipation Near Acapulco

A reversal of tectonic plate motion near Acapulco, Mexico, in 2006 (colored arrows) as measured by GPS satellites did little to ease seismic strain in the region and the potential for a large earthquake that could impact Mexico City 175 miles away, according to a new study led by CU-Boulder.
A reversal of tectonic plate motion near Acapulco, Mexico, in 2006 (colored arrows) as measured by GPS satellites did little to ease seismic strain in the region and the potential for a large earthquake that could impact Mexico City 175 miles away, according to a new study led by CU-Boulder.

A reversal of tectonic plate motion between Acapulco and Mexico City in the last half of 2006 probably didn’t ease seismic strain in the region or the specter of a major earthquake anticipated there in the coming decades, says a University of Colorado at Boulder professor.

Instead of creeping toward Mexico City at about one inch per year – the expected speed from plate tectonic theory – the region near Acapulco moved in the opposite direction for six months and sped up by four times, said CU-Boulder aerospace engineering Professor Kristine Larson. The changes in motion were detected by analyzing data from GPS satellite receivers set up in Guerrero, Mexico, that were installed by the National Autonomous University of Mexico (UNAM) under the direction of UNAM geophysicist Vladimir Kostoglodov and augmented by CU-Boulder.

“The million-dollar question is whether the event makes a major earthquake in the region less likely or more likely,” said Larson, whose research is funded in part by the National Science Foundation. “So far, it does not appear to be reducing the earthquake hazard.”

A paper on the subject by Larson, the University of Tokyo’s Shin’ichi Miyazaki and UNAM’s Kostoglodov and José Antonio Santiago was published Aug. 1 in Geophysical Research Letters.

Scientists use GPS satellite receivers to record laser pulses from spacecraft to measure tiny movements in Earth’s crust.

The question of earthquake hazard is particularly important for Guerrero, since it is located 175 miles southwest of Mexico City, Larson said. “A very large earthquake in Guerrero would produce seismic waves that would travel quickly to the Mexican capital, and since Mexico City is built on water-saturated lakebed deposits that amplify seismic energy, the results would be catastrophic,” she said.

In 1985, a magnitude 8.1 earthquake triggered by the Cocos Plate dipping under the North American Plate off the west coast of southern Mexico struck along the coast north of Guerrero and killed 10,000 people in Mexico City, injured about 50,000 and caused an estimated $5 billion in property damage.

Since the last major earthquake in northwest Guerrero was a 7.6 magnitude event in 1911, many scientists think the area is ripe for a much larger earthquake, likely in the range of 8.1 to 8.4, Larson said. Geophysicists refer to the impending earthquake as the “Guerrero Gap,” she said.

“Before GPS we thought the ground moved at a constant speed between earthquakes,” Larson said. “The recognition of these transient events where the plate reverses direction is arguably the most important geophysical discovery that has stemmed from the introduction of GPS measurements.”

The Guerrero slip events recorded by Larson and Kostoglodov’s research team in 2006 are the largest ever reported in the world.

Studies of the Guerrero Gap are helping scientists better understand other subduction zones around the world, including the Cascadia region off the coast of Washington and Oregon, Larson said. Smaller but much faster backwards slip events have occurred there, as have very large earthquakes in previous centuries.

Alaskan Earthquake In 2002 Set Off Tremors On Vancouver Island

Alaskan Coast Line - Photo Credit: Web Doodle, LLC
Alaskan Coast Line – Photo Credit: Web Doodle, LLC

Perhaps it was just a matter of sympathy, but tremors rippled the landscape of Vancouver Island, the westernmost part of British Columbia, in 2002 during a major Alaskan earthquake. Geoscientists at the University of Washington have found clear evidence that the two events were related.

Tremor episodes have long been observed near volcanoes and more recently around subduction zones, regions where the Earth’s tectonic plates are shifting so that one slides beneath another. Tremors in subduction zones are associated with slow-slip events in which energy equivalent to a moderate-sized earthquake is released in days or weeks, rather than seconds.

Now researchers studying seismograph records have pinpointed five tremor bursts on Vancouver Island on Nov. 3, 2002, the result of a magnitude 7.8 earthquake on the Denali fault in the heart of Alaska.

As surface waves, called Love waves, shook Vancouver Island they triggered tremors underneath the island in the subduction zone where the Explorer tectonic plate slides beneath the North American plate. The tremors were measured by seismometers along roughly the northern two-thirds of the island.

“What we found is that when the waves pushed the North American plate to the southwest, the tremor episode turned on and when the motion reversed it turned off,” said Justin Rubinstein, a UW postdoctoral researcher in Earth and space sciences and lead author of a paper describing the work published in the Aug. 2 edition of Nature.

Though the Denali quake was mostly felt in Alaska, its effects were apparent thousands of miles away. It sloshed lakes from Seattle to Louisiana, muddied wells as far east as Pennsylvania and triggered small earthquakes in seismic zones across the Western United States.

Still, finding evidence of tremors on Vancouver Island was unusual.

“A few people have seen tremor episodes triggered by earthquakes, but not as clearly as we have. This is by far the clearest and easiest to interpret,” said co-author John Vidale, a UW professor of Earth and space sciences and director of the Pacific Northwest Seismic Network.

“This shows us it’s just like a regular fault — you add stress and it slips,” Vidale said. “It’s like regular faulting but on a different time scale.”

Other authors are Joan Gomberg of the U.S. Geological Survey in Seattle and UW researchers Paul Bodin, Kenneth Creager and Stephen Malone.

An earthquake typically will appear suddenly on a seismograph, while the much more subtle ground motion from a tremor burst gradually emerges from the background noise and then fades again, Rubinstein said.

By comparison, tremors typically produce the strongest seismic signals in a slow-slip event, in which seismic energy is released very gradually during periods as long as three weeks.

In this case, the authors suggest that the force of the Love waves induced slow slip on the interface between the North American and Explorer tectonic plates near Vancouver Island and triggered the tremor bursts, each lasting about 15 seconds.

“That made it easier for us to observe because there were these five distinct bursts,” Rubinstein said. “Normally you are not going to feel these tremors. The shaking in the tremors we observed was 1,000 times smaller than the surface waves from the earthquake.”

Being able to spot the tremors was largely a matter of distance and timing, Vidale said.

“We were able to separate the tremor signal from that of the distant earthquake because the surface waves had traveled more than 1,200 miles, losing the high-frequency vibrations that would have masked the high-frequency tremor vibrations,” Vidale said.

While the tremors were recorded a great distance from the rupture that triggered the Denali earthquake, the scientists suggest the same process could occur closer to the fault and might actually be important in the rupture process.

Seismograph data for the research came from the Canadian National Seismograph Network and was distributed by the Geological Survey of Canada.

Scientist Studies Minnesota’s Rock In Antarctica

An intrusion (the forcible entry of molten rock or magma into or between other rock formations) in Antarctica. Unlike Minnesota, geologists get a perfectly clear view of intrusions in Antarctica.
An intrusion (the forcible entry of molten rock or magma into or between other rock formations) in Antarctica. Unlike Minnesota, geologists get a perfectly clear view of intrusions in Antarctica.

Geologists learn by looking at rocks. Of course, it’s not that simple. Here in Minnesota, the tapestry of mineral-laden geology lies buried under forests, soils and parking lots. This makes Dean Peterson’s job difficult. As one of the economic geologists at the University of Minnesota, Duluth’s Natural Resources Research Institute (NRRI), his job is to understand the state’s geology–where and what types of ore minerals were deposited some 1.1 to 2.7 billion years ago. In Minnesota, geologists figure it out by reading scattered outcroppings and drilling holes. It’s doable, but it’s difficult.

So when Peterson was offered an opportunity to spend a month in Antarctica’s Dry Valleys, he jumped at the chance. Yes, that’s a long way from Minnesota, but surprisingly, the geology is the same. Both areas were focal points of dynamic magmatic systems associated with continental rifting-molten rock flowed up from the earth’s mantle, forming intrusions in the upper crust. The geologic setting was the same.

But the beauty of Antarctica for geologists is the 100 percent exposure of rock. They can look at layer upon ancient layer of deposits, up to 10,000 feet high. In Minnesota, the Duluth Complex, a large, composite of mafic rocks (rich in dark-colored minerals like magnesium and ireon) in northeastern Minnesota, was the hot spot for dynamic magmatic molten movement. It’s where NRRI’s economic geologists go to identify valuable mineral deposits.

Understanding local deposits

“In the Duluth Complex, I study the ‘plumbing’ of the intrusions. That’s the key to finding the higher grade ore deposits,” says Peterson. “So in the Dry Valleys I can actually see how the magma moves up from the earth’s crust, how it crosses certain rock bodies, and where it picks up sulfur to form sulfide minerals. In Antarctica I could see the ‘plumbing’ that I can’t see in Minnesota.”

If that wasn’t exciting enough for Peterson (and it was) he also spent a month with one of the most renowned geologists in the country, Bruce Marsh of Johns Hopkins University.

Did you know?

Antarctica is the coldest, windiest, and harshest continent. The continent is covered in continuous darkness during the austral winter and continuous sunlight in the summer. (The average annual temperature is -56°F at the Amundsen-Scott South Pole Station, the southernmost continually inhabited place on the planet).

Source U.S. Antarctic Program

“Spending time seeing this fabulous geology and learning from Dr. Marsh is really something special,” says Peterson.

Paul Morin, a visualization expert in the geology and geophysics department on the U’s Twin Cities campus, and researchers from Poland and Slippery Rock University in Pennsylvania joined Peterson on the expedition. The trip was funded by a grant from the National Science Foundation.

From Peterson’s travel notebook:

  • Antarctica is not as cold as people might think. Temperatures were, on average, in the 20s to 30s Fahrenheit and sometimes down to 10 at night, but we got used to it right away. After a day we were in shirtsleeves and a windbreaker. The sun is always out and intense.

  • When the wind stops blowing there is utter silence. There is nothing to make a noise. It’s eerie at first, but then I got used to it. The silence really gives you time to think. When we went back to McMurdo (U.S. Field Station) the noise created by 1,100 people living in close quarters was unbelievable.

  • Humans have evolved in humid environments where water vapor in the atmosphere selectively absorbs light–as you look into the distance things get bluer and bluer. We unconsciously perceive distance using the air’s absorption of light. Antarctica is the driest place on earth. The humidity in the Dry Valleys averages about 1 or 2 percent. The air’s dryness adds an additional dimension to an Antarctic experience–light doesn’t change color with distance. Mount Erebus, 120 miles away, will look exactly like it would if you were right next to it. It’s hard to visually calculate any distance.