News FocusSolid-Earth Science

Geophysical Exploration Linking Deep Earth and Backyard Geology

Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1283-1285
DOI: 10.1126/science.340.6138.1283

Big Science came to solid-Earth studies when the $400 million EarthScope program offered a sharper view of the interior that could help geologists; it's working, mostly.

RALEIGH—Geologists tramping over the volcanic rocks of Montana's Yellowstone or the high Colorado Plateau in the 1960s had little patience with the geophysicists trumpeting what would soon be called plate tectonics. What could seafloor spreading at the bottom of the Atlantic Ocean possibly have to do with the maddeningly complicated jumble of rocks that is North America?, the continental geologists asked.

Soon enough, however, the geophysicists won them over. Plate tectonics did neatly explain some major geologic features, such as the Appalachian Mountains and the volcanoes of the Pacific Northwest. But 30 years after the plate tectonics revolution, geologists still weren't content. What about fiery Yellowstone in midcontinent, the cryptically elevated Colorado Plateau, or any number of other quirky geologic features that still weren't fitting into the big picture of plate tectonics?

That's better.

Seismic data from EarthScope's Transportable Array sharpened existing imaging (left) so that colder (blue) and hotter (red) features in the mantle stand out (right).


So around the turn of the century, geophysicists decided how they might finally satisfy the geologists. EarthScope, a proposed four-pronged program, would point a geophysical "telescope" inward to bring the subsurface of North America into unprecedentedly sharp focus. A wave of 400 seismometers would roll from coast to coast imaging the deep interior. A net of GPS instruments and a radar satellite would precisely gauge the grinding of plates along the West Coast from Mexico to Alaska. And a single borehole in central California would probe the heart of the threatening San Andreas fault.

No one could say just what these largely undirected explorations would discover, geophysicists conceded, but explanations for some of geologists' lingering mysteries would surely emerge as geophysicists brought into focus deep details approaching the scale at which geologists work on the surface.

Ten years on, EarthScope is looking like a good investment. "The amount of very high-quality data and science generated by EarthScope is spectacular," says geophysicist Jean-Bernard Minster of the Scripps Institution of Oceanography (SIO) in San Diego, California. "EarthScope was the right idea. We've learned a lot more about Earth the last 10 years than the previous 30 to 40 years. I can say that because I don't get any money from EarthScope."

Beneath North America, it turns out, creatures of the deep Earth roam. From a hot, rising, deep-rooted plume fueling Yellowstone to a cold falling "drip" helping unloose the Colorado Plateau, these deep features do indeed shape geology at the surface. Not that all was smooth sailing. Drilling into even tiny earthquakes proved particularly challenging. But as the program's wave of traveling seismometers washes up on the East Coast, the pleasant surprises are far outweighing the shortfalls.

Going big science …

From the start, EarthScope was unlike other projects built through the National Science Foundation's (NSF's) Major Research Equipment and Facilities Construction account. At $197 million for construction over 5 years, it was certainly major. But compared with physicists building a gravity wave detector or astronomers an observatory, geoscientists have "a cultural core that is fundamentally different," geophysicist David Simpson, president of the Incorporated Research Institutions for Seismology in Washington, D.C., told the biennial EarthScope National Meeting here last month.

EarthScope's facility has not one but thousands of instruments of several sorts distributed coast to coast and from the Mexican border to Alaska. There are no EarthScope principal investigators in line for a Nobel Prize, only a community-based management structure. EarthScope data are not held for analysis by a select few, but immediately available to anyone anywhere in the world. And its "scope" will not be scanning the interior for any particular Holy Grail discovery, just looking around.

"If you have lots of good data," observed seismologist Edward Garnero of Arizona State University, Tempe, "discoveries flow." Those discoveries, researchers reason, will almost certainly help them understand the North American continent and help predict earthquakes and volcanic eruptions as well.

To get the big picture

For EarthScope's deepest and broadest look into Earth, seismologists rolled out USArray: 500 seismometers to record seismic waves passing through and altered by the rock beneath North America. Plenty of other seismometers were already in operation, but most were bunched around dangerous, quake-producing faults in the West.

US Array ($69 million for construction and $88 million for operations so far) took a much broader view using more sophisticated instruments. Researchers spread 100 seismometers evenly across the lower 48 states in a regular grid with 300 kilometers between them. Those would remain where they were installed. But then, hard against the Pacific coast, geoscientists deployed 400 more seismometers in an 800-kilometer-wide swath between the Canadian and Mexican borders with 70-kilometer spacing.

Got it covered.

The EarthScope program was intended to sharpen geophysicists' view of the interior by using more uniform and denser instrument networks. GPS stations (yellow circles) gauge building crustal strain. Seismometers allow mantle imaging (triangles: black, green, and red are in use; pink are future and white past transportable sites; blue are in temporary local study networks). Red star is core-drilling site.


Then they marched this 400-strong Transportable Array (TA) eastward, one station at a time. After a seismometer had been operating 2 years, field engineers would extract it from its half-buried vault on the trailing edge of the TA and reinstall it to the east on the leading edge, one instrument every day or two for 10 years until a 2000-strong instrument grid reaches the Atlantic coast; that should happen in September. Just as more pixels in an electronic camera yield a sharper image, the dense, continent-spanning TA grid would produce clearer views of the subsurface.

And it's working. "The things being seen are much smaller," Garnero says, by an order of magnitude. And the greater detail is showing that "the mantle is doing stuff." Seismic analyses can construct 3D images of the interior based on how hotter or colder rock slows or speeds up seismic waves from distant quakes. Earlier, fuzzier seismic images had already shown hotter-than-normal rock a few hundred kilometers beneath Yellowstone. But images constructed with the sharper TA data show that the hot rock extends through the upper mantle beneath the North American plate and into the lower mantle at least 900 kilometers down. That is the first rising, hot-rock plume that most seismologists can agree is rooted in the lower mantle (Science, 5 April, p. 22).

The Yellowstone plume most likely starts at the mantle's bottom, hard against the molten-iron core, but it obviously hasn't had an easy rise. As imaged by seismologist Brandon Schmandt of the University of New Mexico, Albuquerque, and his colleagues, the plume seems to have encountered resistance about 600 kilometers down that created a gap in the plume. The obstacle appears to have been a sheet of tectonic plate that dove into the mantle beneath the western edge of North America tens of millions of years ago and then broke apart. In another study using TA data, seismologist Eugene Humphreys of the University of Oregon in Eugene and his colleagues suggested that the sinking slab broke apart when a chunk of particularly thick descending plate jammed at the continent's edge about 50 million years ago (Science, 14 January 2011, p. 142).

Indeed, slabs of sunken plate can now be clearly seen dangling here and there around the western lower 48. Schmandt and others have pointed out that sinking slabs would have let hot mantle rock invade the shallow mantle until the hot, melting mantle rock came up against the continental plate. TA's improved seismic images do seem to imply such local mantle upwellings. These often coincide in space and time with major episodes of volcanic outpourings such as the outbursts that formed the Idaho Batholith and the Columbia River flood basalt, suggesting a connection, researchers say.

And then there are the drips. These are the antiplumes, the bottommost layers of the continental plate that peel away or slough off and sink because they are colder and therefore denser than adjacent mantle. Relieved of the burden, the overlying plate can rise. One drip clearly seen in TA data probably let the southern Sierra Nevada float upward. Using TA data, seismologist Alan Levander of Rice University and colleagues have shown how drips off the bottom of the Colorado Plateau have helped let it rise to its 2000-meter altitude. The current drip has been falling away for the past 6 million years, they find. What with a rising plume, sinking drips, and melting mantle boring into the continent, Humphreys says, "we're remaking continents."

Probing the plate boundary

The rest of EarthScope took a broad but exceedingly shallow look at the western boundary of the North American plate, with a single, pinpoint boring into the San Andreas. In addition to the seismic networks of USArray, the 2001 EarthScope Project Plan proposed two components that would focus on the zone from Mexico to Alaska where the Pacific plate grinds by the North American plate. Only one of the proposed components came to pass.

The Plate Boundary Observatory (PBO) component of EarthScope ($100 million for construction and $58 million for operations so far) consists of 1100 GPS instruments installed in a broad swath of the western United States. They continuously measure their changing position to about 1 millimeter per year. Over the long haul, the two plates are moving by each other at about 30 millimeters per year. But if the plates snag on the San Andreas or on any of the broad maze of faults spanning the plate boundary, the stress would squeeze or stretch the surrounding crust, moving the GPS instruments. All told, PBO GPS continuously records the building strain—or sudden release of strain in quakes—at points separated by tens to hundreds of kilometers.

A deep grab.

Workers extract a core from 3 kilometers down in the SAFOD drill hole into the San Andreas fault. The core showed clay weakening the fault.


Unlike GPS, the second planned geodetic component—a satellite-borne interferometric synthetic aperture radar (InSAR)—would have mapped changing strain everywhere across the plate boundary, not just at widely separated points. An InSAR satellite can gauge the motion of contiguous spots on the surface a few tens of meters across with a precision of a few millimeters. With such spatial continuity, geodesists could have been far more precise about gauging the strain building toward failure on a particular segment of fault, geodesist David Sandwell of SIO told the meeting.

But a U.S. InSAR satellite with a focus on the western plate boundary was not to be, at least not yet. Although NSF could chip in some seed funding, researchers had to sell NASA on the idea. Minster has been the principal investigator on four InSAR proposals, "none of which worked," he says. "I'm not quite sure why."

But even without InSAR, EarthScope's PBO has been delivering. PBO observations have revealed the often complex release of quake energy along a fault. PBO strain data are also being fed into calculations of fault-by-fault earthquake hazards that will produce the next Uniform California Earthquake Rupture Forecast gauging risk.

And it turns out there's more shakin' than the San Andreas. In another bit of EarthScope serendipity, PBO observations have helped reveal a jiggling of the Great Basin of Utah and Nevada. The crust there is stretching east to west over millennia, but GPS is showing that over a few years, a part of the Great Basin can go from extending to contracting or the entire basin can shift to the east or west. Tectonophysicist Brian Wernicke of the California Institute of Technology in Pasadena and colleagues have argued that somewhere tens of kilometers beneath the surface, the rock above is able to slip across the rock below. That would mean the whole system could be responding to the churning of the mantle brought on by the disrupting sunken plate imaged by USArray.

A hole not far enough

The final EarthScope component took a far narrower view of North America: down a 15-centimeter-wide hole drilled next to the San Andreas in central California. Planners intended that the $27 million San Andreas Fault Observatory at Depth (SAFOD) would help understand the chemical and physical processes that cause the opposing sides of the San Andreas to quietly slip by each other in some places and stick, build stress, and fail in an earthquake in other places. Those are good things to know if you're wondering about the next Big One.

So SAFOD workers drilled a hole straight down 1.8 kilometers west of the fault and then angled it to intercept the fault at a depth of just over 3 kilometers. The plan was to punch out cores of rock across the fault and bring them to the surface, along with fault fluids, for analysis. Then workers would lower an instrument package down the hole to monitor fault conditions and to listen to the tiny quakes that pop off nearby.

Drilling through hard, broken rock is neither easy nor cheap, but SAFOD drillers managed to recover core from a weak section of the fault. Fifty years ago, geologists proposed that clay was lubricating the sides of the fault in such spots so the fault would not stick. Tectonophysicist David Lockner of the U.S. Geological Survey in Menlo Park, California, and colleagues confirmed that idea by showing that clay formed by alteration of the mineral serpentine had rendered the fault there slippery and "profoundly weak."

SAFOD efforts to understand how and why other fault segments stick and then break did not work out so well. Drilling problems created extra costs, according to SAFOD co-leader Mark Zoback of Stanford University. On top of that, the cost of even routine drilling soared as the growing Chinese economy sucked up raw materials and an oil-drilling boom in North Dakota drove up the cost of leasing a drilling rig. So NSF money for drilling ran out before a nearby stuck patch of fault could be cored. Then a leaky seal wrecked the instrument package days after it was lowered into the hole.

Still, "overall, I view EarthScope as very successful," says geophysicist Seth Stein of Northwestern University in Evanston, Illinois, excluding the satellite component that was never attempted. He was among the half-dozen researchers who first took the idea for an inward-looking scope to NSF. "Two out of three components were on time and on budget and are doing everything they were supposed to. Two point five out of three ain't bad."

And it isn't over. The USArray is only just now reaching the East Coast, and then it will be moved on to Alaska. For that matter, seismologists are still swamped by the volume of USArray data and are only beginning to look farther east and even deeper while applying more sophisticated analytical techniques. The mantle aquarium may soon have more inhabitants.


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