Cracking Quakes and Other Earthy Matters
The past few years have witnessed a number of devastating earthquakes, such as the Great Sumatra-Andaman earthquake in 2004 that generated tsunamis as high as 100 feet, shook the planet by as much as 1 centimeter and triggered other earthquakes as far away as Alaska. That earthquake and others — more recently, several that hit China — have caused billions of dollars in damage and widespread loss of life. While most occurred in Asia, few places are immune. Tremblors have been felt in the United States — not just in expected locales such as California and Alaska, but also in the Midwest.
We may not be able to control cataclysmic earth events, but understanding them can lead to prediction that could minimize their impacts. Rice Earth scientists and others have made some headway toward deciphering how and why earthquakes and volcanic activity take place, and one may just have found a way to predict when a quake is imminent.
Continental Staying Power
Continents seem so substantial that it’s easy to forget they’re simply big chunks of rock floating around on the Earth’s viscous mantle. The movement can lead to earthquakes, but other processes also are at work. Geologists know, for example, that continents have come and gone during the Earth’s 4.5 billion years. As of now, however, there are more theories than hard data about some of the key processes that govern continents’ lives.
One thing we do know is that continents ride higher than oceans. This is partly because the Earth’s crust is thicker beneath continents than it is beneath oceans. It also is the result of the continents losing their magnesium and calcium to the oceans, leaving behind lighter, silicon-rich rock that is buoyed up by the denser rock beneath the Earth’s crust.
Rice geologist Cin-Ty Lee always assumed that processes deep within the Earth accounted for most of the magnesium loss. One of these processes is delamination, in which dense, magnesium-rich magma wells up beneath continent-feeding volcanoes then ultimately sinks back into the Earth’s interior. Lee already had learned from previous research that about 40 percent of the magnesium in basaltic magma was lost to delamination, but a bit of laboratory serendipity made him take a closer look.
In measuring the lithium content of the granitic rocks, Lee noticed that lithium tends to behave like magnesium, and he realized he could use lithium as a proxy to find out how much magnesium continents had lost due to chemical weathering. He was surprised to find that chemical weathering alone accounted for 20 percent of the magnesium loss.
“Weathering occurs in just the top few meters or so of the Earth’s crust, and it’s driven by the hydrosphere, the water that moves between the air, land and oceans,” Lee said. “It appears that our planet has continents because we have an active hydrosphere — if we want to find a hydrosphere on distant planets, perhaps we should look for continents.”
Too Hot to Move
The planet Venus provides evidence for Lee’s hypothesis. Venus’ surface, which shows no outward signs of tectonic activity, is bone dry. It also is heavily scarred with volcanoes. Scientists have long believed that Venus’ crust, lacking water to help lubricate tectonic plate boundaries, is too rigid for active plate tectonics. But Rice Earth scientist Adrian Lenardic thinks something else might be at work there.
Conventional wisdom holds that plate tectonics is both stable and self-correcting and that the stresses generated by a flowing mantle help keep tectonic plates in motion. But that view relies on the assumption that excess heat from a planet’s mantle can efficiently escape through the crust. Lenardic has recently completed a study that suggests a planet’s mantle can become less viscous if it heats up. In fact, prolonged heating of a planet’s crust via rising atmospheric temperatures can shut down plate tectonics and cause a planet’s crust to lock in place.
“The heat required is far more than anything we expect from human-induced climate change,” Lenardic said, “but things like volcanic activity and changes in the sun’s luminosity could lead to this level of heating. Our goal was to establish an upper limit of naturally generated climate variation beyond which the entire solid planet would respond.”
Lenardic said the research team wanted to better understand the differences between the Earth and Venus and establish the potential range of conditions that could exist on Earth-like planets in the solar system and elsewhere in the universe. The findings may explain why Venus evolved differently from Earth. The two planets are close in size and geological makeup, but Venus’ carbon dioxide–rich atmosphere is almost 100 times more dense than the Earth’s and acts like a blanket. As a result, Venus’ surface temperature is hotter than that of even Mercury, which is only half as far from the sun.
The research team wanted to better understand the differences between the Earth and Venus and establish the potential range of conditions that could exist on Earth-like planets in the solar system and elsewhere in the universe. The findings may explain why Venus evolved differently from Earth.
The team discovered that Earth’s plate tectonics could become unstable if the surface temperature rose by 100 degrees Fahrenheit or more for a few million years.
“The time period and the rise in temperatures, while drastic for humans, are not unreasonable on a geologic scale, particularly compared to what scientists previously thought would be required to affect a planet’s geodynamics,” Lenardic said.
One of the most significant findings in the new study is that the atmospheric heating needed to shut down plate tectonics is considerably less than the critical temperature beyond which free water could exist on Earth’s surface.
“The water doesn’t have to boil away for irrevocable heating to occur,” Lenardic said. “The cycle of heating can be kicked off long before that happens.”
The researchers also found that a spike in volcanic activity could accompany the initial locking of the tectonic plates. This might explain the large percentage of volcanic plains that are present on Venus.
Targeting a Tsunami Zone
Volcanoes generally seem innocuous compared with the devastating shakings that emanate from subduction zones — places where one tectonic plate slides beneath another and recycles back into the Earth’s molten mantle.
Earthquakes often occur in subduction zones when plates that normally move smoothly across one another lock, causing stress to build. When the lock breaks and the plates jolt past each other, the sudden release causes the earth to shake. If the shock occurs beneath the sea floor, it can create a tsunami.
One infamous subduction zone, the Nankai Trough, located on the floor of the Pacific Ocean about 100 miles from Kobe, Japan, has been responsible for numerous earthquakes and tsunamis and likely will cause more.
The morphology of the Nankai Trough interests Rice Earth scientist Dale Sawyer, who was part of an international team that spent eight weeks aboard the new scientific drilling vessel Chikyu collecting data on a particularly troublesome zone deep beneath the trough.
“Earthquakes don’t nucleate just anywhere,” Sawyer said. “While the slip zone for quakes in this region may be hundreds of kilometers long and tens of kilometers deep, the initiation point of the big quakes is often just 5 to 6 kilometers below the seafloor. We want to know why.”
The drilling done by Sawyer and colleagues marked the beginning of a massive project dubbed the Nankai Trough Seismogenic Zone Experiment. The project is organized by the Integrated Ocean Drilling Program (IODP), an international scientific research program dedicated to advancing scientific understanding of the Earth by monitoring and sampling subseafloor environments. In addition to drilling across the fault in the Nankai Trough, the scientists also hope to sample the rocks and fluids inside the fault, and they want to place instruments within the fault zone to monitor activity and conditions leading up to the next great earthquake.
Sawyer said scientists with IODP plan to return to the Nankai Trough each year through 2012, with the ultimate goal of drilling a six-kilometer-deep well to explore the region where the quakes originate. If they succeed, the well will be more than three times deeper than previous wells drilled by scientific drill ships, and it will provide the first direct data from this geological region.
Seeking Seismic Signs
Rice seismologist Fenglin Niu and his colleaques have used a similar well in California to uncover information that could change quake prediction forever.
The famed San Andreas fault runs through San Francisco and around Los Angeles, and about halfway between the cities lies the town of Parkfield, where the U.S. Geological Survey has been collecting seismic data for almost 40 years. Niu and his colleagues set up shop there, at the San Andreas Fault Observatory at Depth, a deep well seismologists use to make direct measurements of the fault.
“Almost everything we know about the deep interior structure of the Earth comes from seismic waves, the elastic waves of energy that are released during earthquakes,” said Niu, whose work has earned a prestigious Early CAREER Development Award from the National Science Foundation’s EarthScope Program.
Changes in the rock structure are critical for predicting earthquakes. When rocks are compressed, the stress forces air out of tiny cracks, causing seismic waves to travel slightly faster through the rock. Detecting stress changes before an earthquake has been the Holy Grail of earthquake seismology.
Today’s state-of-the-art earthquake warning systems give only a few seconds’ warning before a quake strikes. These systems detect P-waves, the fastest-moving seismic waves released during a quake. Like a flash of lightning that arrives before a clap of thunder, P-waves precede slower-moving but more destructive waves.
In an attempt to obtain readings that would give warning much further in advance of the event itself, Niu and his colleagues employed precision instruments built by collaborators at Lawrence Berkeley National Laboratory (LBNL). These sensors can measure the minute changes in the time — sometimes just tens of billionths of a second — that it takes seismic waves to travel through the rock along a fixed pathway beneath Parkfield. The instruments are so sensitive that, although they were more than half a mile below ground, they could measure fluctuations in air pressure at the Earth’s surface.
Changes in the rock structure are critical for predicting earthquakes. When rocks are compressed, the stress forces air out of tiny cracks, causing seismic waves to travel slightly faster through the rock. Detecting stress changes before an earthquake has been the Holy Grail of earthquake seismology. Only recently, however, has technology improved sufficiently to make the necessary precision and reliability possible.
In analyzing the data from the new sensors, Niu and his colleagues found that a distinct change occurred in the rock before each of the minor Parkfield-area earthquakes during the test period. A measurable change preceded a magnitude 3 quake by 10 hours. This was the largest local event during the observation period. A smaller but closer magnitude 1 temblor five days later was preceded by a signal about two hours before the quake.
“We’re working with colleagues in China and Japan on follow-up studies to determine whether this physical response can be measured in other seismically active regions,” Niu said. “Provided the effect is pervasive, we still need to learn more about the timing of the signals if we are to reliably use them to warn of impending quakes.”
The study’s other co-authors include Paul Silver of the Carnegie Institution for Science’s Department of Terrestrial Magnetism, Rice graduate student Xin Cheng and LBNL scientists Tom Daley and Ernest Majer. The research was supported by the National Science Foundation, Rice, the Carnegie Institution and LBNL, and it appeared in the journal Nature.