Volume 6, Issue 48 November 2009
Listening to the Earth
Barbara Romanowicz is a Berkeley professor of earth and planetary science and director of the Berkeley Seismological Laboratory. Photo courtesy Barbara Romanowicz
The last time you saw the doctor, chances are that he or she planted the chilly face of a stethoscope on your chest and listened. It's a technique Barbara Romanowicz can appreciate, though not a physician herself.
A Berkeley professor of earth and planetary science and director of the Berkeley Seismological Laboratory, Romanowicz has worked to deploy state-of-the-art seismic sensors to listen in on the planet's rumbles. "Just like you use x-rays and magnetism in medicine to find out what's inside the body, we use seismic waves—earthquakes—to image the deep structure of the Earth," Romanowicz says.
This method of studying the earth has produced the familiar "onion" model of its interior: a solid inner and liquid outer core, primarily of iron, surrounded by the solid mantle and surface crust. Romanowicz is developing ever more sophisticated methods to decipher seismic waves and identify places where textures and materials stray from the model's ideal.Her findings have yielded some spectacular insights into the ground beneath our feet. For example, earth scientists had identified two massive superplumes of upwelling rock, each 5,000 kilometers across at their origins but 10,000 km in diameter at the surface. One is located beneath the South Pacific, centered in the vicinity of Tahiti, and the other in southern Africa. By measuring the amplitudes of seismic waves, Romanowicz and colleagues showed that the plumes reach right to the base of the lithosphere, the rigid outer shell that makes up the tectonic plates.
The Berkeley Seismological Laboratory and Monterey Bay Aquarium Research Institute buried a broadband seismometer (within the cylinder) in the seabed 1000 m below the surface of Monterey Bay in 2002. In March of 2009, the MOBB (Monterey Bay Ocean Bottom Broad Band Observatory) was connected to the MARS data cable and now provides data in real time to the Northern California Earthquake Data Center. Image credit: courtesy Paul McGill, MBARI
Seismic waves travel at extremely low velocities through both areas, as would be expected of hot, possibly molten rock. Each is located at the same latitude but on opposite sides of the globe. "We're now struggling with the chicken and egg problem," Romanowicz says. "Are they there because plate tectonics have pushed them where they are, or are they driving how the plates move around?"
In 2003, a year later, she and her colleagues toppled dogma about how deep down the continental roots extend. Some seismologists had argued for lithospheric thicknesses of over 400 km. Romanowicz and students realized that their conclusions were due to the directional properties of rocks in the soft layer beneath the lithosphere. These asthenospheric rocks are strongly deformed, which affects how they transmit earthquake tremors. The researchers then reexamined the data, and calculated that the asthenosphere meets the lithosphere no more than 200-250 km deep.
These same directional properties are now helping Romanowicz spot new layers beneath the very oldest portions of North America. Called cratons, these geological formations tend to be located in the interiors of continents, and are surrounded by areas that get progressively younger nearer the ocean's edge. Some of the cratons in North America are estimated at 2.5 billion years old or more. But given that land is constantly being formed and recycled along the edges of tectonic plates, scientists have puzzled over why cratons have persisted for so long.
The extra layers of rock Romanowicz and colleagues have found beneath North America's cratons coincide with areas where the rocks were depleted of certain chemical elements due to melting. As a result, says Romanowicz, "they're actually lighter. These older portions of the continents are floating."
Romanowicz and colleagues traced the background hum detected by Pacific seismometers to seasonal storm strength. This image compares significant wave height, as measured by satellite (bottom plots) with maps of source regions for the hum (top plots) heard during northern hemisphere winter (left) and summer (right). Image credit: Barbara Romanowicz
This suggests that the continents were built in a process Romanowicz likens to jam making. "If you heat the fruit up very high, scum will form at the top that you have to stir to get rid of. At some point in the past, the Earth was very hot, and the scum that was formed were the cores of these cratons," she says.
Only after this period, she argues, did the present regime of plate tectonics begin. "Since then, 2.2 billion years ago, it's been the same as now, with oceanic plate subduction and the recycling of the oceanic lithosphere. But the continents have stayed because they formed around the scum."
Several years ago, Japanese scientists noticed a low background signal in their seismic recordings. Romanowicz used two seismic networks, one in Japan and the other in California, to listen in on the hum for herself. She found that the noise appears to come from the North Atlantic in our winter, but the Southern Ocean during our summer, implying the sounds come from storm waves. She is now developing an explanation for how horizontal wave energy is converted to vertical forces that pound the ocean floor like a drum.
When Romanowicz became director of Berkeley's Seismological Laboratory in 1991, she advocated keeping a continuous record of seismic recordings, including data that did not contain earthquakes. The discovery of the incessant hum, plus episodic, non-volcanic tremors that occur along the San Andreas Fault, validates Romanowicz's stance. "It pays not to throw things out," she says.