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Reconstructing the History of Earthquakes, Mountains, and Volcanoes

A visit with Mian Liu, Professor of Geological Sciences

By Tammy Ritterskamp
Published: - Topics: San Diego Supercomputer Center geology physics database geophysics
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Becoming a geologist was not the original aspiration for Mian Liu, Professor of Geological Sciences. The Chinese government assigned him to the discipline when he was 17 years old, a course of study he later followed at Nanjing University. His initial lack of interest in geology had much to do with the way the subject was taught. “The focus was not on understanding the processes; we were forced to memorize lots of facts,” he explains. Instead, Liu’s earliest interest was in physics, which “just seemed more intuitive.” He began sitting in on a variety of lectures and found that he preferred learning about geophysics, the physics of the Earth, eventually earning a Ph.D. in that area from the University of Arizona.

Liu is currently a teacher and researcher in the interdisciplinary field of geophysics. He explains to his students that “anything you are interested in you can find in geosciences. If you want to study physics, we have the most interesting physics on Earth. You can study fluid dynamics in the Mechanics Department, but you can also come here to study fluid mechanics deep inside the Earth. The flow of Earth’s mantle is responsible for a lot of things you see on Earth today, including earthquakes, mountain building, and volcanic eruptions.” Even biology: if you like biology, says Liu, you can immerse yourself in geobiology, and use biological processes to treat environmental contaminations such as those around gas stations. He concludes that for “almost any discipline you can find an application in this Earth system.”

Earthquake studies are a relatively new interest for Liu. His doctoral dissertation research focused on Hawaiian “hotspots”—areas that have experienced active volcanoes over a long period of time. He describes the process as follows: “Basically we think the Pacific plate has been moving over a hotspot. When the plate is sitting on the hotspot, such as the Big Island is right now, the hotspot is pumping heat into the crust and producing these volcanoes. As the plate moves away, old volcanoes wane and new ones form. You can see from the submarine topography a very nice chain of volcanoes formed in the center of the Pacific Ocean.”

As a postdoctoral fellow, Liu studied mantle convections, trying to understand convective flow in the Earth’s mantle, a force that amounts to the “primary driving mechanism of everything we see on Earth today.” The planet is restless with volcanoes, earthquakes, and mountain-building, and all of these phenomena happen because the inside of the earth is too hot, causing its natural cooling process to drive convection in the mantle.

When he came to MU in 1992, Liu shifted his interest once again, this time to continental dynamics. With his background in geology, he was able to employ geophysical methods to study the formation of the Himalayan-Tibetan Plateau, the Andes, and the North American Rockies.

It was easy for Liu to start studying earthquakes in Missouri because of the famous New Madrid seismic zone in the southeastern part of the state. “In 1811 and 1812, during a period of three months, that area experienced at least three major earthquakes,” he recounts. “The estimated magnitude was originally over eight, which is very large. [One earthquake] rang the bell in Philadelphia and momentarily reversed the water flow in the Mississippi river. Since then, we have had more than a dozen middle-sized earthquakes, enough to shake your bookshelf, cause books to drop, or knock things off the wall.” Since 1977, more than 4,000 small earthquakes have been recorded in the New Madrid area.

The puzzling geological question is why these earthquakes keep occurring in a place where they aren’t supposed to occur. Liu explains that the mainstream geological theory of plate tectonics “predicted that most earthquakes would happen along the boundaries of tectonic plates.” For example, many earthquakes occur in California, where the Pacific plate moves against the North American plate along the San Andreas Fault. “The same theory says the interior of these plates should be stable, and the New Madrid seismic zone is in the middle of the North American plate,” he continues. But the theory doesn’t answer all of our questions; intraplate earthquakes do happen, and, as Liu realistically observes, “many aspects of intraplate earthquakes we just don’t know about.”

Two methods have been used to study intraplate earthquakes in the New Madrid seismic zone. One is to measure precise crustal motion around the fault zone via GPS (Global Positioning System) technology. Another is to reconstruct the history of earthquakes from the actual geological record. The U.S. Geological Survey has been doing such reconstructions by digging trenches in certain areas and looking for sand blows (sand squeezed out of the earth as a result of large earthquakes). Interestingly, these two research methods have produced quite different results. The GPS measurements indicate little to no strain accumulation in the area, suggesting little chance for repeating a large earthquake in the area soon, while trench-digging has revealed evidence of large earthquakes happening numerous times during past millennia, recurring on average about every 500 years.

Liu seeks to address these problems by studying other areas of the world where the same odd seismic phenomena occur. One of the most promising places is in his native country. Northern China has the largest number of active intraplate earthquakes in the world, and scholars have maintained a relatively complete historical record of these events that dates back about 2,000 years, much further than the records we have in North America. In 1556, for instance, an earthquake in northern China killed 830,000 people and holds the record as the deadliest in human history. In 1976 an earthquake killed 244,000 people and wiped out the entire industrial city of Tangshan. All together, over 50 large earthquakes have been recorded in northern China during the last 700 years.

Liu and colleagues at MU have been conducting a pilot study of this region with Chinese partners, looking for various kinds of evidence. “An earthquake is a result of a sudden slip on fault zones,” he observes. “So you need to first know exactly where the fault zone is and what is the fault structure.” For the past two summers, his group has traveled to China to install seismometers to gather data. Since they cannot view the earth’s interior directly, they employ these devices to take images of the subsurface structure. As he puts it, “we use seismic waves similar to what a doctor uses in X-rays or CAT scans. Earthquakes send seismic waves traveling through different parts of the earth and they reach your stations. So you play detective to reconstruct the Earth structure.”

The team then applies GPS technology to analyze earthquakes through their visible symptoms. Because “an earthquake results from relative crustal motions across fault zones,” says Liu, “we can monitor crustal motion using GPS.” Their Chinese partners have set up a network of these stations in northern China to make measurements in tiny increments–millimeters per year–deriving “a very precise velocity field to see exactly how the crust has been moving.” But even that level of precision is not enough, since this method covers an extremely short period of time. In New Madrid, for example, the GPS work has been going on for only a decade. Describing the situation in geological terms, Liu cautions that “ten years is an instant for fault motion that is slow, long, and time scale-dependent.”

To make up for that built-in shortcoming, a third approach, Liu’s specialty, involves computer simulations. As he notes, “we don’t have time to wait for [earthquakes] to happen again in a thousand years!” The data from studies of Earth structure, rate of crustal motion, and geometry of the faults are entered into a computer, and Liu and his students then mathematically simulate the long-term process of fault motion and earthquakes. In this regard Liu feels lucky to have an excellent team to work with at the University of Missouri-Columbia. While he specializes in computer modeling, his colleague Dr. Eric Sandvol is a seismologist who images Earth structure using seismic waves, and Dr. Francisco Gomez studies crustal motion using GPS and other techniques. The newest member of the team, Dr. Marie Cormier, is a marine geologist who studies earthquakes from marine records.

As if Liu’s “plate” isn’t full enough, he also has his hand in several research “side dishes,” one being a collaboration with another MU colleague, Dr. Peter Nabelek, on metamorphism, “the change in mineral composition of rocks caused by pressure and temperature changes” and “contact metamorphic aureole” caused by magma intrusion into the crust. Liu has another project supported by the National Science Foundation: GEON (Geoscience Network, www.geongrid.org), a five-year, $12-million dollar initiative involving dozens of institutions along with the San Diego Supercomputer Center. “The goal of this research is to establish a prototype cyberinfrastructure for geosciences,” Liu explains. He compares GEON to the search engine Google, in the sense that one can enter keywords and search the database for a particular type of material, with the result that “any scientist can have the desired data at his or her fingertips.” The GEON project is currently in its final year, and the prospects are promising. At the outset Liu wasn’t certain how long the project would take, but now, he says, “I am now very optimistic about it.”