Safety first, science second when the lab shakes, rattles and rolls

If you are a geophysicist, catching an earthquake in the act is science heaven. And Chuck DeMets has been there twice.

For DeMets, a professor of geology and geophysics, his most recent brush with divine opportunity came on Jan. 22 in a Colima, Mexico, hotel room when a 7.8-magnitude earthquake sent a barefoot DeMets and his Mexican field assistant Patti Zamora scrambling for the safety of the open street.

“I felt this vibrating, and the hotel just started humming. I knew it was an earthquake, and all I could think of was that we had to get out of the building before the surface waves hit,” DeMets says.

In an earthquake, the surface waves are the last to arrive, after the primary and secondary waves. The surface waves, which roll across the landscape like Malibu breakers, are the waves to fear. They take the greatest toll on life and property.

“We got about 40 feet down the hall” when the surface waves struck, DeMets recalls. The hotel was only a few miles from the “damage epicenter” of the temblor.

With glass chandeliers swinging wildly and furniture crashing to the floor, the Wisconsin earthquake expert struggled to stay upright and escape the swaying building that was falling down around him. “It was hard to stand up. The floor was really moving.”

In the street, there was predictable chaos — broken buildings, water pouring from scores of ruptured rooftop cisterns, and people crying and standing in stunned silence. Thirty people died, more than 300 were injured and an estimated 10,000 people were left homeless, the greatest loss occurring among those who lived in the traditional adobe structures of Mexico.

In 1995, a magnitude-8 temblor — roughly the same size as the one that destroyed much of San Francisco in 1906 — killed 49 people in the same area and occurred along the same fault that DeMets has been studying for nearly a decade.

“This most recent event is actually the second large earthquake we’ve caught in the area,” DeMets says. “In October of 1995, the first large earthquake in 60 years occurred along this fault. For us, it was serendipitous because the goal of our experiment is to study precisely the fault where these earthquakes occurred.”

The area under DeMets’ microscope is a zone that stretches more than 900 miles along the west coast of Mexico. In this zone, the ocean bottom is sliding beneath the North American continent. The process is far from smooth, continuous or predictable. Great slabs of rock get hung up for decades, sometimes centuries, and eventually pop loose, triggering earthquakes like the one that chased DeMets into the streets of Colima.

“Ultimately, we’re trying to understand the earthquake cycle,” DeMets explains — how the earth behaves not only during earthquakes but how the earth along these fault lines creeps and slips between large seismic events. To help take the measure of that cycle, DeMets has set up a series of 30 geodetic stations above the fault, a place where three tectonic plates converge making the area a well-known seismic danger zone.

For the geoscientist, a large temblor marks the beginning and end of an earthquake cycle. It becomes a geophysical bookmark from which to begin new measurements and analyses along a fault. So, for DeMets and his colleagues, the latest earthquake in Colima marked the beginning of a new chapter in the study of this important fault.

“We had to rearrange the experiment to account for the earthquake,” DeMets says. “After getting through that 20 seconds of terror in the hotel, it became very exciting.”

For DeMets and Zamora, re-establishing contact with the three other field teams involved in the experiment, including UW–Madison research program manager Bill Unger and graduate student Stuart Schmitt, was the first priority.

The two-person teams were scattered across parts of the Mexican states of Colima and Jalisco. Each was responsible for tending to six geodetic stations. A station consists of a simple steel pin — one-half inch in diameter and 6 to 9 inches long — cemented into bedrock or concrete footings. The network set up by DeMets and colleagues is part of a large study of seismic activity along important faults.

“The trick is to find these things year-after-year and measure their positions,” says Unger, a man whose work in the field for more than four decades has taken him from Peru to Kenya to Mexico and the shores of Asia’s Lake Baikal, the world’s deepest lake.

“What we do,” said DeMets, “is track how these sites, these pins in the earth, move through time and to do that we use global positioning system technology. The technology is so precise you can track, in three dimensions, the very slow motion of the plates of the Earth.”

By tracking the movement of the pins over a period of years and analyzing the pattern defined by the motions of many sites, geophysicists can determine the relative motion of the plates along a fault.

For DeMets and others, trying to read the geophysical fine print of earthquakes and colliding tectonic plates, GPS technology has been a godsend: “This works really well. It has completely revolutionized geophysics over the past 15 years.”

The technology, says DeMets, promises new insight into plate motion and its relationship to earthquakes. The big question is, do earthquakes account for all of the motion across faults, or are there other processes that account for a lot of the motion that occurs as the big plates that make up the crust of the Earth slip past one another?

“What we’ve found is that earthquakes are not a major part of the story. Earthquakes, we think, account for only about 30 percent of the “slip budget’ of a fault.”

The bulk of motion across a fault, according to the latest thinking, can be attributed to “aseismic activity” — a spectrum of processes that occur so slowly they leave no seismic signature. A very slow heave of a plate, for example, may cause one or more of the pins in DeMets’ network to move a few millimeters or more in a year. Over time, this type of aseismic process could help balance the geophysical books as one adds it to the more violent earthquake column.

“We now have the technology to study these processes,” DeMets says. “Seismometers, the classic tool of geophysics, can only measure shorter impulse waves. They don’t give you resolution over time.”

But while GPS technology can fill in the temporal gray areas of the tectonic waltz, the earthquake provides the seismic exclamation point. And being on hand to witness nature’s application of it, and to rearrange one’s experiment to account for such a violent event, helps geophysicists like DeMets get a more accurate read of the processes that threaten the lives and homes of millions of people.