Shifting Ground: GNSS for Earthquakes
This is the third article in a series in xyHt introducing readers to scientists and technologists who work in pursuit of early-warning capabilities for earthquakes and volcanoes—in the increasingly aligning fields of seismology, geodesy, and surveying. You can access this series on xyHt‘s website by searching my name.
Glen Mattioli has been using geodetic techniques since 1995 to study plate kinematics, plate rigidity, inter-seismic and co-seismic plate boundary deformation, and active and quiescent volcanic systems.
Since June 2012, he has been director of geodetic infrastructure at UNAVCO, Inc., a non-profit university-governed consortium that facilitates geoscience research and education using geodesy. Previously, he was a professor of geosciences for nearly 25 years, most recently in the Department of Earth and Environmental Sciences at the University of Texas at Arlington.
Benchmarks and Time
Professional land surveyors and other surveyors who support the building of large engineering infrastructure—such as roads, dams, and bridges—have always used National Geodetic Survey (NGS) benchmarks and referenced their surveys to those fiducial points.
“In the last decade,” Mattioli says, “NGS, as well as various states—particularly California—have moved to an active spatial reference network, as opposed to one that depends on position estimates at some fixed epoch in time. That means that they’re defining not only the nominal position of some benchmark; they’re also defining how that benchmark is moving in space as a function of time.”
This is important, he argues, in places that are deforming substantially, especially when building very large, critical infrastructure, which requires knowing the positions of structural elements to better than a centimeter. This can only be achieved by referencing an external frame.
“In the past, most surveyors have let NGS define both the horizontal frame as well as the vertical frame. Both of those things are being redone.”
NGS announced in February that it is moving toward a model of an active spatial reference network for the entire country, to be implemented by 2022.