| Changes in river channel steepness were seen in the area where the quake had struck |
Terrain Tales: The May 12 quake provides compelling evidence that landscapes can encode information about the rates and patterns of tectonic activity.
The May 12 earthquake that struck the Sichuan Province in China appears to indicate that topography, or more specifically, changes in landscape gradients can be used to reveal seismic hazards of a region. Geophysical methods have traditionally been used to study tectonic activity and earthquakes.
When two continental plates push against each other, as in the case of the Indian and Eurasian plates, the crust crumples, resulting in the formation of mountain ranges.
Sudden steepness
But even in such rugged mountainous regions, there can be places where the ranges suddenly become hundreds of metres steep. Tectonic geomorphology, as it is called, is based on the premise that such sudden change in gradients reflects the underlying tectonic activity.
This is particularly derived by studying the relief and elevation of bedrock-based river channel network.
Dr. Eric Kirby, in a paper published online in the Nature Geosciences journal, reported systematic changes in river channel steepness and topographic relief in the area where the quake had struck.
The anomalous steepness of the river-channel profile “coincides almost exactly with the surface trace of the Yingxiu-Beichuan fault” that ruptured on May 12, reports the paper.
Dr. Kirby is from the Department of Geosciences, Penn State University, Pennsylvania, U.S., and is the first author of the paper.
The May 12 quake provides the “most compelling evidence to date that the landscape itself encodes information about the rates and patterns of tectonic activity,” the paper states.
First observed in 2003
The sudden change in topography in the Sichuan Basin was seen by Dr. Kirby and others way back in 2003. Their findings, published in the Journal of Geophysical Research, in 2003 noted that the Yingziu-Beichuan fault was more active than the surrounding faults.
Despite reservations by those relying on traditional methods in interpreting topography to understand tectonics, the researchers insist that the May 12 event “makes clear that landscape analysis can help with initial identification of active structure.”
Global positioning system (GPS) is one of the tools used for understanding the rate at which two plates move towards each other.
But the precision of GPS is highly dependent on the nature of the measurement and the length of time that an instrument occupies a position.
Campaign mode
Though the Sichuan Province has more than 20 GPS stations that were installed in the early 1990s, the measurements were not taken continuously.
“Measurements were all conducted on a campaign mode,” Dr. Kirby noted in his email communication to this Correspondent. In a campaign mode, a marker is installed in the ground, and is measured by GPS for 24-72 hours to obtain a precise position. It is then remeasured several years later, to see how that position has changed.
Dr. Kirby notes that measurements were taken 3-4 times after they were installed in the 1990s. “As far as I am aware, there are no continuous stations in this region,” he pointed out.
The GPS had recorded that the plate convergence was occurring at a slow rate of 2 mm per year. When the lateral movement of the plates is small, the vertical motion of the crust is even smaller.
Though GPS can detect both lateral and vertical movement, it becomes challenging to detect very small movements, particularly the vertical component.
Uncertainties magnified
“It is simply difficult [for the GPS] to detect rates of vertical motion of the crust that are less than 1 mm/year,” noted Dr Kirby. And when done on a campaign mode the uncertainties become larger, and the vertical uncertainties are in the order of many centimetres.
And because GPS is not able to resolve differential vertical motions of the crust (because of large uncertainties), the geomorphic analysis provides a clue to where this is happening.
Unlike GPS that provides very precise estimates of the position at a single point where the GPS station is located, digital elevation models provide an estimate of the topography across an entire region. And this helps in identifying the seismic hazard areas.
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