New technique developed to track slow slip of Earth's crust

A new University of Washington (UW) study finds that the same technique used by traders to track a stock can help detect gradual movement of tectonic plates, or what are called "slow slip" earthquakes.

Designed to quickly pinpoint slow slips from a single Global Positioning System (GPS) station, the technique borrows the financial industry's relative strength index, a measure of how quickly a stock's price is changing, to detect slow slips within a string of GPS observations.

These movements do not unleash damaging amounts of seismic energy, but researchers are beginning to understand how they may be linked to a major earthquake known as the Big One.

"I've always had an interest in finance, and if you go to any stock ticker website there's all these different indicators," said Brendan Crowell, a UW research scientist in Earth and space sciences and lead author of a paper published in the Journal of Geophysical Research: Solid Earth.

"This particular index stood out in its ease of use, but also that it needed no information -- like stock volume, volatility or other terms -- besides the single line of data that it analyzes for unusual behavior."

The study tests the method on more than 200 GPS stations that recorded slow slips between 2005 and 2016 along the Cascadia fault zone, which runs from northern California on the U.S. West Coast up to northern Vancouver Island in Canada.

As Cascadia Subduction Zone is the most-studied slow slip area in the world, Crowell said it was "a good way to validate the methodology."

Discovered in the early 2000s, slow slips are a type of silent earthquake in which two plates slip harmlessly past one another over weeks or months. In Cascadia, the slipping runs backward from the typical motion along the fault.

A slow slip slightly increases the chance of a larger earthquake. It may be providing clues, which researchers don't yet know how to decipher, to what is happening in the physics at the plate boundary.

Regular earthquake monitoring relies on seismometers to track the shaking of the ground. That doesn't work for slow slips, which do not release enough energy to send waves of energy through the Earth's crust to reach seismometers. Instead, detection of slow slips relies on GPS data.

At GPS stations, the same type of sensors used in smartphones are secured to steel pipes that are cemented at least 35 feet, or about 10 meters, into solid rock.

By minimizing the noise, these stations can detect millimeter-scale changes in position at the surface, which can be used to infer movement deep underground. Using these data to detect slow slips currently means comparing different GPS stations with complex data processing.

However, thanks to the efforts of stock traders who want to know quickly whether to buy or sell, the new study shows that the relative strength index can detect a slow slip from a single one of the 213 GPS stations along the Cascadia Subduction Zone, and this simple new technique's estimates for the size, duration and travel distance for major slow slip events match the results of more exhaustive analyses of observations along the fault.

The initial success suggests the method could have other geological applications. "I want to be able to use this for things beyond slow slip," Crowell was quoted as saying in a news release from UW.

"We might use the method to look at the seismic effects of groundwater extraction, volcanic inflation and all kinds of other things that we may not be detecting in the GPS data." Endit