Evidence mounts that slow-slip seismic events follow a deterministic pattern

If there is one word you are not supposed to use when discussing serious earthquake science, it is "predict." Seismologists cannot predict earthquakes; instead they calculate how likely major earthquakes are to occur along a certain fault over a given period of time.

It is a matter of debate among seismologists whether the process that drives earthquakes—the loading of strain along a fault followed by the sudden, sharp release of energy as two tectonic plates grind against one another—is a stochastic (random) process, for which only an estimate of the probability of occurrence can be made, or whether it is a deterministic, and potentially predictable, process.

Seismologists at Caltech studied a decade's worth of so-called "slow-slip events," which result from episodic fault slip like regular earthquakes but only generate barely perceptible tremors, in the Cascadia region of the Pacific Northwest. Their analysis shows that this particular type of seismic event is deterministic and potentially could be predictable days or even weeks in advance.

A paper about the work was published in the journal Science Advances on July 1.

"Deterministic chaotic systems, despite the name, do have some predictability. This study is a proof of concept to show that friction at the natural scale behaves like a chaotic system, and consequently has some degree of predictability," says Adriano Gualandi, the lead and corresponding author of the paper. Gualandi was a postdoctoral scholar in the lab of Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering, while working on this research. Gualandi and Avouac collaborated with Sylvain Michel, who worked on this project as a graduate student at Caltech, and Davide Faranda of Institut Pierre Simon Laplace in France on the study.

Slow-slip events were first noted about two decades ago by geoscientists tracking otherwise imperceptible shifts in the earth using global positioning system (GPS) technology. The events occur when tectonic plates grind incredibly slowly against each other, like an earthquake in slow motion. A slow-slip event that occurs over the course of weeks might release the same amount of energy as a one-minute-long magnitude 7.0 earthquake. However, because these quakes release energy so slowly, the deformation that they cause at the surface is on the scale of millimeters, despite affecting areas that may span thousands of square kilometers.

As such, slow-slip events were only discovered when GPS technology was refined to the point that it could track those very minute shifts. Slow-slip events also do not occur along every fault; so far, they have been spotted in just a handful of locations including the Pacific Northwest, Japan, Mexico, and New Zealand.

Slow-slip events are useful to researchers because they build up and reoccur frequently, making it possible to study how strain loads and releases along a fault. Over a 10-year period, 10 magnitude 7.0 or greater slow-slip earthquakes might occur along a given fault. By contrast, most regular earthquakes of that magnitude only reoccur on the order of hundreds of years. Because of this time lag between regular large earthquakes and the lack of instrumental records from hundreds of years ago, it is impossible to precisely compare past events with recent ones.