Seismologists at Caltech analyzed 10 years' worth of slow-slip events that result from episodic fault slip, like regular earthquakes, but only produce barely perceptible quakes in the Cascadia region of the Pacific Northwest. They found that this particular type of seismic event is deterministic and could be predictable-- days or weeks in advance.
"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," said lead author Adriano Gualandi.
Slow-slip events were first noted about 20 years ago by geoscientists tracking imperceptible shifts in the Earth with GPS. Such events happen when tectonic plates grind slowly against each other, like a slow-moving earthquake.
A slow-slip event that takes place over the course of weeks may release the same amount of energy as an M7.0 quake for one minute.
However, since these tremors release energy slowly, the deformation they cause is on the scale of millimeters, despite impacting areas that may be thousands of square kilometers away.
Slow-slip events were only discovered when GPS technology was improved to the point that it could detect the slow shifts. Such events do not occur along every fault and have only been spotted in a few locations so far, including the Pacific Northwest, Mexico, New Zealand, and Japan.
In a short time frame of around a decade, seismologists using high-end GPS equipment can observe the cycle repeat itself several times.
It also represents a 'forced non-linear dynamical system.' The motion of the tectonic plates is the force driving the system, while the friction between the plates makes the system non-linear.
Despite the fact that both the motion and friction can be starting conditions of the system, how much strain under has a major impact on long-term results. Not knowing those exact starting conditions is one of the probable reasons that the overall system is unpredictable in the long run.
However, a study of the fault slip history can show how often and for how long similar patterns recurred over time. The team was able to assess the predictability horizon time of slow-slip events this way.
"This result is very encouraging. It shows that we are on the right track and, if we manage to get more precise data, we could attempt some real-time prediction experiments for slow earthquakes," Gualandi added.
Gualandi likens the probable prediction of a slow-slip event to the current science of weather forecasting, which also involves predictions about a chaotic and complex process.
"We already know that approximately every 12 to 14 months there will be a new slow earthquake, but we do not know exactly when it will happen."
"What we have shown is that it seems to be possible to determine when the fault will slip some days before it happens, similar to the way the weather can be forecast fairly accurately a couple of days in advance."
One major question is whether the findings can translate to the regular earthquakes that endanger lives and properties. Last year, Gualandi and colleagues reported evidence that slow-slip earthquakes are a good analog for more destructive seismic events.
"If the analogy that we're drawing between slow earthquakes and regular earthquakes is correct, then regular earthquakes are predictable," said co-author Jean-Philippe Avouac.
"But even if regular earthquakes are deterministic, the predictability horizon may be very short, possibly on the order of a few seconds, which may be of limited utility. We don't know yet."
"The predictable chaos of slow earthquakes" - Gualandi, A. et al. - Science Advances - DOI: 10.1126/sciadv.aaz5548
Slow earthquakes, like regular earthquakes, result from unstable frictional slip. They produce little slip and can therefore repeat frequently. We assess their predictability using the slip history of the Cascadia subduction between 2007 and 2017, during which slow earthquakes have repeatedly ruptured multiple segments. We characterize the system dynamics using embedding theory and extreme value theory. The analysis reveals a low-dimensional (<5) nonlinear chaotic system rather than a stochastic system. We calculate properties of the underlying attractor like its correlation and instantaneous dimension, instantaneous persistence, and metric entropy. We infer that the system has a predictability horizon of the order of days weeks. For the better resolved segments, the onset of large slip events can be correctly forecasted by high values of the instantaneous dimension. Longer-term deterministic prediction seems intrinsically impossible. Regular earthquakes might similarly be predictable but with a limited predictable horizon of the order of their durations.
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