Slow earthquakes are predictable in Cascadia

GPS stations detect activity below Cascadia as the ocean floor slides beneath North America. The front of the plate is secured at shallow depths (shaded area), but we see repeated slow slide events (in blue) that release the plate front pressure, and generate tremors (black dots). Credit: California Institute of Technology

If there is one word that you are not supposed to use when discussing serious seismology, it is “expectation”. Seismologists cannot predict earthquakes. Instead, they calculate the probability of major earthquakes occurring along a given error over a given time period.

It is a matter of controversy among seismologists whether the process that drives earthquakes – loading a strain along a fault followed by sharp and sudden energy release with two tectonic plates against each other – is a random (random) process, which can only estimate the probability of occurrence, or whether it is a process Imperative and predictable.

Caltech seismologists have studied up to a decade of so-called “slow slip events”, which result from accidental slip-ups like regular earthquakes but generate barely recognizable tremors, in the Cascadia region of the Pacific Northwest. Their analysis shows that this special type of seismic event is inevitable and can be expected for days or even weeks in advance.

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

Adriano Gualandi, the group leader and corresponding author of the paper says. Gualandi was a postdoctoral researcher at the Jean-Philippe Avoic Laboratory, Earl C. Anthony Professor of Geology, 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 from the Pierre Simon Laplace Institute in France to study.

Slow events were observed for the first time in about two decades by geologists who tracked imperceptible shifts in Earth using GPS technology. Events occur when tectonic plates grind incredibly slowly against each other, such as an earthquake in slow motion. A slow slip event that occurs over a period of weeks may release the same amount of energy as a 7.0 minute earthquake for one minute. However, because these earthquakes release energy very slowly, the deformation they cause on the surface is on a millimeter scale, despite affecting areas that may span thousands of square kilometers.

As such, only slow events are detected when GPS technology is optimized to the point that it can track these very subtle shifts. Slow events also don’t happen along every error. To date, it has been spotted in a few locations including the Pacific Northwest, Japan, Mexico and New Zealand.

GPS stations detect activity below Cascadia as the ocean floor slides beneath North America. The front of the plate is secured at shallow depths (shaded area), but we see repeated slow slide events (in blue) that release the plate front pressure, and generate tremors (black dots).

Slow events are useful to researchers because they accumulate and repeat frequently, making it possible to study how loads are loaded and released along holidays. Over a 10-year period, 10-degree slow earthquakes of 10 or more magnitude may occur along a given error. By contrast, most regular earthquakes of this magnitude are repeated only over hundreds of years. Because of this time lag between large regular earthquakes and lack of useful records for hundreds of years, it is impossible to accurately compare past events with recent events.

Despite their names, the slow events of seismologists offer a way to press “rapid progress” in the loading / sliding process that drives earthquakes. Within a short period of time of about 10 years, seismologists who use modern GPS equipment can observe the cycle again several times.

Slow events represent what is known as a “nonlinear coercive dynamic system.” The movement of tectonic plates is the force that drives the system, while the friction between the plates, which causes the pressure to increase and eventually release in the event of slipping, makes the system non-linear; In a non-linear system, the change in output is not proportional to the change in the input. Despite the fact that both motion and friction can be modeled using fully deterministic differential equations, the conditions of starting a system – how much pressure is actually under error – for example – have a significant impact on long-term results. Not knowing these exact starting conditions is one of the possible reasons why public order is not expected in the long run. However, checking the history of the defect’s slip can reveal the number of times and the length of time for similar patterns to repeat over time. In this way, the team was able to assess the prediction horizon time for slow events.

“This result is very encouraging,” says Gualandi. “It shows that we are on the right track, and if we can get more accurate data, then we can try some real-time forecasting experiments for slow earthquakes.”

Gualandi is similar to the potential prediction of a slow-moving event with the current science of weather forecasting, which also includes predictions of a complex and chaotic process (similarly it drops in accuracy after a week or so). “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 appears it is possible to determine when the error will occur in some days before this occurs, similar to the way in which the weather can be predicted” Somewhat delicate two days ago. “

One of the main questions is whether the results of slow earthquakes can translate into regular earthquakes that rock cities and endanger life and property. Last year, Michelle, Afwak and Jawalandi reported evidence that slow-slip earthquakes are a good counterpart to their most devastated cousins.

Afwak says: “If the measurement we draw between slow earthquakes and normal earthquakes is correct, then regular earthquakes are predictable.” “But even if regular earthquakes are inevitable, the forecast horizon may be very short, perhaps within a few seconds, which may be of limited benefit. We do not know yet.”

The paper is entitled “Expected Chaos in Slow Earthquakes”.

Slow motion earthquakes: studying “slow slip” events can shed light on devastating tremors. More information: A. Gualandi et al. Projected Chaos of Slow Earthquakes, Advances Science (2020). DOI: 10.1126 / sciadv.aaz5548 Provided by California Institute of Technology

Quote: Predictable Slow Earthquakes in Cascadia (2020, July 2) Retrieved 2 July 2020 from https://phys.org/news/2020-07-earthquakes-cascadia.html

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