Small and large earthquakes can have similar beginnings

Earthquakes often start with a short wave phase of low amplitude, before turning into an event of final size.^2,3. One mechanism that could explain this observation is a cascading failure, in which the stress changes due to a randomly failing geological failure patch result in the failure of other patches, such as domino rollover. In this case, the magnitude of an earthquake is controlled by the dynamic conditions as the event develops, and is impossible to predict until the earthquake slows down or s'. do not stop.

An alternative possibility involves a slow slip – the relative motion of rocks on either side of a fault. This slippage, undetectable by the seismometers, could progressively accelerate in a limited area of ​​the fault before reaching a critical speed and reach the size of the final earthquake. If correct, the magnitude of the earthquake could be determined by the size of the preceding previous slide region or by the characteristics of the initial waves. and if these properties could be observed and understood, a short-term prediction would be possible.

Some seismometric archive studies have shown that the magnitude of an earthquake is independent of the first hundredths of a second.⁴ or longer³. However, these analyzes were limited to a few earthquakes. Other studies^5,6 suggested a dependence of the size of the final earthquake at the beginning. But these analyzes imply indirect parameterization of the data and may not accurately account for the loss of seismic wave energy as they pass through the Earth.⁷.

Ide compared the onset of close earthquakes of different magnitudes to determine whether these earthquakes provide an indication of the final size of the earthquake. It conducted a detailed analysis of all significant earthquakes recorded in sufficient detail between June 2002 and April 2018, about 1,100 kilometers of the Japan Trench, a subduction zone in which the Pacific Plate is embedded. the Pacific Ocean. under the Okhotsk plate under Japan. He followed a procedure that was used to identify phenomena called repetitive earthquakes⁸. These are similar sized tremors whose seismometric recordings are so similar that the events must involve a similar repeated movement of the same fault plates.⁸.

Instead of looking for events of similar size, Ide started with 1,654 large earthquakes (greater than 4.5 magnitude) and compared them to all known small events (magnitude less than 4) located close enough (at about 100 meters) for their locations are indistinguishable. He then calculated the similarity between the first 0.2 seconds of the seismometric recordings of these earthquakes.

Ide discovered 390 pairs of large and small earthquakes that have very similar beginnings. The beginnings of 200 significant events are indistinguishable from those of similar co-located events. He interpreted this conclusion as indicating that the beginnings of large earthquakes may be identical to those of small ones, and that therefore the initial conditions and dynamics of an earthquake do not determine its magnitude.

By separating earthquakes into subduction-like events – similar to the 2011 Tohoku-Oki earthquake (Figure 1) – and to other types, Ide found that subduction-type earthquakes are more likely than other types of earthquakes to occur. 39, have related events. He also found that subduction-type couples can be separated by more than ten years, while those of other types are limited to small earthquakes occurring shortly after the big event.

The author has interpreted these results with the help of a model in which a seismic fault consists of plates having a relatively constant range of sizes and fracture characteristics. Sliding one of these patches could trigger the sliding of a larger neighboring patch, and so on. This image is compatible with the numerical models that have been proposed to explain the repetition of earthquakes⁹. The similarity of the beginnings over extended periods of time implies that a long-term characteristic structure is present and capable of repeatedly hosting large and small earthquakes.

By taking into account the localized earthquakes, in which the seismic waves of small and large events take the same paths to get to the measuring stations, Ide has eliminated the biases due to waves traveling different paths across the Earth.^3,4. His conclusion, that the beginning of an earthquake does not control its final size, is consistent with detailed observations of well-recorded earthquakes on the San Andreas Fault in Parkfield, California.^ten. It is also compatible with global statistical compilations of major earthquakes^5,7,11 which revealed that all earthquakes develop at approximately the same rate and begin to differ only when the break shows signs of slowing down.

Most earthquakes around Japan occur offshore or in the underground depths and are therefore not close to the seismometers, which limits the spatial coverage and frequency range of recording. Ide's analysis focuses on high frequency waves (1 hertz and above). Therefore, there is no apparent difference in the low frequencies at which most of the energy is released by large earthquakes. It would also lack any previous slippage such as that observed in laboratory experiments and numerical models.¹² – reliable and consistent observations of this slow slide before earthquakes remain elusive.

Another problem is that, while Ide was looking for comparisons of large earthquakes, about 60% have a magnitude difference of less than 1.5, similar to the variation observed in repeated earthquake sequences.⁸. About one-eighth of the paired events have a magnitude difference greater than 2.

For the moment, earthquake-prone populations need to rely on early warning systems, long established in Asia and established in recent years in California. These warning systems involve estimating the magnitude of earthquakes using seismometers close to the source and sending this information to vulnerable populations before the slower and more destructive seismic waves.¹³. Ide's results are a key step towards a better understanding of earthquake initiation – knowledge that could improve the speed and accuracy of these warnings.