Reaching the bottom of slow-motion earthquakes

Earthquakes can be devastating flows that last for a few seconds or minutes at most. But not all pent-up seismic energy release is fast or violent. Slow earthquakes – also called slow slipping events – can last for days or even months, with small amounts of energy released over time. Slow events are so light that they can only be detected by GPS networks. Although slow events were first recognized in nearly two decades, researchers are still learning about the mechanisms that cause them.

The relatively shallow depth of the subduction zone Hikurangi allowed researchers to dig and collect samples from Pacific plate sites shortly before (geologically) before moving below the Australian plate, and a new study published in Science Advances suggests that geological homogeneity may be at the bottom of some Slow events at least. This study focused on the subduction zone Hikurangi off the coast of North Island of New Zealand, where the tectonic plate in the Pacific Ocean slides beneath the Australian plate. The subduction zone Hikurangi is one of the most shallow sites where researchers have observed slow slipping events – at depths of less than 15 km – opening up some unique research opportunities.

“Previously, many of the areas where we discovered slow events were not completely subject to direct geological observation,” said Philip Barnes, lead author of the study and geologist at the National Water and Atmospheric Research Institute in New Zealand. For example, slow-slide events occur in the Cascadia subduction zone in North America, at depths of about 40 km.

The relatively shallow depth of the subduction zone Hikurangi allowed researchers – on two expeditions from the International Oceanographic Discovery Program (IODP) – to dig and collect samples from Pacific Rim plate soon (geologically) before moving under the Australian plate. By analyzing these basic samples, the researchers determined the structure and physical properties of the plate before moving to the areas where slow slipping events were observed. Then they combined these analyzes with previously acquired seismic reflection data – measurements of how sound waves are reflected from different types of rocks.

Parts of the volcanic rock core collected on JOIDES Resolution regarding IODP 375 flight from the top of a sea mountain at the margins of subduction Hikurangi. Such rocks are believed to form part of the slow-slipping subduction zone. Credit: Philip Barnes (Newa)
Hikurangi is heterogeneous

These experiments showed that the Pacific plate region located before the Hikurangi subduction zone was very heterogeneous. The core samples contained several different types of rock, which had very variable properties, such as strength, porosity, and texture. “Some of the rocks were soft and weak, while others were solid, durable and strong,” Barnes said in a press release.

In some regions, properties such as porosity – a measure of empty distances within a rock type – varied roughly two slits within tens of centimeters. The Pacific plate not only consisted of heterogeneous rock types, but its geology was also variable, including seamounts that rise more than a kilometer above the sea floor.

This geological and physical homogeneity can explain the slow, noticeable events observed in the area, according to Barnes. He said: “Many people believe that the errors of slow events are in a state of transition friction.” “They are very close to failure (which could cause a typical earthquake), but something hinders this failure.”

A mixture of rock types and geological features that enter the subduction zone Hikurangi can mean that some parts of the error periodically malfunction and falter (the so-called stick slip), while other parts slip along without much ado (seismic or faulty crawl), eventually resulting in Its a public area with slow events.

Direct notes

The direct observations from the study will help to improve numerical and computer models attempting to solve the slow slide events. Roland Bürgmann, a geologist at the University of California, said: “Studies of slow environments have been either from different geophysical observations, which tend to be hazy, or from ancient geological exposures, which may be difficult to relate to active systems.” Berkeley who was not related to the study. “This study provides new information, samples are taken directly, and a beautiful illustration of things that are implanted in a slow-slipping area.”

But of course, this study provides only a few data points. “More comprehensive work should be done on multiple subduction zone sectors with or without slow slipping areas to confirm the slow slipping correlation with rock and gradient heterogeneous structures,” said Portman.

Other factors, such as fluid pressure changes, may influence slow slipping events, either in Hikorangi or elsewhere. The degree of heterogeneity found by the authors may also make the interpretations more difficult. “The heterogeneity and randomness of the Pacific plate means that it is simply difficult to conclude that what the researchers see on the next plate will be the same as it enters the subduction zone,” said Jean-Philippe Avoac, a geologist at the California Institute of Veterinary Medicine. Technology is not related to the study. “While this study is an impressive effort to distinguish the next sea floor [at the Hikurangi subduction zone]How to play to explain slow events, not yet fully defined. “

Barnes agrees that more work needs to be done. “This paper is one of the first outputs of a very large research project,” he said. “We are in the early stages of what I hope will be a much fuller exploration of slow events.”

Aditarub Chakravorty ([email protected]), Science writer

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