Scientists get first look at cause of 'slowmotion' earthquakes25 March 2020, by Michael Bishop

Credit: CC0 Public Domain

An international team of scientists has for the firsttime identified the conditions deep below theEarth's surface that lead to the triggering of so-called 'slow motion' earthquakes.

These events, more commonly known as slow slipevents, are similar to regular sudden andcatastrophic earthquakes but take place on muchlonger timescales, usually from days to months.

By drilling down to just over 1km deep in waterdepths of 3.5km off the coast of New Zealand, theteam have shown that the fault zone areas in whichslow slip events occur are characterised by a 'mashup' of different rock types.

The results, published today in the journal ScienceAdvances, showed that the areas are comprised ofextremely rough sea floor topography made ofrocks that varied markedly in size, type and physical characteristics.

The lead author of the paper, Dr. Philip Barnes ofNew Zealand's National Institute of Water andAtmospheric Research (NIWA), described that'some rocks were mushy and weak, whilst otherswere hard, cemented and strong.'

This has given scientists the first-ever look at thetypes and properties of rocks directly involved in slow motion earthquakes and begins to answersome of the major outstanding questionssurrounding these unique events, such as whetheror not they can trigger larger, more damagingearthquakes and tsunamis.

The UTIG-led expedition drilled cores from the

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subduction zone and revealed a surprising diversity in therocks buried half a mile beneath the seafloor. This mash-up of rocks means a mix of weak and strong points in theEarth's crust which the scientists say influences theoccurrence of earthquakes. Credit: IODP JRSO

Co-author of the study Dr. Ake Fagereng, fromCardiff University's School of Earth and OceanSciences, said: "This was the first effort to samplethe rocks that host slow slip events, and thestriking, immediate observation is that theirstrengths are hugely variable. One can thereforevisualise the slow slip source as a mixture of hardand weak rocks, and use this as a starting point formodels of how slow slip occurs."

First discovered on the San Andreas fault inCalifornia, but since 2002 found to occur in severalother locations, slow slip events remain a relativemystery to scientists, who are endeavouring to findout how, where and why they occur and whatdrives their behaviour.

As part of their study, the international teamundertook two International Ocean DiscoveryProgram (IODP) expeditions aboard the JOIDESResolution research vessel to the Hikurangisubduction zone off the east coast of the NorthIsland in 2017 and 2018.

This was the first time that scientists had studied,and directly sampled, rocks from the source regionof slow slip events using ocean floor scientificdrilling methods.

A map of New Zealand showing the Hikurangi subductionzone off the coast of New Zealand's North Island. Thered circles are the sites where the UTIG-led scienceexpedition drilled and recovered cores of rocky materialfrom inside the subduction zone. Credit: IODP JRSO

The Hikurangi subduction zone is New Zealand'slargest earthquake fault and is one of the bestplaces in the world to study slow slip because herethese events occur close to the sea floor whichmakes drilling to collect rock samples a lot easier.

For instance, Laura Wallace of GNS Science, NewZealand, describes that the 2016 Kaikouraearthquake triggered a series of major slow slipevents on the Hikurangi subduction zone—wherethe Pacific Plate dives beneath the eastern NorthIsland—and was the most widespread episode ofslow slip seen in New Zealand since they were firstdiscovered in the country.

These slow slip events following the Kaikouraearthquake released a large amount of built-uptectonic energy and lasted over the weeks andmonths following the earthquake.

During the expedition the team drilled twoboreholes to obtain a sequence of rocks andsediments on the incoming (Pacific) plateapproaching the North Island.

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The drilling data were interpreted together withseismic reflection profiles—or pictures of the layersunder the surface of the earth which are created atsea by sound waves.

The study has indicated that the co-existence ofthese contrasting rock types in the fault zone maylead to the slow slip movements observed offshorefrom Gisborne, and perhaps elsewhere atsubduction boundaries around the world.

Indeed, Dr. Barnes says that the research will havedirect relevance not only to New Zealand, but toareas like Japan and Costa Rica, which sit on theRing of Fire—the perimeter of the Pacific Oceanbasin where many earthquakes and volcaniceruptions occur.

"We now know that a highly variable mixture of rockstrengths is part of the recipe for slow slip. Thisopens for new studies of how such mixturesdeform, why they can generate slow slip, and underwhat conditions (if any) they can also generatedamaging earthquakes. This may help address theoutstanding question of how earthquakes and slowslip events interact," continued Dr. Fagereng.

More information: "Slow slip sourcecharacterized by lithological and geometricheterogeneity" Science Advances (2020). DOI:10.1126/sciadv.aay3314 , advances.sciencemag.org/content/6/13/eaay3314

Provided by Cardiff UniversityAPA citation: Scientists get first look at cause of 'slow motion' earthquakes (2020, March 25) retrieved 24December 2021 from https://phys.org/news/2020-03-scientists-motion-earthquakes.html

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