Rare 'boomerang' earthquake observed along Atlantic Ocean fault line


Date:
August 11, 2020
Source:
Imperial College London
Summary:
Scientists have tracked a 'boomerang' earthquake in the ocean
for the first time, providing clues about how they could cause
devastation on land.



FULL STORY ========================================================================== Scientists have tracked a 'boomerang' earthquake in the ocean for the
first time, providing clues about how they could cause devastation
on land.


========================================================================== Earthquakes occur when rocks suddenly break on a fault -- a boundary
between two blocks or plates. During large earthquakes, the breaking
of rock can spread down the fault line. Now, an international team of researchers have recorded a 'boomerang' earthquake, where the rupture
initially spreads away from initial break but then turns and runs back
the other way at higher speeds.

The strength and duration of rupture along a fault influences the among
of ground shaking on the surface, which can damage buildings or create tsunamis.

Ultimately, knowing the mechanisms of how faults rupture and the physics involved will help researchers make better models and predictions of
future earthquakes, and could inform earthquake early-warning systems.

The team, led by scientists from the University of Southampton and
Imperial College London, report their results today in Nature Geoscience.

While large (magnitude 7 or higher) earthquakes occur on land and have
been measured by nearby networks of monitors (seismometers), these
earthquakes often trigger movement along complex networks of faults,
like a series of dominoes.

This makes it difficult to track the underlying mechanisms of how this
'seismic slip' occurs.

Under the ocean, many types of fault have simple shapes, so provide the possibility get under the bonnet of the 'earthquake engine'. However, they
are far from large networks of seismometers on land. The team made use of
a new network of underwater seismometers to monitor the Romanche fracture
zone, a fault line stretching 900km under the Atlantic near the equator.



==========================================================================
In 2016, they recorded a magnitude 7.1 earthquake along the Romanche
fracture zone and tracked the rupture along the fault. This revealed that initially the rupture travelled in one direction before turning around
midway through the earthquake and breaking the 'seismic sound barrier', becoming an ultra-fast earthquake.

Only a handful of such earthquakes have been recorded globally. The team believe that the first phase of the rupture was crucial in causing the
second, rapidly slipping phase.

First author of the study Dr Stephen Hicks, from the Department of Earth Sciences and Engineering at Imperial, said: "Whilst scientists have found
that such a reversing rupture mechanism is possible from theoretical
models, our new study provides some of the clearest evidence for this
enigmatic mechanism occurring in a real fault.

"Even though the fault structure seems simple, the way the earthquake
grew was not, and this was completely opposite to how we expected the earthquake to look before we started to analyse the data." However,
the team say that if similar types of reversing or boomerang earthquakes
can occur on land, a seismic rupture turning around mid-way through an earthquake could dramatically affect the amount of ground shaking caused.

Given the lack of observational evidence before now, this mechanism has
been unaccounted for in earthquake scenario modelling and assessments
of the hazards from such earthquakes. The detailed tracking of the
boomerang earthquake could allow researchers to find similar patterns
in other earthquakes and to add new scenarios into their modelling and
improve earthquake impact forecasts.

The ocean bottom seismometer network used was part of the PI-LAB and
EUROLAB projects, a million-dollar experiment funded by the Natural
Environment Research Council in the UK, the European Research Council,
and the National Science Foundation in the US.


========================================================================== Story Source: Materials provided by Imperial_College_London. Original
written by Hayley Dunning. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Stephen P. Hicks, Ryo Okuwaki, Andreas Steinberg, Catherine
A. Rychert,
Nicholas Harmon, Rachel E. Abercrombie, Petros Bogiatzis, David
Schlaphorst, Jiri Zahradnik, J-Michael Kendall, Yuji Yagi, Kousuke
Shimizu, Henriette Sudhaus. Back-propagating supershear rupture
in the 2016 Mw 7.1 Romanche transform fault earthquake. Nature
Geoscience, 2020; DOI: 10.1038/s41561-020-0619-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200811153918.htm

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