New Zealand’s worst
earthquake fear confirmed by sediment survey
By Michael Slezak
New Zealand’s worst earthquake fears might
be a real possibility. The first geological evidence of a huge megathrust
earthquake crossing fault boundaries there is giving credence to worst case
scenarios of a potential magnitude 8.9 quake.
Previous modelling found
that such a quake would kill more than 3000 people and could cause A$13 billion
of damage in Wellington alone.
The 2010 earthquake there was
magnitude 7.1 and the following year there was a
quake at magnitude 6.3, which killed 185
people and will cost an estimated A$40 billion to
repair the damage in Christchurch alone.
Off the east coast of New Zealand, the
Pacific plate is colliding with, and subducting under, the Australian Plate in
what’s called the Hikurangi Margin. The fault is divided into three segments
and the worst-case-scenario model imagines that all three rupture at once –
something so far thought to be unlikely.
Some of the world’s biggest earthquakes –
including the ones in Sumatra in 2004, Chile in 2010 and Japan in 2011 –
occurred at such subduction zones. The subduction zone quakes are deep,
powerful and can extend along large distances. But usually they occur at single
segments.
Until recently, geologists weren’t even
sure that they could cross segment boundaries. But then it happened in the Solomon Islands in 2007, Tohoku in 2011 and
geological evidence suggests it happened in the past in Cascadia,
off the West Coast of the US and Canada.
Same time, same quake
“It
seems like each new big subduction interface earthquake that we’ve had is
coming out with surprises, showing that we really don’t know how they behave,”
says Kate Clark of
GNS Science research consultancy in New Zealand.
To get a better sense of the risks in New
Zealand, Clark and her colleagues examined sediments in salt marshes in New
Zealand, and used radiocarbon dating to figure out the age of the layers. This
allowed them to find the first direct evidence of two ruptures in the southern
segment of the fault in the past 1000 years.
The more recent of those two quakes –
occurring about 500 years ago – seems to coincide with a previously known
rupture of the central segment, suggesting they might have happened together.
The carbon dating can only place them within about 150 years of each other, but
that’s enough to suggest they might be connected.
“If
the earthquake 500 years ago ruptured across one segment boundary, there’s
really no reason it couldn’t rupture across another one,”
says Clark. And if it happened in the past, it could happen in the future, she
says.
Pushing the limits
“There
has been a tendency to divide plate margins into segments and assume that
generally each segment ruptures independently of others, thus limiting the size
of the earthquake,” says Kevin Furlong from
Pennsylvania State University in the US, one of the researchers that showed a
cross-segment rupture had happened in 2007 in the Solomons.
He says that reasoning led to the
assumption that any earthquake in Tohoku could only be up to magnitude 8. “Unfortunately we know that in fact a
larger area could – and did – rupture, leading to a magnitude 9 earthquake,” says
Furlong.
He adds that, similarly, conventional
wisdom is that Hikurangi subduction zone in New Zealand can only rupture in one
segment at a time. “But I think that view
is being challenged as more information becomes available.”
Clark says she needs to gather more
sediments from a larger area to figure out how big the newly discovered quakes
were, and whether they did occur at the same time. But she guesses that if the
two segments ruptured simultaneously, it would have been bigger than a
magnitude 8 earthquake.
Related Posts:
https://www.newscientist.com/article/dn27550-new-zealands-worst-earthquake-fear-confirmed-by-sediment-survey/
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