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ust after midnight on Nov. 14,
2016, the northern end of New
Zealand’s South Island was hit
by a magnitude-7.8 earthquake.
Epicentered about 60 kilometers
southwest of the popular tourist town of
Kaikoura, the quake was the strongest
the area had seen since the 1855 mag-
nitude-8.2 Wairarapa quake struck the
Cook Strait. The Kaikoura quake led to
two deaths as well as extensive damage
to roads, rails and buildings.
Scientists investigating the earthquake,
including Ian Hamling, a geophysicist at
New Zealand-based research and con-
sulting company GNS Science, who was
awakened in Wellington that night by the
nearly two minutes of shaking, soon real-
ized the Kaikoura quake involved a chain
reaction of ruptures on multiple faults.
“Nothing of this complexity with such
good robust documentation has been
documented before,” says Hamling, who
analyzed the quake in a recent study
in Science.
Hamling and his colleagues used GPS
and seismic data, as well as satellite-based
InSAR measurements to study the rup-
ture pattern during the event, finding that
the earthquake propagated from north
to south over 170 kilometers. Surface
rupture occurred along at least 12 major
faults — which varied in orientation
and included two that were previously
unknown — with displacements upwardsof 10 meters on several. The ruptures also
propagated between faults separated by
15 kilometers or more.
It was previously thought that gaps
larger than about 5 kilometers between
faults would prevent a rupture on one
fault from triggering ruptures on other
faults. The new findings challenge that
conventional wisdom, and Hamling says
New Zealand is already changing its
seismic hazard modeling to include the
possibility of multifault ruptures on faults
separated by more than 5 kilometers.
“Hopefully, we will get people talking, and
also re-examine the paleoseismic record,”
Hamling says. The record could reveal
other examples of single earthquakes
involving widely spaced faults, which
could improve understanding of how a
rupture can step across wide gaps. He
says the former notion that a 5-kilometer
gap would prevent the spread of ruptures
between faults “is far too restrictive,”
adding that allowing for triggering over
greater distances will hopefully result in
more accurate seismic models.
In the United States, the U.S. Geo-
logical Survey (USGS) and the Working
Group on California Earthquake Probabil-
ities (WGCEP) have begun incorporating
scenarios of multifault ruptures involving
relatively distant faults into their haz-
ard assessment models. “The Kaikoura
earthquake could be the documented
record holder for how far ruptures can
jump,” says Ned Field,
a geophysicist with
USGS in Golden,
Colo., and leader of
WGCEP, who was not
involved with the new
study. The compre-
hensive record of the
Kaikoura earthquake,
compiled and analyzed
by Hamling and his
team, reinforced the
WGCEP’s most recent
hazard assessment,Uniform California Earthquake Rupture
Forecast (UCERF3), Field says, which, for
the first time, accounted for the jumping
of complex ruptures between faults.
WGCEP included multifault ruptures
with gaps up to 5 kilometers in UCERF3,
and may include multifault rupture sys-
tems with gaps up to 20 kilometers in
their next rupture forecast, Field says.
Such an update won’t necessarily affect
California hazard models a great deal,
he says. But, as the documented seismic
record continues to grow, and geophys-
icists have more data from multifault
earthquakes, earthquake rupture fore-
cast models worldwide will become
more comprehensive.
Gretchen MillerJ
Rivers get salty near
mountaintop coal
mining
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