Wednesday, February 6, 2019 The Nugget Newspaper, Sisters, Oregon
13
‘Silent slip’ along fault line may serve as prelude to big quakes
By Steve Lundeberg
Correspondent
CORVALLIS 4 Big earth-
quakes appear to follow a brief
episode of <shallow mantle
creep= and <seismic swarms,=
suggests new research at
Oregon State University that
offers an explanation for the
foreshocks observed prior to
large temblors.
Published today in Nature
Geoscience, the findings are
an important step toward
understanding the relation-
ship and interactions between
aseismic slip and seismic slip.
Also known as silent slip or
slow slip, aseismic slip is dis-
placement along a fault that
occurs without notable earth-
quake activity.
The research involved the
Blanco Transform Fault off
the coast of Oregon; a trans-
form fault is a plate boundary
at which the motion is mainly
horizontal.
Under the sea, transform
faults connect offset mid-
ocean <spreading centers,=
places at seafloor ridges where
new oceanic crust is formed
through volcanic activity and
gradually moves away from
the ridge.
<Slow slip directly trig-
gers seismic slip 4 we can
see that,= said co-correspond-
ing author Vaclav Kuna, a
graduate student in geology
and geophysics in OSU9s
College of Earth, Ocean and
Atmospheric Sciences. <The
findings are very interesting
and may have some broader
implications for understand-
ing how these kinds of faults
and maybe other kinds of
faults work.=
Researchers deployed 55
seismometers on the ocean
bottom on and around the
Blanco fault for a year.
<It9s a very seismically
active fault that generates sig-
nificant earthquakes at higher
rates than the majority of
faults on land, making it ideal
for studying the process of
earthquake generation,= Kuna
said.
The seismometer deploy-
ment 4 from September 2012
to October 2013 4 resulted
in the detection of more than
1,600 earthquakes at the
Blanco Ridge, a 130-kilome-
ter segment of the Blanco fault
that served as the study area.
Two distinct asperities
4 basically rough edges
4 along the ridge rupture
roughly every 14 years with
quakes in the magnitude 6
range.
<Our work was enabled by
recent advances in long-term
ocean-bottom seismometer
deployments and is only the
second major project targeting
an oceanic transform fault,=
said co-corresponding author
John Nabelek, professor of
geology and geophysics at
OSU.
At its southernmost point,
the Blanco Transform Fault
is about 100 miles from Cape
Blanco, Oregon9s western-
most location, and the fault
runs northwest to a point about
300 miles from Newport.
The Cascadia Subduction
Zone, a fault that extends from
British Columbia to northern
California, lies between the
Blanco fault and the coast-
line. The fault was the site
of a magnitude 9 earthquake
in 1700 and is building up
stress where the Juan de Fuca
Plate is sliding underneath the
North American Plate.
Some scientists predict a
40 percent chance of another
magnitude 9 or bigger quake
occurring along the fault in
the next 50 years.
<The Blanco fault is only
400 kilometers offshore,=
Nabelek said. <A slip on
Blanco could actually trigger
a Cascadia Subduction slip;
it would have to be a big one,
but a big Blanco quake could
trigger a subduction zone
slip.=
The Earth is put together
in layers beneath the crust, the
outermost skin that varies in
thickness from about 40 miles
(continental crust at moun-
tain ranges) to about 2 miles
(oceanic crust at mid-ocean
ridges).
The boundary between the
crust and the next layer, the
upper mantle, is known as the
Moho.
<We see slow, aseismic
slips that occur at depth in
the fault beneath the Moho
and load the shallower part
of fault,= Nabelek said. <We
can see a relationship between
mantle slip and crust slip. The
slip at depth most likely trig-
gers the big earthquakes. The
big ones are preceded by fore-
shocks associated with creep.=
Kuna explains that the
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layers have different levels of
seismic <coupling,= the ability
of a fault to lock at asperities
and accumulate stress.
<The crust is fully cou-
pled 3 all slip is released in
a seismic way,= Kuna said.
<Fault in the shallow mantle
is partly coupled, partly not,
and releases slip both seismi-
cally and aseismically. The
deep mantle is fully creeping,
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uncoupled, with no earth-
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Our results also show that an
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The National Science
Foundation supported this
research.
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