Geotechnical News • March 2012
47
wisdom, the Cascadia subduction
zone slipped without earthquakes. But
in the last 30 years, geologists have
uncovered sedimentary records as well
as historical records in Japan show-
ing that indeed, the fault repeatedly
had these huge earthquakes with big
tsunamis.
Cascadia’s last big event occurred in
1700 and was likely very similar to
the March 2011 Japanese earthquake
– a magnitude 9 quake and tsunami
that traveled all the way across the
Pacific. This similarity is forebod-
ing for earthquake scientists, as a
key scientific lesson of the Japanese
earthquake has been that the standard
datasets collected onshore are com-
pletely inadequate for characterizing
the upcoming ruptures on an offshore
subduction zone thrust fault.
One key limitation in the seismic haz-
ard estimation for subduction zones
is the use of geodetic data recorded
onshore – primarily GPS data – to
determine the extent to which offshore
faults are locked and building up
strain for the next big earthquake. GPS
can detect surface motion to unprec-
edented precision – a fraction of a mil-
limeter per year – but land-based GPS
is too far away from offshore faults to
be sensitive enough to that motion.
We know the fault is locked around
the coast but we don’t know how
far offshore it’s locked. One goal
is to determine if the fault really is
locked all the way to the trench or not.
Instruments are needed out there to
be really sensitive to it. One reason
that’s important is for understanding
what the next tsunami will be like. The
March Japan earthquake had such a
big tsunami because most of the fault
motion was really shallow and close
to the seafloor. Figuring out exactly
where the locking starts at the shallow
end of the fault is a primary goal.
To do this, tiltmeters will be installed
at a location approximately 4 kilome-
ters above the Cascadia subduction
zone thrust interface. Tiltmeters are
standard instruments on land – most
volcano observatories have them.
These instruments are very, very sensi-
tive to tiny little deformations that
occur in the rock. The movements can
be subtle. They can be slow. Some-
thing a seismometer is not sensitive to.
The tiltmeters will be located within
a 300 meter-deep borehole, a study
site established by the Integrated
Ocean Drilling Program, and will take
advantage of an existing seafloor cable
infrastructure – NEPTUNE Canada
– enabling immediate access to the
data collected by the instrument. The
instrument array should be installed
and returning data by summer 2013.
If such a data stream had been avail-
able in real time from the Japanese
subduction zone in the days preced-
ing the March 11 quake, the scientific
community might have known that
the potential for a large earthquake
was very high because the fault was
already slipping slowly.
Part of the reason for installing a tilt-
meter in a borehole is because of inter-
esting signals collected in boreholes in
the past, signals that provide clues to
better understanding of earthquakes. It
all feeds back into understanding the
fault system – how the stress changes
over time in the fault system.”
The Woods Hole Oceanographic
Institution is a private, independent
organization in Falmouth, Mass., ded-
icated to marine research, engineer-
ing, and higher education. Established
in 1930 on a recommendation from
the National Academy of Sciences, its
primary mission is to understand the
oceans and their interaction with the
Earth as a whole, and to communicate
a basic understanding of the oceans’
role in the changing global environ-
ment. For more information, please
visit
MILESTONES
Hayley Croteau joins RST
Instruments as Sales Engineer
Hayley Croteau, P. Eng. has joined
RST Instruments Ltd., in Maple
Ridge, British Columbia, as Sales
Engineer. Hayley has 5 years of
experience in geotechnical consult-
ing and mining and completed her
B.A.Sc. in Geological Engineering at
The University of British Columbia.
She has experience with geotechnical
instrumentation, geotechnical design,
landslide monitoring, slope stability
analysis and geotechnical field review.
