SZO Seminar Notes - April 22, 2015: Potential
earthquakes along
the Japan Trench - Geologic clues before 3/11
Discussion leaders: Brian Atwater & Jody
Bourgeois
Papers: Sugawara, D., K.
Goto, F. Imamura, H. Matsumoto and K.
Minoura (2012), Assessing the magnitude of the 869 Jogan tsunami using
sedimentary deposits: Prediction and consequence of the 2011
Tohoku-oki
tsunami, Sed. Geol., 282, 14-26.
For the first part of the seminar Jody described
her time in Japan
before, during, and in the aftermath of the Tohoku earthquake. She
suggested that the impacts of the Tohoku earthquake would not
have been very
different even if information about the 869 AD Jogan earthquake
had been
differently interpreted and more widely considered. She noted
that many people were
already very well prepared and the most disastrous impact was
the destruction
of the nuclear power plant, which likely would have happened
under many
circumstances. More
importantly, we
should consider how can tsunami deposits and other
geologic/stratigraphic
records might best be used to learn and prepare.
Jody summarized what she learned on a field trip to
the Sendai
region in 2010. At that time scientists
knew about the Jogan
earthquake and the large run-up its tsunami left, as documented
in a sand layer
clearly visible in cores, just beneath an ash layer dated at 915
AD. The Jogan
tsunami deposits had been mapped and
modeled as a likely a M8.3 event.
The
lateral extent of the earthquake is not well known, largely
because evidence to
the north is difficult to obtain; going northward access becomes
difficult,
there is no ash marker, and the more rugged (steep walled
valleys) topography
makes interpretation more difficult.
After the Maule earthquake, the company that owns
the Fukashima
nuclear plant, Tepco, reviewed the tsunami vulnerability of its
power plants
and estimated a maximum run-up for Fukashima of 5.7 m from a
M8.4, although
they considered larger slip values and possible 8 m of run-up.
The run-up from
Tohoku exceeded 12 m.
Jody also noted that when Tohoku happened forecasts
of the tsunami
were issued, but these may have led to a false sense of security
because the
estimates were too small and the precision reported of 10 cm was
misconstrued
as accuracy.
We discussed the paper by Aria et al., which
documents and
interprets recordings of turbidity currents.
Notably, the study shows that the turbidity currents were
triggered by
backwash from the tsunami, not the shaking.
This possibility has not been considered previously
(shaking usually
assumed to be the trigger).
It was noted that the potential for turbidites
depended strongly
on the sediment supply, which may change drastically over time. This complicates
interpretations of turbidite
deposits spanning intervals thousands of years.
We discussed the potential of using bathymetry to
map old
turbidity flows and provide baseline information for future
ones. Surface
roughness and other analyses could be
useful with such data. Paul
Johnson
emphasized that while done routinely, acquisition of standard
swath bathymetric
data had to be done very carefully. He
noted that only recently has it been possible to collect such
data offshore of
Washington State, as it was previously prohibited by the Navy as
it is/was the
corridor for Trident subs.
He also
informed us that funding to use the Thompson ship for
educational purpose was available
from the State for a month each year, and suggested that 80 km
wide
margin-perpendicular swaths could be acquired in a 10-day
cruise, so the entire
margin could be mapped over several years.