It appears that major subduction zone quakes around the ring of fire
occur more or less every five hundred years or so. In fact it should
be treated as inevitable. There may be specific locales that are
generally immune for one reason or another, but that also appears to
be the exception. All this means that the local problem cannot be
ignored.
The good news for the North West is that the urban areas have a large
slice of rock between themselves and the natural epicenter. On top
of that the exposed coastal zone is lightly occupied.
What is needed though is the mapping of exclusion zones in which
insurance is mandatory and prohibitive. We watched 200,000 die in
Sumatra and saw towns disappear. The same risks exist for the mouth
of the Columbia river. In a couple of generations the only
structures in such a zone will be strictly those that are necessary.
The key point is that we now understand the risk and understand its
inevitability. We also understand that the events are associated
with serious tsunamis. It is now criminal to not pursue remedies at
the policy level. The solutions are even easy over the long term.
I have watched vigorous earthquake resistance building codes been
steadily adopted here in Vancouver over what must now be two
generations and that captures everything simply because of the normal
cycle of renovation. There is little in the way of cast iron drain
pipes out there either. Once recognized, ownership change usually
triggers activity.
My point is that a major rule set can be deployed over two
generations. This is not perfect, but it is practical for this type
of problem and those unaware become aware of the live risk.
The same should be applied to areas vulnerable to storm surges. Once
again, geology can confirm the risk if there is any doubt. Why
common sense has not already dealt with this escapes me.
13-year Cascadia
study complete - and earthquake risk looms large
by Staff Writers
Corvallis OR (SPX) Aug 07, 2012
Structures such as
the Coos Bay bridge are among the major infrastructure that will face
risks when a subduction zone earthquake strikes the Pacific
Northwest. (Photo by Lynn Ketchum, courtesy Oregon State University).
A
comprehensive analysis of the Cascadia Subduction Zone off the
Pacific Northwest coast confirms that the region has had numerous
earthquakes over the past 10,000 years, and suggests that the
southern Oregon coast may be most vulnerable based on recurrence
frequency. Written by researchers at Oregon State
University, and published online by the U.S. Geological Survey, the
study concludes that there is a 40 percent chance of a major
earthquake in the Coos Bay, Ore., region during the next 50 years.
And that earthquake
could approach the intensity of the Tohoku quake that devastated
Japan in March of 2011.
"The southern
margin of Cascadia has a much higher recurrence level for major
earthquakes than the northern end and, frankly, it is overdue for
a rupture," said Chris Goldfinger, a professor in OSU's College
of Earth, Ocean, and Atmospheric Sciences and lead author of the
study.
"That doesn't
mean that an earthquake couldn't strike first along the northern
half, from Newport, Ore., to Vancouver Island.
"But major
earthquakes tend to strike more frequently along the southern end
- every 240 years or so - and it has been longer than that since
it last happened," Goldfinger added. "The probability for
an earthquake on the southern part of the fault is more than double
that of the northern end."
The publication of the
peer-reviewed analysis may do more than raise awareness of earthquake
hazards and risks, experts say. The actuarial table and history of
earthquake strength and frequency may eventually lead to an update in
the state's building codes.
"We are
considering the work of Goldfinger, et al, in the update of the
National Seismic Hazard Maps, which are the basis for seismic design
provisions in building codes and other earthquake risk-mitigation
measures," said Art Frankel, who has dual appointments with the
U.S. Geological Survey and the University of Washington.
The Goldfinger-led
study took four years to complete and is based on 13 years of
research. At 184 pages, it is the most comprehensive overview ever
written of the Cascadia Subduction Zone, a region off the Northwest
coast where the Juan de Fuca tectonic plate is being subducted
beneath the continent. Once thought to be a continuous fault line,
Cascadia is now known to be at least partially segmented.
This segmentation is
reflected in the region's earthquake history, Goldfinger noted.
"Over the past
10,000 years, there have been 19 earthquakes that extended along most
of the margin, stretching from southern Vancouver Island to the
Oregon-California border," Goldfinger noted. "These would
typically be of a magnitude from about 8.7 to 9.2 - really huge
earthquakes.
"We've also
determined that there have been 22 additional earthquakes that
involved just the southern end of the fault," he added. "We
are assuming that these are slightly smaller - more like 8.0 - but
not necessarily. They were still very large earthquakes that if
they happened today could have a devastating impact."
The clock is ticking
on when a major earthquake will next strike, said Jay Patton, an OSU
doctoral student who is a co-author on the study.
"By the year
2060, if we have not had an earthquake, we will have exceeded 85
percent of all the known intervals of earthquake recurrence in 10,000
years," Patton said. "The interval between earthquakes
ranges from a few decades to thousands of years. But we already have
exceeded about three-fourths of them."
The last
mega-earthquake to strike the Pacific Northwest occurred on Jan. 26,
1700. Researchers know this, Goldfinger said, because written
records in Japan document how an ensuing tsunami destroyed that
year's rice crop stored in warehouses.
How scientists
document the earthquake history of the Cascadia Subduction Zone is
fascinating. When a major offshore earthquake occurs, Goldfinger
says, the disturbance causes mud and sand to begin streaming down the
continental margins and into the undersea canyons.
Coarse sediments
called turbidites run out onto the abyssal plain; these sediments
stand out distinctly from the fine particulate matter that
accumulates on a regular basis between major tectonic events.
By dating the fine
particles through carbon-14 analysis and other methods, Goldfinger
and colleagues can estimate with a great deal of accuracy when major
earthquakes have occurred over the past 10,000 years.
Going back further
than 10,000 years has been difficult because the sea level used to be
lower and West Coast rivers emptied directly into offshore canyons.
Because of that, it is difficult to distinguish between storm debris
and earthquake turbidites.
"The turbidite
data matches up almost perfectly with the tsunami record that goes
back about 3,500 years," Goldfinger said. "Tsunamis don't
always leave a signature, but those that do through coastal
subsidence or marsh deposits coincide quite well with the earthquake
history."
With the likelihood of
a major earthquake and possible tsunami looming, coastal leaders and
residents face the unenviable task of how to prepare for such events.
Patrick Corcoran, a hazards outreach specialist with OSU's Sea Grant
Extension program, says West Coast residents need to align their
behavior with this kind of research.
"Now that we
understand our vulnerability to mega-quakes and tsunamis, we need to
develop a culture that is prepared at a level commensurate with the
risk," Corcoran said.
"Unlike Japan,
which has frequent earthquakes and thus is more culturally prepared
for them, we in the Pacific Northwest have not had a mega-quake since
European settlement. And since we have no culture of earthquakes, we
have no culture of preparedness.
"The research,
though, is compelling," he added. "It clearly shows that
our region has a long history of these events, and the single most
important thing we can do is begin 'expecting' a mega-quake, then we
can't help but start preparing for it."
No comments:
Post a Comment