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THE ASTORIAN • SATURDAY, SEPTEMBER 21, 2019
Scientists probe lakes to learn about quakes
Research could improve
computer models
By TOM BANSE
Northwest News Network
Multiple teams of earthquake research-
ers are looking in what may seem like an
unlikely place to fi gure out how strongly
the Pacifi c Northwest shook during great
quakes in the past.
They’re poking around the bottom of
lakes in w estern Washington and Oregon.
It turns out lakes preserve a nifty earth-
quake record that can shed light on the next
“Really Big One.”
“The lake records are the missing link,”
Oregon State University marine geologist
Chris Goldfi nger said. “We don’t know
very well how hard it is going to shake as
you move inland. We have models, which
are great, but we don’t have very much
direct evidence.”
Goldfi nger leads one of four separate
teams of geologists that are probing differ-
ent Northwest lakes for traces of ancient
earthquakes or tsunamis. He said a key
goal of his lake investigations is to “ground
truth” computer models of the potential
shaking under the region’s major cities,
which are more than 100 miles east of the
dangerous offshore Cascadia fault zone.
“A good way to test that simply is to do a
transect of lakes across the land and go east
away from the fault far enough until the
signal goes away,” Goldfi nger explained.
“That mostly worked.”
The existence or absence of underwa-
ter landslides allows geologists to extrap-
olate “how hard did it actually shake” in
past great earthquakes at specifi c distances
from the epicenter. Goldfi nger’s prelim-
inary assessment is that the current U.S.
Geological Survey seismic hazard maps
capture the earthquake risk pretty well for
the Northwest’s population centers.
Over the past fi ve years, Goldfi n-
ger and a cadre of students and techni-
cians deployed sonar to map a selection of
lake basins and extracted cores from the
lake bottoms. The best results came from
Leland Lake, northwest of Seattle in Jeffer-
son County, from Lake Sawyer, southeast
of Seattle near Black Diamond, and from
Bull Run Lake in the foothills of Mount
Hood east of Portland.
Evidence of ancient earthquakes is
recorded in thin, almost invisible, stripes in
the tubes of chocolate-colored mud pulled
from the lake fl oors.
“If you shake the whole lake basin, a
lot of sediment along the shoreline will fail
Tom Banse/Northwest News Network
Curator Val Stanley and geologist Chris Goldfi nger examine a sediment core in the vast
collection of the Marine and Geology Repository at Oregon State University.
and just run to the bottom of the basin and
leave what is called a turbidite, which is
just a submarine landslide deposit,” Gold-
fi nger said in an interview at Oregon State’s
vast geologic core repository. “It can also
fail just by having the very top (layer of)
fl uff sediment fail – just a few centimeters
of sediment — which still leaves a deposit
that you can fi nd.”
Carbon dated
The events that leave a stripe can be car-
bon dated. When the same date shows up in
lake after lake over a wide area, he infers
the cause was most likely a major earth-
quake or volcanic eruption. Analysis of the
lake sediments yielded a geologic record
of 16 to 25 earthquakes going back as far
as 10,000 years. Some of the hand-pushed
cores were stopped by a concrete-like layer
of ash left behind by the cataclysmic erup-
tion of Mount Mazama, which created Ore-
gon’s Crater Lake, about 7,600 years ago.
When Goldfi nger studied the sidewalls
of these lakes to estimate how much shak-
ing it took to unleash underwater land-
slides, his team found the strength of shak-
ing diminished considerably with distance
from the offshore Cascadia fault. This
could be reassuring for people in the Inter-
state 5 corridor worried about the next “Big
One.”
“It is going to shake very gently,” Gold-
fi nger predicted. “People will be standing
there talking about it going, ‘Should we do
something? Is this a big earthquake or just
a little one?’”
But Goldfi nger isn’t about to give the
inland cities of the Northwest a pass on
earthquake preparation.
“The thing about the ‘Big One’ is that
it will go on for minutes and minutes and
minutes, which are an eternity when you’re
in it,” he said. “The fragile infrastructure
that we have — the unreinforced masonry
buildings, the pipelines built in the 1930s,
all these sorts of things — really can’t han-
dle even light shaking for less than one
minute, let alone four or fi ve minutes.
That’s the thing.”
As for the other research teams, one is
from the U.S. Geological Survey’s Santa
Cruz offi ce. It is focused on Floras Lake,
in Oregon, and Lake Ozette, Washington.
Those lakes are so close to the coast, they
can show how often Cascadia earthquakes
trigger tsunamis. A tsunami leaves behind
a layer of beach sand in the lake sediments.
A different team led by University of
Oregon geologist Joshua Roering is scour-
ing lakes in the Oregon Coast Range. The
premise here is that great earthquakes
should trigger landslides. Landslides could
block streams and drown forests — or
dump trees into a lake. Close examination
of the tree rings in those so-called “ghost
forests” can tell you exactly when the trees
died. Then you construct a chronology for
ancient catastrophes that you can project
forward.
Yet another set of researchers is target-
ing underwater “ghost forests” in the Puget
Sound region. The U.S. Geological Sur-
vey’s regional earthquake hazards project
chief, Brian Sherrod, is hopeful that evi-
dence found in lakes can help him fi gure
out how often and how hard the ground
shakes from earthquakes spawned from
widespread shallow, crustal faults on land,
such as the Seattle Fault, Tacoma Fault and
the Southern Whidbey Island Fault.
“We have a whole bunch of those in
the Puget Lowlands,” Sherrod said. “Any
of those could produce fairly signifi cant
earthquakes and could be quite damaging
to the urban area.”
Sherrod said previous studies have estab-
lished pretty well that the offshore Casca-
dia fault zone generates a magnitude eight
or nine megaquake about every 500 years
on average. Sherrod says the lake evidence
may show damaging crustal earthquakes
happen locally much more often than Cas-
cadia Subduction Zone megaquakes.
Wood samples
University of Arizona Professor Bryan
Black is helping Sherrod collect wood
wedges from ghost forests in lakes on
many sides of Puget Sound — including
Price Lake, Mill Pond and Catfi sh Lake —
as well as some nearby saltwater inlets.
Black said the wood samples he has
analyzed so far are well preserved. He is
keen to fi nd if there is a connection to the
last full rip on the Cascadia fault, which
happened in the year 1700. Another high
priority is to determine the extent of strong
shaking from the last rupture of the shal-
low Seattle fault, approximately 1,100
years ago.
Black said the Seattle fault quake trig-
gered landslides that sent forests into
nearby Lake Sammamish and Lake Wash-
ington. He said a lot of sunken wood in
tidewater inlets down by Olympia can be
radiocarbon dated to around 1,100 years
ago too, which suggests the Seattle quake
might be connected to a more massive
event that touched a wider area.
“We’re trying to get an exact calendar
year for the Seattle quake and fi gure out if
it was coincident with events to the south,”
Black said . “Was it one really big quake
that involved many faults at once or maybe
these faults ruptured separately all within
a few years of each other — or possibly
decades apart. If it was at all it once, that
would have implications for the worst case
from crustal earthquakes.”
RECOLOGY WESTERN OREGON
TACKLES CONTAMINATION AT THE CURB
Contamination at the curb - What’s the big deal? Recycling works best when done properly.
Let’s all do our part to keep the recycle stream free of contamination to ensure an efficient
recycling system. Check out this list of common contaminants and how to dispose of
them appropriately.
contamination toensure an efficientrecycling system. Check outthislist of common contaminants andhowtodisposeofthemappropriately.
Contaminant
Pet food bags
Plastic bags
Plastic Clamshells
Styrofoam
Snack bags &
wrappers
Food Residue
Napkins & Paper
Towels
Why not at the curb?
Lined with plastic.
Nearly impossible to
separate the plastic
from the paper fiber
for recycling.
Clogs machinery at
sorting facility.
Therefore it needs
to be collected
separately.
No market for this
low-grade plastic.
Made with a mixture
of material - such
as plastic and alumi-
num, making it very
difficult to recycle.
Breaks into tiny piec-
es when compacted
in recycle truck.
Therefore it needs
to be collected
separately.
Food residue inside
containers leads to
mold and germs.
Always rinse or wipe
out containers
before placing in
the cart.
Paper fibers get
shorter each time
they are recycled.
By the time they
become a towel or
tissue, the fibers
are too short to be
recycled again.
Where should it go?
Place in your trash
cart. Consider buy-
ing in bulk to reduce
the number of pet
food bags.
Take to a partici-
pating retails store.
Consider using
canvas or other
reusable bags.
Place in your trash
cart. Consider less
prepackaged foods
to reduce plastic
clamshell waste.
Place in your trash Rinse or wipe to
Put in your home
Place in your trash
cart.
remove food residue compost system or
cart. Consider using
before recycling.
in your trash cart.
reusable snack bags.
Reduce by using
cloth towels.
For more information on recycling in McMinnville please got to Recology.com,
call 503-861-0578 or email rwoinfo@recology.com