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A6 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
