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Wednesday, February 6, 2019 The Nugget Newspaper, Sisters, Oregon 13 ‘Silent slip’ along fault line may serve as prelude to big quakes By Steve Lundeberg Correspondent CORVALLIS 4 Big earth- quakes appear to follow a brief episode of <shallow mantle creep= and <seismic swarms,= suggests new research at Oregon State University that offers an explanation for the foreshocks observed prior to large temblors. Published today in Nature Geoscience, the findings are an important step toward understanding the relation- ship and interactions between aseismic slip and seismic slip. Also known as silent slip or slow slip, aseismic slip is dis- placement along a fault that occurs without notable earth- quake activity. The research involved the Blanco Transform Fault off the coast of Oregon; a trans- form fault is a plate boundary at which the motion is mainly horizontal. Under the sea, transform faults connect offset mid- ocean <spreading centers,= places at seafloor ridges where new oceanic crust is formed through volcanic activity and gradually moves away from the ridge. <Slow slip directly trig- gers seismic slip 4 we can see that,= said co-correspond- ing author Vaclav Kuna, a graduate student in geology and geophysics in OSU9s College of Earth, Ocean and Atmospheric Sciences. <The findings are very interesting and may have some broader implications for understand- ing how these kinds of faults and maybe other kinds of faults work.= Researchers deployed 55 seismometers on the ocean bottom on and around the Blanco fault for a year. <It9s a very seismically active fault that generates sig- nificant earthquakes at higher rates than the majority of faults on land, making it ideal for studying the process of earthquake generation,= Kuna said. The seismometer deploy- ment 4 from September 2012 to October 2013 4 resulted in the detection of more than 1,600 earthquakes at the Blanco Ridge, a 130-kilome- ter segment of the Blanco fault that served as the study area. Two distinct asperities 4 basically rough edges 4 along the ridge rupture roughly every 14 years with quakes in the magnitude 6 range. <Our work was enabled by recent advances in long-term ocean-bottom seismometer deployments and is only the second major project targeting an oceanic transform fault,= said co-corresponding author John Nabelek, professor of geology and geophysics at OSU. At its southernmost point, the Blanco Transform Fault is about 100 miles from Cape Blanco, Oregon9s western- most location, and the fault runs northwest to a point about 300 miles from Newport. The Cascadia Subduction Zone, a fault that extends from British Columbia to northern California, lies between the Blanco fault and the coast- line. The fault was the site of a magnitude 9 earthquake in 1700 and is building up stress where the Juan de Fuca Plate is sliding underneath the North American Plate. Some scientists predict a 40 percent chance of another magnitude 9 or bigger quake occurring along the fault in the next 50 years. <The Blanco fault is only 400 kilometers offshore,= Nabelek said. <A slip on Blanco could actually trigger a Cascadia Subduction slip; it would have to be a big one, but a big Blanco quake could trigger a subduction zone slip.= The Earth is put together in layers beneath the crust, the outermost skin that varies in thickness from about 40 miles (continental crust at moun- tain ranges) to about 2 miles (oceanic crust at mid-ocean ridges). The boundary between the crust and the next layer, the upper mantle, is known as the Moho. <We see slow, aseismic slips that occur at depth in the fault beneath the Moho and load the shallower part of fault,= Nabelek said. <We can see a relationship between mantle slip and crust slip. The slip at depth most likely trig- gers the big earthquakes. The big ones are preceded by fore- shocks associated with creep.= Kuna explains that the HAPPY HOUR MON-FRI 4-7pm ROCKIN ROBIN’S KARAOKE NIGHTS! layers have different levels of seismic <coupling,= the ability of a fault to lock at asperities and accumulate stress. <The crust is fully cou- pled 3 all slip is released in a seismic way,= Kuna said. <Fault in the shallow mantle is partly coupled, partly not, and releases slip both seismi- cally and aseismically. The deep mantle is fully creeping, Entertainment & Events FEB 6 WED FEB 8 FRI Prime Rib Fridays 5pm! 175 N. Larch St. t. hardtailsoregon.com Facebook darcymacey FEB 9 SAT FEB 10 SUN Feb. 15 / Fri / 7 PM Dry Canyon Stampede 7-piece Country Western dance band performing classics to contemporary with a blend of rockin’ rhythms & moving ballads. Feb. 21-24 / Th-Sun Sisters One Acts Play Mar. 16 / Sat / 8 PM Tommy Castro and the Sisters Saloon Poker Night 7 p.m. Every Wednesday! $20. For information call 541-549-7427 or go to sisterssaloon.net. Cork Cellars Tasty Thursday Hosted Wine Tasting 5 to 7 p.m. For additional information call 541-549-2675 or FEB go online to corkcellarswinebistro.com. 7 THUR Sisters Saloon Karaoke Night 9 p.m. to midnight. Every Thursday, no cover! For additional information call 541-549-7427 or go to sisterssaloon.net. FRIDAY • SATURDAY 541-549-6114 uncoupled, with no earth- quakes. But the fault is loaded by this creep from beneath 3 it9s all driven from beneath. Our results also show that an aseismic fault slip may trigger earthquakes directly, which may have implications for active faults on land.= The National Science Foundation supported this research. FEB 11 MON Paulina Springs Books Author Presentation with Pam Houston 6:30 p.m. “Deep Creek” is a memoir that tells the story of “that girl who dared herself to buy a ranch...” For more info call 541-549-0866 or go to paulinasprings.com. Hardtails Bar & Grill Karaoke Night with Rockin Robin 9 p.m. Every Friday, no cover! For additional information call 541-549-6114 or go to hardtailsoregon.com. Cork Cellars Live Music with Brian Odell and Fiddler Bob Baker 6:30 to 8:30 p.m. No cover! For information call 541-549-2675 or go online to corkcellarswinebistro.com. Hardtails Bar & Grill Karaoke Night with Rockin Robin 9 p.m. Every Saturday, no cover! For additional information call 541-549-6114 or go to hardtailsoregon.com. 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