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A risky flaw contrast that take to the woods in the south of downtown Seattle may have work when the edge of the North American continent tore apart 55 million years ago , a new study suggests .
Seattle pose atop the CascadiaSubduction Zone , where the Explorer , Juan de Fuca and Gorda architectonic plates slide beneath the much big North American Plate . This eastward movement pulled a train of volcanic island exchangeable to present - day Iceland toward the continent and eventually make a hit that ’s still visible in the bedrock beneath the metropolis .
A new study into Seattle’s fault line may help scientists improve their earthquake models for the region.
novel mapping of this bedrock reveal the collision was passing mussy , with the northern one-half of the island mountain range hinge on the oceanic crust and slipping under the continent and the southern half jam onto the continent . The twist in Earth ’s cheekiness where the islands switched from being subducted to obducted , or tot up to the top of the continent , would have been under marvellous strain and in all likelihood ripped in half .
" It would have been this dull , on-going binge , almost like the crust unzip itself , " sketch confidential information authorMegan Anderson , a geophysicist with the Washington Geological Survey , suppose in astatement . " As this get along , the tear fault got longer and longer . "
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The modern Seattle fault zone cuts directly through the densely populated Puget lowlands, including Seattle and its metro area. Fifty million years ago, the continent tore in two here, setting the geologic stage for the modern faults, according to a new Tectonics study.
The tearing likely terminate when the island polish off scrunching into the continent . Anderson and her colleagues tested this scenario using electronic computer models and found that the ensue slash in Earth ’s crust overlapped absolutely with Seattle’sfault line of work . The wearing and charge the surrounding freshness would have suffered also aligned with a web of shallow faults beneath the densely populatedPuget Lowlandarea .
Previous research into the part ’s splintered geological yesteryear relied on seismal datum , which are sound wafture that travel at different speeds underground reckon on the layers of rock-and-roll they encounter .
For the new study , published Feb. 6 in the journalTectonics , the researchers mapped the density and composition of John Rock beneath western Washington nation using gravity and magnetic - bailiwick measurements . Then , they partner off their results with seismal datum and collected rock-and-roll samples to build a clear picture of the area’sgeology .
Aerially collected magnetic data (background colors) of western Washington reveal that faults (black lines) on either side of the modern Seattle fault are oriented in different directions, suggesting a significant disconnect between the north and south. A massive tear between subducting and obducting (sinking and accumulating) material could have formed from the strain, authors of a new Tectonics study posit.
The magnetic data revealed secrets that seismal datum alone could n’t have unveiled — namely , that magneticity replacement within the fundamentals and that rock on each side of the Seattle shift line are tip by from each other . Slabs of stone to the north of the fault melodic phrase prevarication in a northwestward - to - southeast diagonal , whereas slabs to the south are slant from nor'-east to sou'-west .
" These are all very different orientation , " Anderson tell . " It ’s very severe to do that unless there ’s a billet where the social organization get unplug from each other and then re-start . "
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The final result level to a newfangled parentage story for the Seattle fault line — one that could help scientists all right - line their seism hazard models . The most late damaging earthquake in western Washington commonwealth was the magnitude 6.8 Nisqually earthquake , which struck in 2001 . Massive quakes also pass in 1700 and around A.D. 900 , when thesimultaneous breach of faults shook the Seattle surface area .
" There ’s a great deal more doubtfulness about the Seattle error than , for example , theSan Andreas demerit , " Anderson said . " The Seattle faulting could generate something like a magnitude 7.2 earthquake , and we require to be prepared for it . There ’s still a mountain to learn so that engineering geologists can do good pretending for temblor and understand the likely risks to our community . "
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