CORVALLIS, Ore. – Major earthquakes seem to follow a brief episode of "shallow-mantled sneak" and "seismic swarming," suggesting new research at Oregon State University that provides an explanation for the tremors observed before major tremors.  The results published on Monday in Nature Geoscience are an important step in understanding the relationships and interactions between aseismic and seismic slip. Aseismic slip, also known as silent slip or slow slip, is a shift along an error that occurs without appreciable earthquake activity.
The research concerned the Blanco transformation fault off the Oregon coast; A transformation error is a plate boundary where the motion is mostly horizontal.
Below sea level, transformation faults connect centers offset in the middle of the ocean, at the seafloor backs, where new oceanic crust is formed by volcanic activity
"Slow slip causes seismic failure – we can see that," said the corresponding author Vaclav Kuna, a doctoral student in geology and geophysics at the OSU College of Earth, Ocean and Atmospheric Sciences. "The results are very interesting and could have some more profound implications for understanding how these bugs and possibly other bugs work."
The researchers deployed 55 seismometers on the seafloor and around the Blanco Fault for a year. 19659006] "It is a very seismically active disturbance that causes significant earthquakes in earthquakes at a higher rate than most disturbances on land, making them ideal for studying the process of earthquake generation," said Kuna.
The Seismometer Mission – From September 2012 to October 2013, more than 1,600 earthquakes were reported at Blanco Ridge, a 130 kilometer stretch of the Blanco fault that served as the study area.
Two different bumps – essentially rough edges – along the ridge fracture 14 years with 6 magnitude earthquakes
"Our work has been made possible by the recent advances in long-term seismometer measurements in the seafloor and is underway second major project aimed at an oceanic transformational deficiency, "said John Nabelek, professor of geology and geophysics at OSU.
At its southernmost point, the Blanco Transfiguration fault is located about 100 miles from Cape Blanco, the far west Oregon location, and the fault runs northwest to a point about 300 miles from Newport.
The Cascadia Subduction Zone, a fault that stretches from British Columbia to northern California, lies between the Blanco Fault and the coast. The fault was the site of a magnitude 9 earthquake in 1700 and builds up stress as the plate Juan de Fuca slides under the plate of North America.
Some scientists predict a 40 percent chance of another quake of magnitude 9 or more ahead of the error in the next 50 years.
"The Blanco fault is only 400 kilometers off the coast," said Nabelek. "A slip-up at Blanco could actually trigger a Cascadia subduction spurt; It would have to be a big one, but a big blubber quake could trigger a slip off of the subduction zone.
The earth is arranged in layers below the crust, with the outermost skin varying in thickness from about 40 miles (continental crust) in mountain ranges to about 2 miles (oceanic crust at the crests of the middle ocean).
The boundary between the crust and the next layer, the upper mantle, is known as Moho.
"We see slowly, aseismically, nooks and crannies that occur in depth below the Moho and strain the flatter part of the fault," Nabelek said. "We can see a relationship between cladding and crust transition. The depth slide most likely triggers the big earthquakes. The big ones are precursors associated with creep. "
Kuna explains that the layers have different levels of seismic" coupling ", the ability of a fault to block bumps and build up stress – the entire slip is released seismically," said Kuna. "Mistakes in the shallow mantle are partially coupled, sometimes unreliable, and trigger both seismic and seismic. The deep coat crawls completely without earthquake. But the mistake is loaded by this creep from below – everything is driven from below. Our results also show that an aseismic slip can directly trigger earthquakes, which can affect land-based active errors.
The National Science Foundation supported this research.