The slip events coincide with regular occurrences of what scientists call nonvolcanic tremor, which showed up clearly on seismometers but for which the origins were uncertain.
Now researchers from Italy and the University of Washington have concluded that both phenomena are signs of the same processes taking place about 25 miles deep at what is believed to be the interface between the Juan de Fuca and North American tectonic plates.
"We are now more confident that the tremor and the slip are both products of the same slip process," said Kenneth Creager, a UW professor of Earth and space sciences and a co-author of a paper describing the research being published Jan. 30 in Science.
The findings could have major implications for megathrust earthquakes in the Cascadia subduction zone, an area along the West Coast from northern California to southern British Columbia. Megathrust events are huge earthquakes, often in the range of magnitude 9, that occur in areas where one tectonic plate is forced beneath another.
The slow slip events appear to be building stress on the megathrust fault, where the Juan de Fuca plate is sliding beneath the North American plate, with the two locked together most of the time. That pressure is relieved when the plates slip during megathrust earthquakes such as one determined to have occurred off the coast of Washington on Jan. 26, 1700, estimated at magnitude 9.2. That quake was similar to the great Sumatra-Andaman Islands earthquake the day after Christmas in 2004, which also measured 9.2 and triggered a devastating Indian Ocean tsunami.
In such events, the plates are locked together for hundreds of years and then slip past each other by sliding 50 feet or more during a megathrust earthquake.
"The same amount of slip must also occur onshore along the Washington coast," Creager said. "While megathrust earthquakes account for most of the plate motion offshore, and perhaps slightly onshore, episodic tremor and slip harmlessly accommodates much of the plate motion that is taking place on plate interface just west of the Puget Sound region's major population centers."
The paper's lead author is Mario La Rocca of Italy's National Institute of Geophysics and Volcanology Vesuvius Observatory. Other authors are Danilo Galluzzo, also of Italy's geophysics and volcanology institute, and Steve Malone, John Vidale, Justin Sweet and Aaron Wech of the UW.
Slip events occur on the interface between tectonic plates, but previous research has suggested that nonvolcanic tremor occurs in a broad range of depths from the plate boundary to 15 miles above it. The new research indicates the tremor is at the plate boundary, in essentially the same place as the slip.
The researchers used seismometer arrays at Sequim and Lopez Island in Washington state and at Sooke on the southern edge of Canada's Vancouver Island to record an episodic tremor and slip event in 2004. La Rocca devised a novel method using the different times that specific waves generated by the tremor were detected by seismometers, and the data helped the scientists pinpoint the depth of the tremor. At the same time, GPS measurements recorded the slow plate slippage.
Since they were discovered in the last decade, slow slip and tremor events in western Washington and British Columbia have been recorded on a regular basis about every 15 months. GPS signals indicate slip of about 1 inch during an average episode.
"We are quite confident that each episodic tremor and slip event will increase the stress on the megathrust fault," Creager said. "If a megathrust earthquake were to begin off the Washington coast, one might expect it to occur during one of these slow slip events."But he said the findings demonstrate that much research remains to be done.
Vince Stricherz | EurekAlert!
A promising target in the quest for a 1-million-year-old Antarctic ice core
24.05.2018 | University of Washington
Tropical Peat Swamps: Restoration of Endangered Carbon Reservoirs
24.05.2018 | Leibniz-Zentrum für Marine Tropenforschung (ZMT)
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A...
02.05.2018 | Event News
13.04.2018 | Event News
12.04.2018 | Event News
24.05.2018 | Ecology, The Environment and Conservation
24.05.2018 | Medical Engineering
24.05.2018 | Physics and Astronomy