"Regular tremor and slip goes through an area fairly slowly, breaking it. Then once it's broken and weakened an area of the fault, it can propagate back across that area much faster," said Heidi Houston, a University of Washington professor of Earth and space sciences and lead author of a paper documenting the findings, published in Nature Geoscience.
Episodic tremor and slip, also referred to as slow slip, was documented in the Pacific Northwest a decade ago and individual events have been observed in Washington and British Columbia on a regular basis, every 12 to 15 months on average.
Slow-slip events tend to start in the southern Puget Sound region, from the Tacoma area to as far north as Bremerton, and move gradually to the northwest on the Olympic Peninsula, following the interface between the North American and Juan de Fuca tectonic plates toward Vancouver Island in Canada. The events typically last three to four weeks and release as much energy as a magnitude 6.8 earthquake, though they are not felt and cause no damage.
In a normal earthquake a rupture travels along the fault at great speed, producing potentially damaging ground shaking. In episodic tremor and slip, the rupture moves much more slowly along the fault but it maintains a steady pace, Houston said.
"There's not a good understanding yet of why it's so slow, what keeps it from picking up speed and becoming a full earthquake," she said.
Houston and her co-authors – Brent Delbridge, a UW physics undergraduate; Aaron Wech, a former UW graduate student now at Victoria University of Wellington, New Zealand; and Kenneth Creager, a UW Earth and space sciences professor – analyzed data collected from tremor events in July 2004, September 2005, January 2007, May 2008 and May 2009 (the 2004 and 2005 events took place only toward the north end of the Olympic Peninsula). The five events provided about 110 days' worth of data representing some 16,000 distinct locations.
The scientists found a distinct signal for clusters of tremor moving rapidly backwards from the leading edge of the tremor, through an area of the fault that had already experienced tremor.
They also noted that rapid tremor reversal appears to happen more readily near the Strait of Juan de Fuca, suggesting that stress from tides could play a role in generating the reversal because the interface appears to be more sensitive just after having been ruptured by the initial tremor event.
Houston noted that episodic tremor and slip occurs at a depth of 22 to 34 miles, where high temperatures have made the tectonic plates more pliable and thus more slippery. At a substantially shallower depth, perhaps 12 miles, the plates are not slippery and so are tightly locked together.
In the locked zone, the tectonic plates can hold the buildup of stress for hundreds of years, rather than just 15 months, but when the interface ruptures it can unleash a great megathrust earthquake such as the one that struck off the coast of Japan in March. Such earthquakes occur in the Cascadia subduction zone every 500 years, on average, and the last one – estimated at around magnitude 9.0 – happened in January 1700. Houston noted that the region is within the large time window when another megathrust earthquake could occur.
One key question still to be answered, she said, is what is happening on the plate interface between the locked zone and the depth where tremor occurs. Scientists hope to get a better understanding of the interplay between tremor events and subduction zone earthquakes, including whether the interval between tremor events changes as the end of the 500-year subduction zone earthquake cycle gets nearer.
"Various aspects of the tremor signal may change as the seismic cycle matures," Houston said. "It's also possible that the noise our seismometers detect from tremor events might get louder just before a big earthquake."
The work was funded by a grant from the National Science Foundation.
For more information, contact Houston at 206-616-7092 or firstname.lastname@example.org
Vince Stricherz | EurekAlert!
UCI and NASA document accelerated glacier melting in West Antarctica
26.10.2016 | University of California - Irvine
Ice shelf vibrations cause unusual waves in Antarctic atmosphere
25.10.2016 | American Geophysical Union
Ultrafast lasers have introduced new possibilities in engraving ultrafine structures, and scientists are now also investigating how to use them to etch microstructures into thin glass. There are possible applications in analytics (lab on a chip) and especially in electronics and the consumer sector, where great interest has been shown.
This new method was born of a surprising phenomenon: irradiating glass in a particular way with an ultrafast laser has the effect of making the glass up to a...
Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion
Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how...
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
27.10.2016 | Life Sciences
27.10.2016 | Life Sciences
26.10.2016 | Power and Electrical Engineering