"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 email@example.com
Vince Stricherz | EurekAlert!
Predicting unpredictability: Information theory offers new way to read ice cores
07.12.2016 | Santa Fe Institute
Sea ice hit record lows in November
07.12.2016 | University of Colorado at Boulder
Physicists of the University of Würzburg have made an astonishing discovery in a specific type of topological insulators. The effect is due to the structure of the materials used. The researchers have now published their work in the journal Science.
Topological insulators are currently the hot topic in physics according to the newspaper Neue Zürcher Zeitung. Only a few weeks ago, their importance was...
In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.
Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
09.12.2016 | Life Sciences
09.12.2016 | Ecology, The Environment and Conservation
09.12.2016 | Health and Medicine