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Topic Name: UW researchers have found Rising tides intensify non-volcanic tremor in Earth's crust
Category: Earthquake Engineering
Research persons: Justin Rubinstein, Mario La Rocca
Location: University of Washington, United States
Details
For more than a decade geoscientists have detected what amount to
ultra-slow-motion earthquakes under Western Washington and British Columbia on a
regular basis, about every 14 months. Such episodic tremor-and-slip events
typically last two to three weeks and can release as much energy as a large
earthquake, though they are not felt and cause no damage.
Now University of Washington
researchers have found evidence that these slow-slip events are actually
affected by the rise and fall of ocean tides.
"There has been some previous evidence of the tidal effect, but the
detail is not as great as what we have found," said Justin Rubinstein, a UW
postdoctoral researcher in Earth and space sciences.
And while previous research turned up suggestions of a tidal pulse at 12.4
hours, this is the first time that a second pulse, somewhat more difficult to
identify, emerged in the evidence at intervals of 24 to 25 hours, he said.
Rubinstein is lead author of a paper that provides details of the findings,
published Nov. 22 in Science Express, the online edition of the journal Science.
Co-authors are Mario La Rocca of the Istituto
Nazionale di Geofisica e Vulcanologia in Italy, and John Vidale, Kenneth
Creager and Aaron Wech of the UW.
The most recent tremor-and-slip events in Washington and British Columbia
took place in July 2004, September 2005 and January 2007. Before each,
researchers deployed seismic arrays, each containing five to 11 separate seismic
monitoring stations, to collect more accurate information about the location and
nature of the tremors. Four of the arrays were placed on the Olympic Peninsula
in Washington and the fifth was on Vancouver Island in British Columbia.
The arrays recorded clear twice-a-day pulsing in the 2004 and 2007 episodes,
and similar pulsing occurred in 2005 but was not as clearly identified. The
likely source from tidal stresses, the researchers said, would be roughly once-
and twice-a-day pulses from the gravitational influence of the sun and moon. The
clearest tidal pulse at 12.4 hours coincided with a peak in lunar forcing, while
the pulse at 24 to 25 hours was linked to peaks in both lunar and solar
influences.
The rising tide appeared to increase the tremor by a factor of 30 percent,
though the Earth distortion still was so small that it was undetectable without
instruments, said Vidale, a UW professor of Earth and space sciences and
director of the Pacific Northwest Seismograph Network.
"We expected that the added water of a rising tide would clamp down on
the tremor, but it seems to have had the opposite effect. It's fair to say that
we don't understand it," Vidale said.
"Earthquakes don't behave this way," he added. "Most don't
care whether the tide is high or low."
The researchers were careful to rule out noise that might have come from
human activity. For instance, one of the arrays was near a logging camp and
another was near a mine.
"It's pretty impressive how strong a signal those activities can
create," Rubinstein said, adding that the slow-slip pulses were recorded
when those human activities were at a minimum.
Note for Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.
An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter means the point at ground level directly above this.
Note for Crust
In geology, a crust is the outermost layer of a planet.
The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovicic discontinuity, a boundary defined by a contrast in seismic velocity. Earth's crust occupies less than 1% of Earth's volume.
The oceanic crust of the Earth is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi)
thick and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and it is mostly composed of less dense rocks than is the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust.
About Researchers
Postdoctoral Scholar
Department of Earth and Space Sciences
University of Washington
Johnson Hall -- Room 070
Box 351310
4000 15th Ave NE
Seattle, WA 98195
Office: ATG 225
Phone: (206) 685-7563
Fax: (206) 543-0489
Email: justin@ess.washington.edu
Mario La Rocca
BORN: April 13, 1969, in San Cipriano Picentino (SA), Italy; Male.
E-mail:
EDUCATION:
Degree in Physics, May 1996, from University of Salerno;
Ph. D. in Physics, February 2000, University of Salerno.
SCIENTIFIC CONTRIBUTIONS:
(1) Seismological monitoring of active volcanoes. Field surveys on Vesuvius, Stromboli, Etna, Panarea (Italy), Teide (Spain), Mt. St. Helens (Washington, USA), Deception (South Shetland, Antarctica), Nisyros (Greece), Azores Islands (Portugal).
(2) Array seismology on volcanoes. Wave field composition, polarization analysis, source location of tremor and long period seismic events.
The work was funded by the National Science
Foundation, and instruments were provided by the Incorporated
Research Institutions for Seismology, Istituto Nazionale di Geofisica e
Vulcanologia and Earthscope.
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