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Post Info TOPIC: Seismic Slip Event


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Cascadian subduction zone
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For the past few weeks, seismologists at the University of Washington in Seattle have been on high alert. Any day now, they expect a flurry of microtremors deep under the nearby Olympic Peninsula, just as occurs roughly every 12-14 months. And when that wave of vibrations comes along, the researchers will be ready to catch it.
Over the past year, the seismology team has set up an elaborate net--an array of more than 100 seismic sensors planted in the ground throughout the peninsula's mountains. When those instruments start picking up signs of the tremors, the researchers will rush back out to install extra seismometers above the spots where the earth is shuddering. Every few days the scientists will tend to the sensors, replacing batteries and downloading data, with the hope of capturing as much information as possible about the seismic events underfoot.

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RE: Seismic Slip Event
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Bogus volcano warning linked to computer programming

The Washington State Emergency Management Department says it has found the cause of the errant warning broadcast June 7 during a monthly test of coastal sirens, including those on the North Olympic Peninsula.
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Oregon tsunami evacuation zones
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A Google map of tsunami evacuation zones for the Oregon coast.
The Tsunami Map Viewer returns a map of areas with Tsunami Evacuation Zones based on information that you enter.

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RE: Seismic Slip Event
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Tremors between slip events: More evidence of great quake danger to Seattle

For most of a decade, scientists have documented unfelt and slow-moving seismic events, called episodic tremor and slip, showing up in regular cycles under the Olympic Peninsula of Washington state and Vancouver Island in British Columbia. They last three weeks on average and release as much energy as a magnitude 6.5 earthquake.
Now scientists have discovered more small events, lasting one to 70 hours, which occur in somewhat regular patterns during the 15-month intervals between episodic tremor and slip events.

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Seattle-Tacoma mega-earthquake
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New study shows that two massive tectonic plates colliding 25 miles or so underneath Washington state's Puget Sound basin; findings suggest that a mega-earthquake could strike closer to the Seattle-Tacoma area, home to some 3.6 million people, than was thought earlier

Using sophisticated seismometers and GPS devices, scientists have been able to track minute movements along two massive tectonic plates colliding 25 miles or so underneath Washington state's Puget Sound basin. Their early findings suggest that a mega-earthquake could strike closer to the Seattle-Tacoma area, home to some 3.6 million people, than was thought earlier.
The deep tremors, which humans can not feel, occur routinely every 15 months or so and can continue for more than two weeks before they die back to undetectable levels. The instruments are detecting an inch or two of movement -- known as "episodic tremor and slip" -- as the Juan de Fuca plate grinds and sinks beneath the North American plate. Closer to the surface, the two plates are locked together. When they snap, scientists say, it could produce a massive 9.0 or greater earthquake and a tsunami.

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1700 Cascadia Earthquake
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The 1700 Cascadia Earthquake was a magnitude 8.7 9.2 megathrust earthquake that occurred in the Cascadia subduction zone in 1700. The earthquake involved the Juan de Fuca Plate underlying the Pacific ocean, from mid-Vancouver Island in southwest Canada off British Columbia to northern California, along the Pacific Northwest coast. The length of the fault rupture was about 1000 kilometres with an average slip of 20 meters.

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RE: Seismic Slip Event
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Research links seismic slip and tremor, with implications for subduction zone
Scientists are investigating interactions between the Juan de Fuca and North American Plates. 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.

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An important seismic event imperceptible to humans has begun in the Pacific Northwest as predicted, according to the government agency Geological Survey of Canada.

The event is called episodic tremor and slip (ETS). It involves a slow movement of the Juan de Fuca and North America tectonic plates along the Cascadia margin of southern British Columbia. Faults associated with the plates have been the sites of major earthquakes -- akin to the colossal tsunami-causing quake last December in Indonesia -- every 500 years or so, the geologic record shows. The last such temblor in the area struck on Jan. 26 in the year 1700.

The movement is slower than a traditional earthquake but more rapid than the normal creep associated with the fault. It runs in the reverse direction of the normal creep.
The movement was predicted. Scientists recently learned that these ETS events recur about every 14 months. It has been detected by Global Positioning System instruments.
The event does not mean an earthquake is imminent, but geologists are eager to study it and learn more and they say sooner or later an ETS event is likely to trigger a major quake.

"Compared to the steady year-round stress accumulation, this more rapid stress increase implies that a large subduction earthquake is more likely to happen during the time of an ETS event" the Canadian geologists write.

"The slip event and the associated (minor) tremors are directly related to megathrust (Sumatra-like) earthquake potential. Neither the tremor nor the slip can be felt" statement from lead geologist John Cassidy and a colleague.

The slip began Sept. 3 on the Olympic Peninsula of Washington State and has migrated north to the Vancouver Island area, Cassidy wrote. Victoria moved 3 millimetres to the west over the course of two days. The events are thought to last six to 15 days.

"We think that it's one of these events that will trigger the big megathrust earthquake. We just don't know which one of these events will trigger the giant earthquake'' - John Cassidy.

The immediate importance of the event is that it occurred as predicted and can now be used to improve understanding of the region's seismology.

"By better understanding these events, we will be able to better predict the effects (and perhaps timing) of future magnitude 9 earthquakes along the west coast".

A separate study recently concluded that a major earthquake along the fault could be overdue, given clusters of the events seen in the geologic record. Because the fault is offshore, scientists say its rupture could create a devastating tsunami.

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