Silent earthquakes differ unexpected turn

Cascadia subduction zone (Figure Steve Malone / Central Washington University).

May 23, 2011. Scientists continues to hold secret imperceptible, but the long seismic phenomenon called episodic tremor and slip (ETS, episodic tremor and slip). Recently, the "quiet earthquake" (or, if you like, "slow slip") was discovered a strange twist: a concussion can suddenly change course and return to again go through the rift that it formed a few days earlier, the rate of return movement in the 20-40 times higher than the original — because the area is already weakened.

ETS was first recorded in the north-west of the United States ten years ago and since then has been on a regular basis in the State of Washington and British Columbia on average every 12-15 months. As a rule, the slow slip starts in the south of the region Puget Sound (From Tacoma in the south to Bremerton in the north) and gradually moving to the north-west to the Olympic Peninsula, following the line of contact between the North American plate and the plate Juan de Fuca (Cascadia subduction zone) Towards the island of Vancouver in Canada.

See also: Wellington is under the influence of slow earthquakes

The process usually takes three to four weeks and release as much energy as the magnitude 6.8 earthquake, without causing damage and without feeling the man in the street. In the normal course of the earthquake fissure runs along the fault at great speed. The reasons for the low rate of "quiet earthquake" remain unclear.

Heidi Houston from the University of Washington (USA) and her colleagues analyzed data from five similar phenomena observed in 2004-2009. A clear signal of that part of the seismic wave is separated from the leading edge of a concussion and is moving at high speed in the opposite direction through a region already traversed. This occurs most often in Strait of Juan de Fuca. And it gave scientists the idea that a role in this phenomenon can play the tidal forces.

In addition, it is noted that the ETS is at a depth of 35-55 km, where due to high temperatures lithospheric plates become softer and unstable. At shallower depths (eg, 20 km) plates can restrain the growth of tension for hundreds of years (and not fifteen months), but the situation is resolved by a powerful earthquake. Such disasters occur in the Cascadia subduction zone on average every 500 years, and the last for today with an 9.0 magnitude occurred in January 1700. Today the region is half-way to the next big push.

See also: Study of the Earth's crust secrets

It remains to answer the following questions. What happens to the plate boundary at a depth lying between "hard" area and a place of "silent earthquakes"? How is the interval between the ETS with the approach of the end of five hundred years cycle? How to transform yourself ETS?

The study is published in the journal Nature Geoscience.

Based on: University of Washington
Source: Kompyulenta

 

Seismic discovery about the formation of volcanoes

September 9, 2013. American scientists claim to have made a discovery that helps explain how the chain of islands are born such as Hawaii and Tahiti. Unlike volcanoes, formed by the collision of tectonic plates, hot volcanoes are out in the plate itself. Current theory is that the hot masses rise vertically from the depths of the Earth's mantle, finding the cracks between the core and the crust of the planet, thus fueling the process of eruption.

However, some of the chain of volcanoes can not be explained as a simple model. Here is a more complex interaction in the upper mantle of the Earth, the researchers expressed the University of California, Berkeley.

Have been found previously unknown channels slow seismic waves in the Earth's upper mantle, which can provide an important piece of the puzzle in the formation of these hot volcanoes and other harsh heat fluxes from the ocean floor.

They also found that the velocity of propagation of seismic waves depends on the temperature. According to them, slow waves may cause a rise in temperature to 200 degrees Celsius or 392 degrees Fahrenheit.

The use of seismic data to create computer models of the Earth's interior has allowed scientists to identify places where these waves are extremely slow. As a result, they hope that these data will provide answers to questions about education volcanic islands of Tahiti, Reunion and Samoa.

Source: Rage Of Nature

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