Why Earthquakes Happen: Water Pressure Under Earth’s Mantle May Be Cause For Major Earthquakes
In February 2010, a massive earthquake struck central Chile and caused untold damage to life and property. It ranks as the sixth largest earthquake ever to be recorded by a seismograph. The earthquake, like all big earthquakes occurred due to collision of oceanic and continental plates. Now, a team of scientists are trying to analyze how water trapped between the two plates contributes to the buildup of pressure along the fault lines, ultimately resulting in an earthquake, according to a press release Friday..
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Led by Onno Oncken, scientists from the GFZ German Research Centre for Geosciences and from Liverpool University have used the 2010 Chile earthquake as a case study to explain how water pressure in the pores of the rocks lining the plate boundary zone causes an earthquake.Mechanical coupling between the oceanic and continental plates results in one plate moving under the other. This process is call subduction and occurs at the boundaries of the plates. Subduction results in build-up of stress along the fault lines and the subsequent release of seismic energy in the form of an earthquake. Lateral extent of the rupture and magnitude of these events ultimately depends on the distribution and pressure of fluids along the plate interface.
"We combined observations of several geoscience disciplines - geodesy, seismology,
petrology. In addition, we have a unique opportunity in Chile that our natural observatory there provides us with long time series of data," say Oncken, director of the GFZDepartment "Geodynamics and Geomaterials". Space-based observations of the Earth such as Earth observation (Geodesy) using GPS technology and radar interferometry allow measuring the movement of tectonic plates in real time. Seismology also provides images of the Earth's sub-surface to understand deep geologic structure.
Earthquake data yield a high resolution three dimensional image of seismic wave speeds and their variations in the plate interface region. Data on fluid pressure and rock properties, on the other hand, are available from laboratory measurements. All these data had been acquired shortly before the great Chile earthquake of February 2010 struck with a magnitude of 8.8.
"For the first time, our results allow us to map the spatial distribution of the fluid pressure with unprecedented resolution showing how they control mechanical locking and subsequentseismic energy release", explains Professor Oncken. "Zones of changed seismic wave speeds reflect zones of reduced mechanical coupling between plates". This state supports creep along the plate interface, which is aseismic fault slip that occurs in the uppermost part of the earth's crust. Zones with a higher degree of locking are interpreted as domains with lower pore fluid pressure. It is these locked domains that subsequently ruptured during the Chile earthquake releasing most seismic energy causing destruction at the Earth's surface and tsunami waves.
The authors suggest the spatial pore fluid pressure variations to be related to oceanic water accumulated in an altered oceanic fracture zone within the Pacific oceanic plate. Upon subduction of the latter beneath South America the fluid volumes are released and trapped along the overlying plate interface, leading to increasing pore fluid pressures. This study has established that spatial pore fluid pressure variations can serve as a useful tool to predict future earthquakes.
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