Slow earthquakes may foretell larger events

Scanning electron microscope images showing localized shear surfaces in cross-section and oblique view. Sense of shear is top to the right Note striations on shear surface.  Similar patterns appear with serpentine. -  Haines, S. H.; Kaproth, B.; Marone, C.; Saffer, D. and B. A. van der Pluijm
Scanning electron microscope images showing localized shear surfaces in cross-section and oblique view. Sense of shear is top to the right Note striations on shear surface. Similar patterns appear with serpentine. – Haines, S. H.; Kaproth, B.; Marone, C.; Saffer, D. and B. A. van der Pluijm

Monitoring slow earthquakes may provide a basis for reliable prediction in areas where slow quakes trigger normal earthquakes, according to Penn State geoscientists.

“We currently don’t have any way to remotely monitor when land faults are about to move,” said Chris Marone, professor of geophysics. “This has the potential to change the game for earthquake monitoring and prediction, because if it is right and you can make the right predictions, it could be big.”

Marone and Bryan Kaproth-Gerecht, recent Ph.D. graduate, looked at the mechanisms behind slow earthquakes and found that 60 seconds before slow stick slip began in their laboratory samples, a precursor signal appeared.

Normal stick slip earthquakes typically move at a rate of three to 33 feet per second, but slow earthquakes, while they still stick and slip for movement, move at rates of about 0.004 inches per second taking months or more to rupture. However, slow earthquakes often occur near traditional earthquake zones and may precipitate potentially devastating earthquakes.

“Understanding the physics of slow earthquakes and identifying possible precursory changes in fault zone properties are increasingly important goals,” the researchers report on line in today’s (Aug. 15) issue of Science Express.

Using serpentine, a common mineral often found in slow earthquake areas, Marone and Kaproth-Gerecht performed laboratory experiments applying shear stress to rock samples so that the samples exhibited slow stick slip movement. The researchers repeated experiments 50 or more times and found that, at least in the laboratory, slow fault zones undergo a transition from a state that supports slow velocity below about 0.0004 inches per second to one that essentially stops movement above that spee

“We recognize that this is complicated and that velocity depends on the friction,” said Marone. “We don’t know for sure what is happening, but, from our lab experiments, we know that this phenomenon is occurring.”

The researchers think that what makes this unusual pattern of movement is that friction contact strength goes down as velocity goes up, but only for a small velocity range. Once the speed increases enough, the friction contact area becomes saturated. It can’t get any smaller and other physical properties take over, such as thermal effects. This mechanism limits the speed of slow earthquakes.
Marone and Kaproth-Gerecht also looked at the primary elastic waves and the secondary shear waves produced by their experiments.

“Here we see elastic waves moving and we know what’s going on with P and S waves and the acoustic speed,” said Marone. “This is important because this is what you can see in the field, what seismographs record.”

Marone notes that there are not currently sufficient measuring devices adjacent to known fault lines to make any type of prediction from the precursor signature of the movement of the elastic waves. It is, however, conceivable that with the proper instrumentation, a better picture of what happens before a fault moves in slip stick motion is possible and perhaps could lead to some type of predictions.

Surprise On Journey To Center Of The Earth: Light Tectonic Plates Lead The Way





Andes Mountains, Peru. When two tectonic plates collide, with one sliding below the other and sinking into mantle, it can lead to the formation of mountain belts, like the Andes.
Andes Mountains, Peru. When two tectonic plates collide, with one sliding below the other and sinking into mantle, it can lead to the formation of mountain belts, like the Andes.

The first direct evidence of how and when tectonic plates move into the deepest reaches of the Earth has been detailed in Nature. Scientists hope their description of how plates collide with one sliding below the other into the rocky mantle could potentially improve their ability to assess earthquake risks.



The UK and Swiss team found that, contrary to common scientific predictions, dense plates tend to be held in the upper mantle, while younger and lighter plates sink more readily into the lower mantle.



The mantle is a zone underneath the Earth’s crust encompassing its super hot molten core. It is divided into an upper and lower area, and is made up of a 2,900 km circumference of churning, viscous rock. It is constantly fed with new material from parts of tectonic plates which slide down from the surface into it.



The researchers’ numerical models show how old, dense and relatively stiff plates tend to flatten upon reaching the upper-lower mantle boundary, ‘draping’ on top of it. Their models are helping to explain plate movements and earthquakes in the Western Pacific, where old plates currently sink below Tonga, the Mariana Islands and Japan.



By contrast, younger more malleable plates tend to bend and fold above the boundary of the lower mantle for tens of millions of years until they form a critical mass that can sink rapidly into the lower mantle.



When this mass moves into the lower mantle, the part of the plate still at the surface is pulled along at high speed. This explains why plate movements below Central and northern South America are much higher than expected for such young plates.


The scientists came to these conclusions by using a numerical model, originally used to show how buildings buckle and fold, which calculates the brittleness, stiffness and elasticity of tectonic plates alongside how the pressures and stresses inside the mantle would affect the plate on its downward descent.



They then compared the modelling with plate movement data. By comparing the two models, the team was able to build up a clear picture of how plates should move when stalled in the upper mantle and also show, for the first time, how tectonic plate rock is mixing within the mantle.



Commenting about the study,* lead researcher Dr Saskia Goes, from Imperial College London’s Department of Earth Science and Engineering, said: “It is exciting to see direct evidence of plates transiting from the upper and lower mantle. This process has been predicted by models before, but no one has been able to link these predictions with observations, as we now do for plate motions.”



When two tectonic plates collide, with one sliding below the other and sinking into mantle, it can lead to the formation of mountain belts, like the Andes, and island arcs, like Japan and, in some places, cause explosive volcanism and earthquakes. Dr Goes say more research is needed, but believes this study could potentially help scientists determine earthquake risks in parts of these zones where none have ever been recorded before.



“The speed with which the two plates converge, and the force with which they are pushed together, determine the size of the largest earthquakes and time between large tremors. Understanding what forces control the plate motions will ultimately help us determine the chances for large earthquakes in areas where plates converge, in places like the northern U.S., Java and northern Peru, but where no large earthquakes have been recorded in historic times,” she adds.


About tectonic plates



There are 8 major and a further 7 minor tectonic plates which cover the Earth’s surface. These plates move across the surface of the Earth. When some plates meet they undergo a process which pushes them upward to create geological formations like mountain ranges. Some plates pull apart, causing fault lines and others undergo a process known as subduction. Subduction occurs when one plate is pushed underneath another and moves into the Earth’s mantle – a rocky zone underneath the crust.



*Journal reference: Saskia Goes, Fabio A. Capitanio and Gabriele Morra. “Evidence of lower mantle slab penetration phases in plate motions.” Nature, 21 February 2008.



This work was supported by a Schweizerischer Nationalfonds Fo¨rderungsprofessur (to S.G.).