Deep earthquakes occur where the oceanic lithosphere, driven by tectonics, plunges under continental plates – examples are off the coasts of the western United States, Russia and Japan.
Some new research is a step toward replicating the full power of these earthquakes to learn what sets them off and how they unleash their violence and was made possible only by the construction of a one-of-a-kind X-ray facility that can replicate high-pressure and high-temperature while allowing scientists to peer deep into material to trace the propagation of cracks and shock waves.
The U.S. Department of Energy's Argonne National Laboratory simulated deep earthquakes by using pressure of 5 gigapascals, more than double the previous studies of 2 GPa. For comparison, pressure of 5 GPa is 4.9 million times the pressure at sea level.
At this pressure, rock should be squeezed too tight to rapture and erupt into violent earthquakes. But it does. And that has puzzled scientists since the phenomenon of deep earthquakes was discovered nearly 100 years ago. Interest spiked with the May 24 eruption in the waters near Russia of the world's strongest deep earthquake – roughly five times the power of the great San Francisco quake of 1906.
These deep earthquakes occur in older and colder areas of the oceanic plate that gets pushed into the earth's mantle. It has been speculated that the earthquakes are triggered when a mineral common in the upper mantle, olivine, undergoes a phase transformation that weakens the whole rock temporarily, causing it to fail.
"We are capturing the physics of deep earthquakes," said Yanbin Wang, a senior scientist at the University of Chicago who helps run the X-ray facility. "Our experiments show that, for the first time, laboratory-triggered brittle failures during the olivine-spinel (mineral) phase transformation has many similar features to deep earthquakes.
"Our current goal is to understand why and how deep earthquakes happen. We are not at a stage to predict them yet; it is still a long way to go."
The work was conducted at the GeoSoilEnviroCARS beamline operated by the University of Chicago at Argonne's Advanced Photon Source.
"GSECARS is the only beamline in the world that has the combined capabilities of in-situ X-ray diffraction and imaging, controlled deformation, in terms of stress, strain and strain rate, at high pressure and temperature, and acoustic emission detection," Wang said. " It took us several years to reach this technical capability."
This new technology is a dream come true for the paper's coauthor, geologist Harry Green, a distinguished professor of the graduate division at the University of California, Riverside.
More than 20 years ago, he and colleagues discovered a high-pressure failure mechanism that they proposed then was the long-sought mechanism of very deep earthquakes (earthquakes occurring at more than 400 km depth). The result was controversial because seismologists could not find a seismic signal in the earth that could confirm the results.
Seismologists have now found the critical evidence. Indeed, beneath Japan, they have even imaged the tell-tale evidence and showed that it coincides with the locations of deep earthquakes.
In the Sept. 20 issue of the journal Science, Green and colleagues explained how to simulate these earthquakes in a paper titled "Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature in the Laboratory".
"We confirmed essentially all aspects of our earlier experimental work and extended the conditions to significantly higher pressure," Green said. "What is crucial, however, is that these experiments are accomplished in a new type of apparatus that allows us to view and analyze specimens using synchrotron X-rays in the premier laboratory in the world for this kind of experiment — the Advanced Photon Source at Argonne National Laboratory."
The ability to do such experiments has now allowed scientists like Green to simulate the appropriate conditions within the earth and record and analyze the "earthquakes" in their small samples in real time, thus providing the strongest evidence yet that this is the mechanism by which earthquakes happen at hundreds of kilometers depth.
The origin of deep earthquakes fundamentally differs from that of shallow earthquakes (earthquakes occurring at less than 50 km depth). In the case of shallow earthquakes, theories of rock fracture rely on the properties of coalescing cracks and friction.
"But as pressure and temperature increase with depth, intracrystalline plasticity dominates the deformation regime so that rocks yield by creep or flow rather than by the kind of brittle fracturing we see at smaller depths," Green explained. "Moreover, at depths of more than 400 kilometers, the mineral olivine is no longer stable and undergoes a transformation resulting in spinel, a mineral of higher density."
The research team focused on the role that phase transformations of olivine might play in triggering deep earthquakes. They performed laboratory deformation experiments on olivine at high pressure and found the "earthquakes" only within a narrow temperature range that simulates conditions where the real earthquakes occur in earth.
"Using synchrotron X-rays to aid our observations, we found that fractures nucleate at the onset of the olivine to spinel transition," Green said. "Further, these fractures propagate dynamically so that intense acoustic emissions are generated. These phase transitions in olivine, we argue in our research paper, provide an attractive mechanism for how very deep earthquakes take place."
"Our next goal is to study the 'real' material, the silicate olivine (Mg,Fe)2SiO4, which requires much higher pressures," Wang said.
Citation: Alexandre Schubnel, Fabrice Brunet, Nadège Hilairet, Julien Gasc, Yanbin Wang, and Harry W. Green II, 'Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature in the Laboratory', Science 20 September 2013: 1377-1380. DOI:10.1126/science.1240206