Tag Archives: spinel

Largest deep earthquake ever recorded still baffles seismologists

A magnitude 8.3 earthquake that struck deep beneath the Sea of Okhotsk on May 24, 2013 still poses a lot of questions to geophysicists. At a depth of about 609 kilometers (378 miles), the kind of rupture which generates an earthquake of this magnitude should just not happen.

earthquake 1

The vast majority of significant earthquakes takes place on shallow depths, usually when at the boundary of two or more tectonic plates – those of course, are the most unstable area. If you correlate a map of tectonic plates with a map of the recent earthquakes at any given time, you’ll find that the vast majority are clustered around those areas. Earthquake also occur at major faults, which are also relatively shallow (in the crust).

Intermediary earthquakes have the focus between 70 (or 40, depending on who you listen to) and 300 km; and deep earthquakes take place at over 300 km depth. Of course, there can be no tectonic boundaries and faults at that depth – we’re talking mantle here.

The cause of deep focus earthquakes is still not entirely understood since subducted lithosphere at that pressure and temperature regime should not exhibit brittle behavior. Probably the most discussed possibility is a mineral transition, like for example olivine undergoing a phase transition into a spinel structure. Still, they may still be influenced by crustal tectonics, and most specifically by what is called the Wadati–Benioff zone.

earthquake

But at these depths, with huge temperatures and pressures, you wouldn’t typically expect such big earthquakes.

“It’s a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 610 kilometers of overlying rock?” said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.

Deep earthquakes occur in the transition zone between the upper mantle and lower mantle and are not usually dangerous for humans, but yield very valuable scientific information. As for the Sea of Ohotsk earthquake, it has some very strange characteristics.

“It looks very similar to a shallow event, whereas the Bolivia earthquake ruptured very slowly and appears to have involved a different type of faulting, with deformation rather than rapid breaking and slippage of the rock,” Lay said.

The precise mechanism for initiating shear fracture under the huge pressure at that depth remains unclear, and unlikely to be solved in the nearby future.

“If the fault slips just a little, the friction could melt the rock and that could provide the fluid, so you would get a runaway thermal effect. But you still have to get it to start sliding,” Lay said. “Some transformation of mineral forms might give the initial kick, but we can’t directly detect that. We can only say that it looks a lot like a shallow event.”

Journal Reference:
L. Ye, T. Lay, H. Kanamori, K. D. Koper. Energy Release of the 2013 Mw 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity. Science, 2013; 341 (6152): 1380 DOI: 10.1126/science.1242032

Alien debris found in lunar craters

Well the title may be a little flashy, but here’s what it’s about: some highly unusual minerals have been found at the centers of impact craters on the moon. Geologists working on the case believe that they may be the shattered remains of the space rocks that made the craters, but didn’t exhume any material from the Moon’s crust.

copernic

The foreign matter is probably asteroid debris, but some of it can even be from Earth – which is known to throw up its fair share of material as it gets hit by asteroids.

Interestingly enough, the discovery of this material didn’t come from analyzing the craters themselves, but rather by looking at a computer model of how meteorite impacts affect the Moon. To be more specific, researchers simulated a high-angle, exceptionally slow impacts — at least slow compared to possible impact speeds. What they found took them completely by surprise.

“Nobody has done it at such high resolution,” said planetary scientist Jay Melosh of Purdue University. Melosh and his colleagues published a paper on the discovery in the May 26 online issue of the journal Nature Geoscience.

They found that when the impact is slow enough (that means speeds of under 43,000 kph), the rocks which are hit don’t really vaporize, as was previously believed. Instead, the mass gets shattered into a rain of debris that is then swept back down the crater sides and piles up in the crater’s central peak.

In the case of the Copernic crater, depicted above, which is estimated to be about 800 million years old, the foreign material stands out because it contains minerals called spinels. Spinel is a magnezium/aluminum mineral that only forms at great pressures, like in the Earth’s mantle, for example (or even in the Moon’s mantle). But spinels are also relatively common in some asteroids which are fragments of broken or failed planets from earlier days of our solar system.

Via Wikipedia

Spinel.Via Wikipedia

Judging by these results, scientists now believe that the unusual minerals observed in the central peaks of many lunar impact craters are not lunar natives, but imports.

“An origin from within the Moon does not readily explain why the observed spinel deposits are associated with craters like Tycho and Copernicus instead of the largest impact basins,” writes Arizona State University researcher Erik Asphaug in a commentary on the paper. “Excavation of deep-seated materials should favor the largest cratering events.”

If true, this also means that pockets of material from the early Earth might be in cold storage on the moon, says Asphaug.

“Even more provocative,” explains Asphaug, “is the suggestion that we might someday find Earth’s protobiological materials, no longer available on our geologically active and repeatedly recycled planet, in dry storage up in the lunar ‘attic’.”

Via Discovery