Instant before earthquake points of highest friction reach 1500 Celsius

It is a trick of Chemistry. From our own view point it seems everything touches. But the plate and your table underneath it only use a fraction of their molecules as contact points. [If you zoom even further you find out atoms have hosophobia - and in fact nothing touches anything. But let's ignore that.]

So it should come as no surprise that in our one big physical universe the same idea applies to Geology.

When during an earthquake tectonic plates slide over each other, they do so at average speeds around 1 meter per second. When however you zoom in during that process, tiny bumps that stick out from these plates take all the force and create all the friction. When these microscopic asperities move they can go faster than the speed of sound – but only after Earth’s unstoppable tectonic forces have sufficiently warmed the contact points, a new Science publication states.

Locally temperatures could rise as high as 2700 degrees Fahrenheit or (close to) 1500 degrees Celsius. Before you now think around a fault line everything must be one big sea of lava – it’s not. The Sun is almost 4 times as hot – across its entire surface. These earthquake heat spikes are very local. Move an inch from the asperity, and you’re back at room temperature, the scientists say. [Having your finger in between would still hurt.]

The geophysicists of Brown University describe the process as ‘flash heating’ – where the extreme temperatures are only reached in tiny spots and the rest of the fault temperature is largely unaffected.

From their laboratory experiment the scientists conclude earthquakes have been misunderstood as ‘high friction events’ – as the flash heating means during the actual movement of the plates the friction is in fact low. Once the plates stop and the faults reassemble, new contact asperities are formed. As long as their temperatures remain low, new slips are unlikely.

Because flash heating is extremely localised it is unlikely measuring fault temperatures will do anything to predict earthquakes.

For damage risk why plates don’t settle after quake can be crucial

It does however seem likely that even though respective plates’ asperities can make new contact within an instant after a quake their sum friction remains lower than (some time) before the initial quake – either because the total number of asperities is lower just after a quake, or perhaps because their temperatures are somehow higher, or perhaps even because the first quake failed to do what Earth commanded: move one plate that way, the other in the opposite direction.

That seems the logical way to imagine why series of aftershocks follow most earthquakes. It also helps to understand the Japanese earthquake of last March. Another Science publication (from last May) taught us that the fact that this quake caused such a devastating tsunami had to do with firstly a geological rebound, whereby one plate slipped back after first moving in the wrong direction. The actual tsunami was however formed as during this process sediment from the ocean floor was scraped together and was suddenly forced to form a large bump at the sea floor – pushing a mass of water upwards, which than radiated out as a giant wave.

© Rolf Schuttenhelm | www.bitsofscience.org

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