On March 15th, 2017 we learned about

Seismic modeling finds new reasons for more movement in the Earth’s mantel

An earthquake, as we normally experience it, is the sum of a variety of different vibrations pulsing through the ground. Most of what we notice are P, S and surface waves, shifting the ground in vertical, shearing and undulating motions respectively. While this activity is apparent at the time of an earthquake, seismologists have also detected vibrations coming from deeper in the Earth’s mantle, even when things are the surface seem nice and stable. They’re of great interest though, since they may be precursors to much more violent activity sometime in the future.

These particular vibrations, called Episodic Tremor and Slip (ETS) are technically caused by seismic tremors. As one tectonic plate is gradually forced under another at a fault, it faces plenty of resistance and friction from it’s neighbor. There is movement, although it’s generally deep and gradual that the ETS waves are hard to monitor on the surface. As such, there’s concern that the pressure and energies at work may surprise us someday with a huge, sudden release, complete with destructive shaking on the surface.

Slipping and sliding

To try to fill in some of these holes, researchers are looking at new modeling techniques to better simulate the forces at work where tectonic plates are squeezing each other. Drawing on various types of data, a point of interest has become the water that can also accumulate where oceanic plates meet. As one plate moves down, water understandably follows, running through small cracks and crevices between the rocks and dirt. The water can also clog up channels with eroded sediment, accumulating between the plates enough to act as a lubricant.

The slightly lubricated plates can then move more easily, which may be a good thing, at least temporarily. The movement deep down likely causes movement above it, but overall this reduces some energy buildup on the plate as a whole. It may even leave pockets and gaps in the rock, which would almost “insulate” some areas from the pressure and stress from ETS waves deeper down, since the wave needs rock and dirt as a medium to move through the ground. This doesn’t mean that the deep, slower seismic tremors can’t lead to built up pressure that could result in a devastating earthquake, but that the process may be more complicated and varied than previously thought.

My second grader asked: So the water letting the plate move is a good thing?

At the risk of oversimplifying things, it seems like it’s more good than bad. Since dangerous earthquakes are the sudden release of built-up energy, it’s probably better for everyone on the surface if that energy is doled out in many smaller events that can create those hard-to-detect ETS waves.

Source: The bangs, crackles and hums of Earth's seismic orchestra by Dr Stephen Hicks, The Guardian

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