Climate & Environment

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A seismic and tectonic model of the Lesser Antilles subduction zone: contribution to seismic hazard assessment

Most of the time, we are blissfully unaware that the individual plates of our planet’s surface are in constant motion. When they collide, one may slide under the other, sinking back into the interior of the earth – but it doesn’t always go quietly. Indeed, this type of interaction, called subduction, is responsible for the most powerful earthquakes, like those striking Sumatra in 2004 and Japan in 2011. This sort of plate boundary is also found in the Lesser Antilles, the island chain in the Caribbean Sea. The region has not seen many huge quakes in recorded history, but that is no reason to sit back and relax. One explanation could be that the plates are sliding past each other, slowly but surely, regularly releasing small amounts of energy. On the other hand, it could also mean they are coupled—stuck to one another—and building up energy that will be released one day in a tremendous earthquake.
The answer depends on the precise nature of the plate boundary here, its structure and physical properties. Dr. Michele Paulatto is using geophysics to create a 3D model of the zone responsible for earthquakes, which lies between 5 and 50 kilometers below the ocean floor. By studying the way waves of elastic energy move through the Earth in this zone, he can tell what kind of material they have passed through and whether they encountered fluids. The latter might be water or carbon dioxide held in the crust. As one plate slides under the other, its fluids will be released, due to pressure and chemical processes. This can change the mechanics of the plate interaction and, thus, its capacity to cause earthquakes: fluids in the subduction zone can reduce the degree of coupling between the plates and allow them to slip without causing an earthquake.
Analyzing seismic wave data from man-made sources lets Dr. Paulatto obtain high-resolution images closer to the surface. Combining that with local earthquake data allows him to see even deeper. So far, his analyses have identified a layer of high-fluid content on top of the subducting plate and shown that this region varies in depth along the subduction zone. His analyses continue, but he suspects that variability in the fluid-rich material entering the zone may play a significant role in the degree of coupling of the two plates and influence which locations are the greater seismic hazards. Dr. Paulatto’s conclusions and their applications for risk assessment and monitoring should prove vital to this earthquake-prone region and to the 4 million people who call it home.

Sliding or Sticking: The Role of Fluids in an Earthquake Source Zone

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