Resource: Predicting Earthquakes

Although scientists have studied earthquakes in great detail for over a century, because they happen so incredibly fast, they are difficult or impossible to predict. The Earth is organized in layers, with the mostly molten core at the center of the Earth, and the crust enveloping the outermost layer. The crust lies at the top of the layer known as the lithosphere, which is not one solid layer, but is rather divided up into approximately 29 tectonic plates, similar to interconnected pieces of a puzzle. These tectonic plates are known to shift relative to each other, resulting in what we know as earthquakes.

Tectonic plates shift along three different types of boundary lines that reflect how the tectonic plates move relative to each other and what the boundary will look like on the Earth’s surface: divergent, convergent, and transform boundaries. The tectonic plates at divergent boundaries spread away from each other. This shift brings up molten lava that cools to partially refill the split in the crust, and over geologic time a ridge is formed. The most pronounced divergent boundary being the Mid-Atlantic Ridge, along which North America separated from North Africa and Europe starting about 200 million years ago. Convergent boundaries are typically found at mountain ranges and/or ocean trenches, which both literally depict the plates colliding. Mountain ranges form where two continents collide, whereas ocean trenches delineate zones where one plate slides below the other and dives deep into the Earth’s interior. Convergent boundaries are also often characterized by the presence of volcanoes. This is because molten lava is given weak areas in the crust to escape to the Earth’s surface. The third boundary, known as the transform boundary, is identifiable by horizontal motions of plates alongside one another.

The San Andreas Fault in California represents the transform boundary between North America and the Pacific Plate to the west, which moves to the north at a rate of almost 2 inches per year, since about 30 million years ago. The San Andreas Fault can be visited in Los Trancos Preserve, where in 1906 the crust shifted over 3 whole feet. At transform boundaries, plates move neither into nor away from each other, but rather rub alongside each other in opposite directions. The edges of the plates are not smooth enough to simply glide past each other. Instead they are stuck to each other for tens to hundreds of years until they suddenly slip, resulting in an earthquake.

All shifts (earthquakes) in tectonic plates release kinetic energy in the Earth’s crust in the form of seismic waves. This energy is felt by us first as a P, or primary wave, and then as an S, or secondary wave. The P wave is very similar to sound waves and its propagation represents instantaneous compressions and extensions of the crust starting right at the site of the earthquake. This would look similar to a slinky if you were holding it against the ground and pushing it forwards and backwards. The S wave felt next is a transverse wave, similar to a wave in a slinky that forms from moving one end rapidly from side to side. The slower S wave carries more energy and causes most of the damage in an earthquake. Although these movements are minute in comparison to the overall size and layout of the Earth, a shift of even 1-2 meters in tectonic plates can translate into an earthquake capable of massive damage.

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