By Andrea Aust, Lindsey Hoshaw and Jenny Oh

You may have felt a jolt in the middle of the night when the South Napa Earthquake struck at 3:20 a.m.

But what causes the earth to shake and how does it happen? Well, the earth is constantly shifting and the Earth’s crust is broken up into many rocky plates, like pieces of a puzzle.

These plates are constantly moving, albeit very slowly, over the softer red-hot rock of the mantle underneath. As the plates move past each other, along fault zones, they sometimes get caught and pressure builds up.

When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. This is an earthquake.

The epicenter of an earthquake is determined by triangulation. This means that seismic data is needed from at least three different locations, and where this data intersects tells us the epicenter. When an earthquake occurs, it is recorded on numerous seismographs located in different directions.

The seismograms at these locations show when the first seismic waves, the primary waves, arrive and then when the next waves, the secondary waves, arrive. Knowing how fast each of these waves travel, scientists can calculate how far away the epicenter was from each seismograph.

What they don’t know is the precise direction the waves came from –- the direction of the epicenter. Scientists then must use a map. Around each of three seismograph locations, a circle is drawn on the map with a radius that equals the known distance to the epicenter. These three circles intersect at a single point. This point is the location of the earthquake’s epicenter.

And while we can pinpoint where an earthquake originated, we haven’t gotten much better at predicting them. We can detect earthquakes several seconds in advance like researchers at the University of California Berkeley did for the South Napa earthquake. But we can’t predict them days or months into the future.

As we better understand how they occur we can construct buildings to withstand a certain amount of shaking and even take fault lines into account as we build new cities. Aside from that, having an emergency kit can help and perhaps one day, the early warning system will give us not just seconds but minutes.

  • Andrew Alden

    I want to correct the mistaken impression that the Earth’s mantle is “molten.” The mantle is solid rock, but it is softer than the rock of the crust because of its high temperature. Like red-hot iron that can easily be shaped with a hammer, the mantle is plastic. This allows it to give under stress in a way that the colder, more rigid rocks of the crust cannot. Thus the crust accumulates stress until it breaks, in earthquakes.

    The Earth’s core, on the other hand, is genuinely molten, a liquid mix of iron and nickel and a lighter element, probably oxygen or sulfur, that we haven’t figured out yet. And inside the core is a central ball of solid metal, where the effects of high pressure overcome the effects of heat.

  • hio;



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