Earthquakes

Bend a stick slowly. It flexes... flexes... then SNAP. That's basically an earthquake. Rocks along a crack get stuck, stress builds up for years, and then they suddenly slip, releasing a burst of energy that shakes the ground.

A fault is a break in Earth's crust where rocks have moved. The energy release follows the elastic rebound theory: rocks bend like a stretched rubber band, then snap back when they break.

Three kinds of faults:

• Normal fault: rocks are pulled apart, and one side drops down. Happens where crust is stretching, like most of Idaho.
• Reverse (thrust) fault: rocks are squeezed together, and one side gets pushed up and over the other. Builds mountains.
• Strike-slip fault: rocks slide sideways past each other. The San Andreas Fault is the classic example.

• The focus is the spot underground where the rock actually breaks.
• The epicenter is the spot on the surface directly above the focus. That's what you hear on the news.

Shaking is usually strongest near the epicenter and fades with distance.

You met P and S waves in Chapter 12. There's a third type:

• P waves arrive first.
• S waves arrive second.
• Surface waves arrive last, rolling along the ground like ocean waves. These cause most of the damage.

Since P waves are faster than S waves, the farther away a station is, the bigger the gap between their arrival times. It's like seeing lightning and then hearing thunder: the longer the gap, the farther away the storm.

One station tells you how far away the quake was, but not which direction. So scientists draw a circle around each station with that distance as the radius. With three stations, the circles cross at one point: the epicenter. This is called triangulation.

• Magnitude measures the energy released. Scientists today use the moment magnitude scale. Each whole number up means about 32 times more energy. So a magnitude 7 releases about 1,000 times more energy than a magnitude 5. The Richter scale is the older, more famous version.
• Intensity measures how much shaking and damage people actually experience in a certain place, using the Modified Mercalli scale (I to XII). One earthquake has one magnitude but different intensities in different places.

• 1983 Borah Peak earthquake (magnitude 6.9): Idaho's largest recorded quake. It hit near Challis and Mackay, killed two children in Challis, and lifted Borah Peak, Idaho's tallest mountain, about a foot higher. It also left a fault scarp you can still visit, a step in the ground several feet high.
• 2020 Stanley earthquake (magnitude 6.5): shook central Idaho and was felt across several states. Thankfully, no one was killed.
• 1959 Hebgen Lake earthquake (magnitude 7.3): just across the border in Montana near Yellowstone. It caused a massive landslide that buried a campground, killed 28 people, and dammed a river to create Quake Lake.

Shaking isn't the only danger:

• Building collapse: the main cause of deaths. Poorly built structures are the biggest risk.
• Liquefaction: shaking makes wet, loose soil act like quicksand. Buildings can sink or tip over.
• Landslides: shaking knocks loose slopes, like at Hebgen Lake.
• Tsunamis: undersea quakes can shove the ocean up into giant waves. In 2004, a tsunami in the Indian Ocean killed over 200,000 people. In 2011, one hit Japan.
• Fires: broken gas lines and electrical wires can spark fires.

Scientists can't predict exactly when an earthquake will strike. But they can map faults, estimate risk, and design safer buildings with flexible frames and base isolators that let buildings sway instead of crack.

If you feel shaking: Drop, Cover, and Hold On. Get under a sturdy desk or table, cover your head and neck, and hold on until the shaking stops. Don't run outside during the shaking.