We live in Earthquake Country where the land beneath us moves, controlling where we have mountains and valleys, and shaping our lives. It is important for us to learn what causes earthquakes and how we can keep our ourselves and our families safe during an earthquake.
Faults Move and Cause Earthquakes
Just like the peel is the outer layer of an orange, the earth has an outer layer called the crust. Unlike the peel of an orange, however, the earth’s crust is slowly shifting. Much of this shifting occurs on faults, which are cracks in the earth’s crust. The largest pieces of the earth’s crust are called tectonic plates .
We feel the ground shake when stress underground causes a fault near us to rupture, so that the two sides of the fault slip abruptly past each other. If the rupture extends to the surface, we see a fault scarp where the fault has moved (called surface rupture). Because faults are weak zones in the rock, earthquakes tend to occur over and over on these same faults. Strike-slip faults have horizontal slip: the rocks around the fault can be weakened by the fault slip and eroded into long valleys. Other faults have both vertical and horizontal slip, and can build mountains. The blind thrust fault beneath Mt. Diablo is one of these. Take a trip to see this fault in action!
Why do I need to care about faults?
If a fault extends to the surface, it not only ruptures the ground, but also breaks pipelines and closes roads!
That’s why you need to have food and water for a few days, and know how to use a temporary toilet. You also need to have plans for lots of people to have permission to pick you up from school or day care if your parents can’t get to you quickly.
Do you live near a fault? Find out!
To see where a fault has moved a fence -
Visit the earthquake trail near Point Reyes Visitor Center to see where the San Andreas fault moved a fence 16 feet in 1906 and learn other cool facts about earthquakes.
Earthquakes Shake Us Up!
When you throw a stick in a pool of water, ripples form and move away from the stick. When the fault breaks, it snaps and causes ripples in the ground that we feel and call an earthquake.
The different types of waves generated by the earthquake are P-waves or primary waves and S-waves and or secondary waves. See P- and S-waves in action .
P-waves, also called compressional waves, are the fastest moving. You will feel them first as compress and expand the ground in the same direction that the wave is moving. P-waves cause windows to rattle and feel like a truck bumping a building.
Next, the S-waves arrive and shake the ground both horizontally (side-to-side) and vertically (up-and-down), causing far more damage to buildings. These waves are also called shear waves. If you are looking outside during some earthquakes, you can even see the shear waves.
The horizontal component (side-to-side movement) of the S-waves causes more damage than the vertical component (up-and-down motion). This is because buildings are designed to stand up and resist gravity, a vertical acceleration. The horizontal components of shaking (just as with wind) have the potential to cause more damage.
However, newer buildings in “Earthquake Country” are designed to withstand this shaking component without killing people. In spite of this, the buildings can still be heavily damaged and may not be able to be used for days, weeks, or even months after the earthquake.
Have you just felt an earthquake? Find out what the U.S. Geological Survey says.
Watch the USGS simulation of an earthquake on the Hayward Fault (make take a minute to load). View other simulations.
Why do I need to care about earthquake shaking?
While most buildings will be safe, the things in a building can fall on us.
That’s why we practice Drop, Cover, and Hold drills at least once a year. It is also why we need to have a home hazard hunt and fix the hazards we find, particularly in our bedrooms where we may be asleep and not be able to Drop, Cover, and Hold.
How hard can it shake near you? Find out!
Watch a video!
A shake table test shows us what can happen in our bedroom.
Watch more videos
How big is a BIG Earthquake?
Earthquakes are “big” depending on what you are measuring.
For example, the largest earthquake in terms of deaths was the January 23, 1556 earthquake in Shensi, China, where approximately 830,000 people were killed. Magnitude, a measure of the energy released in an earthquake, wasn’t used as a measure of size until the last 100 years or so. An earthquake that had a surface-wave magnitude of about 8.7 occurred on June 12, 1897 in Assam, India. An 8.6 surface-wave magnitude earthquake occurred on Sept. 10, 1899 in Yakutat Bay, Alaska (with no reported deaths), while an 8.6 surface magnitude earthquake occurred on March 28, 1964 (nick-named the Good Friday Earthquake) in Alaska, killing only 131 people.
Scientists who study earthquake physics, called “seismologists” measure the horizontal and vertical accelerations, the shaking velocities, and the displacement of the ground in earthquakes.
The shaking you feel is called shaking intensity. A light bulb has only one wattage (it’s a 100 watt bulb, for example), but the INTENSITY of light you see varies with how close you are to the light bulb. Similarly, the intensity of shaking will vary throughout our Bay Area from each earthquake! Learn how shaking intensity is measured.
Why do I need to care about size?
Any earthquake is too big if it scares you or hurts someone you care about. If you camping out in an open area far away from things that can fall on you with plenty of food and water, they can be more fun than a roller coaster. That’s why it is so important to Be Prepared!
How INTENSE will the shaking be from your favorite fault?
To experience quake shaking for yourself -
Visit shake tables at