Earthquakes 101


We Live in Earthquake Country

Earthquakes Happen in California Every Day!

What is an Earthquake?

Everyone living on our planet lives on top of a tectonic plate – a block of earth. The earth’s crust is made up of a lot of these blocks overlapping, and butting up against each other. Where the blocks meet and overlap, are fault lines. Earthquakes happen when the blocks move against each other and cause ground shaking.

Most of us in California happen to live very close to where two large plates meet – the Pacific Plate stretching west into the Pacific Ocean, and the North American Plate, stretching east toward the rest of the United States. Where these two plates meet is a very long, and very famous fault line called the San Andreas Fault.

Listen Up! Earthquakes Are Sound

Earthquakes are actually seismic waves – yes, that’s sound. The waves are produced when the tectonic plates, that have been stuck to each other through friction, gather up energy and suddenly move, releasing that energy in the form of seismic waves. The energy radiates out from the fault, moving through the solid earth like ripples over a pond. When those ripples of seismic waves reach the earth’s surface, they shake the ground…and us!

It’s Shocking!

The biggest shaking that occurs in an earthquake is called the main shock. Many larger earthquakes have smaller tremors that happen before the main shock. We call those foreshocks. Unfortunately, not all destructive earthquakes have foreshocks big enough to warn us of the main shock that’s coming. Also, it’s only after the main shock occurs, that we know for sure that tremors that came before are foreshocks. This means that foreshocks can’t be used to predict earthquakes.

Many larger earthquakes have tremors that occur after the main earthquake. These are called aftershocks. They can happen months and even years after a large earthquake, and they can cause a great deal of damage to areas already devastated by a large quake.

Not All Faults are Alike

We know that fault lines occur where tectonic plates meet. But not all tectonic plates are moving against one another in the same manner.

  • Thrust Faults – In some cases one plate is moving OVER another plate. The movements are vertical. This creates a thrust fault. That was the type of plate movement that caused the 1994 Northridge Earthquake in California.
  • Strike Slip Fault – In other cases, the plates are sliding against each other in opposite directions. The movement is horizontal, and this is called a strike slip fault. Strike slip faults are known for storing up energy over a longer period of time than most thrust faults. When longer strike slip faults final rip, earthquakes can be longer in duration and more destructive because of the length of shaking. The San Andreas Fault is a strike slip fault.

How We Measure Earthquakes

Scientists use seismographs to measure earthquakes. Seismographs get their information from censors placed below the surface of the earth. The censors measure the amount of shaking on the earth’s surface by measuring the sound waves produced by the movements along fault lines. The United States Geological Survey has placed censors all over California. In fact, California is considered one of the most carefully monitored regions in the world.

Old fashioned seismographs used to have drums of paper, with a needled resting on the cylinder. It would create wiggly lines when it registered the earth’s movements. Short wiggles registered smaller earthquakes. Longer wiggles meant stronger earthquakes. The strength of earth movement during an earthquake is called a magnitude. We still measure the earth’s movement with seismographs in units of magnitude, but today, the movements are measured by computers that are fed data from the many sensors placed all over the earth.

Read More from the California Fire Prevention Organization

Our American Red Cross Partners

The California Fire Prevention Organization (California Fire Prevention Organization) enjoys a rewarding relationship with the American Red Cross. For more than a year, we’ve been partnered with the Red Cross Los Angeles Region, supporting their “sound the alarm” home fire safety initiative. There are some important differences between our organizations. It’s that diversity that makes our...
Read More

Installing Free Smoke Alarms Year Round

The education and public safety teams at California Fire Prevention Organization conduct home inspections and install free smoke alarms in an ongoing basis year round. From a metrics point of view, the program is measured by the number of alarms installed and people impacted. But, there is so much more to the story.  These visits...
Read More

Are Wildfire Residents Ready for the Rain?

Weather forecasts for the coming week predict significant rainfall across California. It should put to bed the last embers from the wildfires that roared across the State during the past 100 days. More than a half inch of rain an hour could launch the next disaster for those affected by the recent fires: flooding and...
Read More

Smoke Alarm Holiday Gifts for CA Fire Agencies

California Fire Prevention Organization, a unit of the California Fire Prevention Organization helped to create a safer holiday season with distribution of fire prevention kits to a number of California fire departments. “California has been hit with a remarkable number of wildfires, an increase in structure fires and in fatalities this year,” noted California Fire...
Read More

New PSA Series for Riverside County Fire Chiefs

The California Fire Prevention Organization video team has developed a series of public service announcements for the Riverside County Fire Chiefs. The short video clips cover kitchen fires and anti-drowning messaging. Drowning has become a year-round tragic reality for many communities. And, as most house fires start in the kitchen, getting the word out about...
Read More

More Fatalities as NorCal Fires Impact Thousands

Fire crews on Wednesday made progress, but in the midst of that good news, another fatality was discovered. The death toll related to the ongoing NorCa fires has now reached 42. Fire officials from CalFire confirmed a 23rd death in Sonoma County. That’s 22 from the Tubbs fire and one from the Nuns fire. In addition,...
Read More

Napa, Calistoga, Santa Rosa Fire Info

Share this post with your friends and neighbors on Facebook and Twitter. There are a number of fires burning in Northern California at the moment. These are dangerous fires, and anyone living anywhere near these incidents should take immediate action to protect their families and pets. Firefighters will take care of property to the extent...
Read More

Fireworks Can Burn You!

Every year, important messages about fireworks are ignored by thousands of people, and often, the result is an unhappy ending. Burns and the scars they leave can last a lifetime. Children are most often those who are injured or killed. Fireworks are fun to watch and represent our country’s independence. MySafe:CA believes that on  Independence Day...
Read More

Are You Prepared This September?

September is National Preparedness Month. You know that family escape plan you keep meaning to draw up and finalize? You know the one, where you make sure you’ve mapped out two ways out of your bedroom and two ways out of you home. And you’ve chosen a safe meeting place where every one in your...
Read More