Understanding Earthquake Waves: How California’s Early Warning System Keeps You Safe

Press release from CalOES:

California is known for its diverse and stunning landscapes, and it’s also known for its variety of natural disasters like earthquakes.  

Have you ever felt the ground shake during an earthquake? When there is an earthquake, different kinds of waves are produced, and these waves shake the ground at different times. These include the fast-moving waves, known as P-waves, followed by slower moving, more damaging waves called S-waves. 

The California Earthquake Early Warning System powered by ShakeAlert, uses seismic sensors to quickly detect these waves caused by earthquakes, estimates the magnitude of the shaking and determines when to activate emergency alerts on phones.  

But what’s the difference between a P-Wave and an S-Wave? 

P-Waves: The Fast Movers 

P-waves, or Primary waves, are the first waves to arrive as soon as there’s an earthquake. They move very fast, about 3.7 miles per second. You may or may not feel these waves, but they are recorded by seismic sensors.    

S-Waves: The Slow Movers  

S-waves, or Secondary waves, are the next waves to arrive after P-waves. They move slower, about 2.5 miles per second. These waves, are the high intensity shaking that we feel and are the more damaging waves that earthquakes produce.  

How The Waves Help The Earthquake Early Warning System 

With over a thousand seismic sensors built across the state to detect earthquakes, Cal OES along with federal, state, and local partners developed the California Earthquake Early Warning System. This system detects the P-waves and immediately transmits data to a ShakeAlert processing center, where the location and size of the quake are estimated and updated as more data becomes available. If the magnitude meets the threshold of M4.5 or higher, automated actions are taken, including alerts to cell phones, slowing trains down and opening first responder bay doors.  

Cal OES along with other federal, state, and local partners continues to maintain and advance the technology of the system to increase advance warning of intense shaking for all Californians.  

So next time you hear about an earthquake, remember: P-waves are the speedy ones that help California’s Earthquake Early Warning System, and S-waves are the slower, but more intense shaking we feel.  

PREPAREDNESS TIPS  

• Sign up for life-saving alerts through Earthquake Warning California  

• Practice life-saving actions like, drop, cover and hold on  

• Develop an emergency preparedness plan  

• Pack an emergency supply kit  

• Understand and mitigate hazards: Visit myhazards.caloes.ca.gov to learn about local risks and how to prepare your home (including securing furniture, appliances, home fuel systems and more)