(KTXL) — A 6.4 magnitude earthquake struck a few miles offshore from Humboldt County and damaged homes and roadways and caused power outages, but did not cause a tsunami, according to the National Weather Service.

According to the National Oceanic and Atmospheric Administration, a tsunami is a series of giant waves caused by earthquakes or volcanic eruptions under the sea. 

While earthquakes that occur below the ocean can trigger a tsunami, there are certain things that have to happen, starting with the type of earth movement, according to the United States Geological Survey.

The USGS states that an earthquake’s magnitude is just one factor that could trigger a tsunami. Other factors are how shallow the water is where the quake occurs and how the ground moves.

California is known for its many faultlines, but most of them are strike-slip, meaning that the ground moves sideways, which is less likely to generate a tsunami, but still possible, the USGS says.

In contrast, a thrust earthquake, where one part of the ground moves up or down in relation to the other can cause any water above to be displaced, possibly generating a tsunami.

At the same time, the USGS says that the movement needs to happen in an area relatively shallow in order to generate the tsunami.

Traditional waves on the coasts are generated in much the same way. As the water moves across ground that is sloped upward, it runs out of space to hold the amount of water and this water collects and produces waves.

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The USGS says that earthquakes below magnitude 6.5 are “very unlikely to trigger a tsunami.”

Magnitudes between 6.5 and 7.5  

Earthquakes of this size don’t usually produce destructive tsunamis, but small sea level changes can happen in the epicenter area.

Tsunamis that can cause damage or casualties are rare in this magnitude range, but have occurred due to secondary effects such as landslides or submarine slumps. 

Magnitudes between 7.6 and 7.8 

Earthquakes in this magnitude range “might produce destructive tsunamis,”, especially near the epicenter.

According to the USGS, small sea level changes might be observed at greater distances. Tsunamis that can produce damage at “great distances” are rare in this magnitude range. 

Magnitude 7.9 and greater

For earthquakes that reach this size of magnitude, destructive tsunamis are possible near the epicenter. “Significant” sea level changes and damage might occur in a broader region due to the magnitude size. 

If an earthquake reaches a magnitude of 9.0, there’s a possibility of an aftershock with a magnitude of 7.5 or greater, according to the USGS. 

How to survive during a tsunami

Following Tuesday’s earthquake, the NWS recommended tips for those who live in a tsunami-risk area and feel an earthquake. 

  • If you are in a tsunami area and there is an earthquake, protect yourself from the earthquake first. Drop, cover, and hold on
  • When the shaking stops, if you are in a tsunami hazard zone, evacuate as soon as it is possible and move inland or to high ground immediately
  • A tsunami may arrive within minutes and damaging surges are likely to occur for at least 12 hours or longer. Stay away from coastal areas until officials permit you to return