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January 16, 2024

Did you feel the 1994 Northridge earthquake?

If you experienced the Northridge earthquake, it was likely a traumatizing event. You probably talked to friends and family about it, but one thing you didn‘t do at the time was submit a USGS ”Did you feel it?” report. That's because the “Did you feel it?” reporting system wasn’t created until 1999. On this thirtieth anniversary of the Northridge earthquake, the USGS is inviting people who felt it, to share their story with us, in English, Spanish, or Chinese.   

The magnitude 6.7 Northridge earthquake shook the greater Los Angeles area on January 17, 1994. Nearly 100 lives were lost and countless vulnerable buildings were damaged in one of the most destructive and costly earthquakes to hit a metropolitan city in the U.S. in the last century.  

At the time of its occurrence, the 1994 Northridge earthquake produced one of the most comprehensive datasets of earthquake ground motion recordings to date. In spite of this, the spatial locations of instrumental observations do not provide sufficient resolution to fully visualize many of the important ground shaking features that occurred during this earthquake. To augment the seismic recordings, this ShakeMovie animation utilizes sophisticated computer models to predict how seismic waves were generated along the fault as it broke, and how those waves would have traveled through the complex three-dimensional sub-surface geology in the greater Los Angeles region. The animation clearly illustrates the phenomena of rupture directivity – the strong build-up of shaking in the direction the rupture is propagating – which in this case creates very large amplitude motions in the region north of the fault. Additionally, the channeling of waves into the deep sedimentary basins in San Fernando, Los Angeles and Ventura is also quite evident. As the waves are channeled into these basins, they begin to propagate more slowly, and their amplitudes and shaking durations increase significantly.


So what happened on January 17, 1994?  

Strong shaking from the Northridge earthquake began around 4:30 am and lasted 10 to 20 seconds depending on your location. The shaking was felt hundreds of miles away in every direction, with the strongest intensity focused around the epicenter in the San Fernando Valley. There, people felt extreme shaking and structures experienced very heavy damage.   

Even though the event was a horrific disaster, we are fortunate it was not a worst-case scenario.  Larger magnitude earthquakes could occur in the region, and depending on the specific location of the fault that causes the earthquake, the consequences could be greater.


Take a protective action when you feel shaking! 

While earthquakes can be a significant hazard, people are safer today as a result of what scientists have learned over the last thirty years. The science conducted at the USGS has improved our understanding of earthquakes and the ground shaking they generate. As new information is gathered, it helps inform decisions in regard to public safety, building codes, and insurance rates. For example, the USGS publishes the National Seismic Hazard Model Map and associated tools to help communities evaluate and design structures to address risk that shaking produces.  

As an individual, one of the most important actions you can take when you feel shaking is a protective one. Being aware of your surroundings, and of the potential hazards you may encounter, can determine the difference between safety and injury. This is especially true for the places where you spend the most time. Ask yourself what the hazards are in your home, your workplace, and other places where you frequently spend time. If you don’t know where to start, take a look at the 7 steps for earthquake safety. Recognizing these hazards in advance and doing what you can to minimize their impact to your safety, is the most important thing you can do. And remember, if you feel shaking, you should take a protective action, which will differ depending on where you are. In most cases, in the U.S., this means you should Drop, Cover, and Hold On



An inforgraphic that shows a person receiving an earthquake early warning after they experience an earthquake.

A recent tool developed to support immediate protective action is the USGS ShakeAlert® Earthquake Early Warning System which lets you know that damaging shaking could be on its way. Based on decades of scientific research, the ShakeAlert System uses data from an extensive seismic monitoring network so that alerts can be delivered telling people to take a protective action like Drop, Cover, and Hold On or to trigger an automated action such as slowing a train. The  ShakeAlert System makes split-second decisions using a myriad of factors so that alerts can be delivered as quickly as possible. Sometimes those alerts don’t result in a person feeling shaking, or they may feel shaking but never receive an alert. The ShakeAlert System is always improving, because seconds matter. Here are examples of how ShakeAlert is being used by hospitals, schools, municipal utilities, and others to help reduce earthquake risk. Think of other occupations or situations where receiving a few seconds of warning time might influence your actions and sign up to get alerts delivered to your phone. 


Earthquake! What now? 

A screenshot from the earthquake event pages of an aftershock forecast
Screenshot from a USGS earthquake event page showing the earthquake aftershock forecast.  

If you experience an earthquake or want to know about one, past or present, you need to know about the USGS earthquake event pages. Event pages exist for most earthquake that have occurred since the beginning of the 20th Century. USGS earthquake event pages include many resources, we call them products, to learn about an earthquake. Depending on the size, location, and when the earthquake occurred, there will be different products available. The USGS produced this short video where you can learn more about these resources. 


Aftershock probability forecasting 

Although it is not possible to predict earthquakes, we can use statistical probabilities to give people an idea of what kind of aftershocks (earthquakes size and location) they may expect after an earthquake. One of the new public tools that have come about since the Northridge earthquake is the USGS Aftershock Forecast that is available on the USGS earthquake event pages for earthquakes larger than magnitude 5. This forecast shows the probability of small, medium, and large earthquakes, and roughly how many aftershocks to expect. The USGS Aftershock Forecast team is multidisciplinary and focuses on making products that communicate clearly to a wide audience, in direct, meaningful ways that help users evaluate risk.  


Measuring the shaking you feel 

While we often hear about magnitude in the news, it is not a metric that gives the public a clear indication of the power of the earthquake. The Modified Mercalli Intensity Scale, however, does! The Modified Mercalli Intensity is related to the effects of the shaking (see figure and descriptions), which is more relatable than an absolute numerical value as used in scientific research. Using several data sources, we rapidly estimate MMI values at many points on a grid and generate a color-coded map. In our products, you will find a ShakeMap that provides a visual indicator of what a specific region may have experienced in terms of shaking.   

earthquakes: what you feel

Did you feel it? 

To end where we began, the USGS collects felt reports from anyone who has experienced an earthquake. Your information you contribute is valuable to that earthquake’s scientific body of information. When you submit a felt report, we convert your information into MMI and produce the Did You Feel It?  Map. We also produce a MMI ShakeMap using information from seismic networks. Combined together, these results help us better understand the shaking across a region. It’s great, because there are a lot more people than we have instruments in our seismic networks. If you felt the Northridge earthquake, please fill-out the DYFI survey.   And the next time there is an earthquake, visit the USGS “Did you feel it” earthquake page and submit your felt (or not felt – that's also very useful to know) report.   

As always, stay safe, and remember, if you feel shaking or if you get a ShakeAlert-powered alert, take a protective action. In the U.S, the recommended protective action is Drop, Cover, and Hold On! 

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