If you didn't get a MyShake app alert before, during or after Tuesday's 4.5 earthquake, here's why

HOLLISTER, Calif. (KGO) -- Tuesday's earthquake was centered near Hollister but it was felt across much of the region.

Many voiced their frustrations online about not getting a ShakeAlert update about the quake on their phone.

"All of a sudden there was a big jolt here and a little bit of shaking following that," said Tim Dawson with the California Geological Survey in San Mateo.

"It started to shake and then it really started to role," said Sgt. Bo Leland of the Hollister Police Department.

VIDEO: 4.5 earthquake near Hollister rattles store shelves, even felt across SF Bay Area

A preliminary magnitude 4.5 earthquake near Hollister, Calif. rattled store shelves, and it was even felt in SF and Fresno, according to the USGS.

Descriptions of Tuesday's earthquake came from all across the Bay Area.

On J.R. Stone's ABC 7 News Facebook page questions and comments came pouring in about the ShakeAlert Earthquake Early Warning system. Some people like Melanie received a ShakeAlert image and wrote. "I felt it in Gilroy. I got the alert on my phone."

But many in other areas, like Vicky and Sheila said they never received an alert. Those from the USGS say there's a reason for that.

"The people that will receive those alerts at magnitude four and a half and higher will be those people that could feel that intensity three level," said Robert de Groot with USGS ShakeAlert Operations.

And while many living in the South Bay, Peninsula, San Francisco, and Tri-Valley area reported feeling the quake, Robert de Groot with USGS says that intensity three level of movement was felt near Hollister, where the 4.5 was centered. That intensity was not felt in San Francisco or San Jose.

"The idea is to send those alerts to the people who could feel the most shaking where the most danger is. We want to prioritize that because of course the basic currency of the shake alert system is time and the more alerts you want to deliver, it takes more time," said de Groot.

MORE: USGS experts predict when magnitude 7.5 earthquake could hit Bay Area

As the MyShake app was put to the test during the 5.1 earthquake near San Jose, experts also discuss when a 7.5 quake could rock the Bay Area.

He says they are constantly improving the alert system which monitors earthquakes in California, Oregon, and Washington. While there is still work to be done, early alerts for lower intensities of a quake are a bit of a challenge.

"Intensity level three, basically a truck passing by your house, is kind of the lowest level we can do this most accurately at this point. When you get to the low shaking levels there are so many factors that influence if you'll feel it or not, it's even kind of difficult for people to feel intensity three in some cases," said de Groot.

Many of the alerts went out just as the shaking was happening or just after. The USGS is working on improvements that could speed up getting the data and getting it out to people.

Click here for the latest stories and videos about earthquakes here in the Bay Area and around the world, and click here for more information on disaster preparedness.

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