A series of earthquakes has Southern California shook. Is a big one coming?

Los Angeles (CNN) — Beautiful beaches. Majestic mountains. Hollywood.

One more thing Southern California is known for: earthquakes.

Yet for a long time, and to the great relief of millions, the many active faults that claw and tear through the earth have been relatively quiet.

That peace has been recently shaken by several quakes that jostled the region to attention, including a 4.4-magnitude under Pasadena in mid-August that sent a jolt across Los Angeles. It was no disaster, but strong enough to rattle nerves. Then came the 4.7-magnitude quake near Malibu exactly one month later.

It was enough shaking to leave people wondering: Is a big one coming?

“Under every hill and mountain we have here in Southern California, there’s an active fault that’s helping to produce that topography,” said Kate Scharer, a research geologist at the United States Geological Survey. While the San Andreas is the most famous, scientists know it’s not the only fault that can produce a powerful earthquake in Southern California.

“Magnitude-7s are very possible in this region along the front of the San Gabriel Mountains,” said Robert de Groot, ShakeAlert operations team lead at the US Geological Survey. “Part of the reason those mountains are there is because there’s a really big fault there called the Sierra Madre fault.”

All of these faults have clashed and scraped against each other through the decades — small shifts on the planetary scale, but massive movements have rocked Southern California. The worst in modern Los Angeles history was the 6.7-magnitude Northridge earthquake in 1994, the first to strike under a major metro area since the 1930s.

Highway overpasses, office buildings and parking garages collapsed in the violent shaking that lasted a painful 10 to 20 seconds along a fault scientists didn’t even know existed. Thousands of people were injured and dozens of people were killed. At tens of billions of dollars, it was one of the costliest natural disasters in American history to that point.

Nothing has compared to that quake since then, and in that time, the population of Los Angeles County has grown from around 9 million to more than 10 million people. Many newcomers have no idea the havoc a big earthquake can wreak, including young Angelinos who grew up in Southern California‘s quiet times.

They’ve gotten a taste in the recent sequence of moderate quakes. Now the question is whether they mean a big one is coming.

“When we look back at the catalog, from like the 1930s to the present, we can see this thing happens every once in a while,” Allen Husker, a research professor at the California Institute of Technology and the manager of the Southern California Seismic Network.

“There’s never a definite sequence where it happens, like, 100% of the time, that we’re going to have a big earthquake (after the sequence of smaller quakes).”

“We can be guaranteed that there’s always going to be a big one” in a cycle that’s gone on for ages, Husker told CNN. What we don’t know is when.

“There will be another big one in California sometime in our lifetimes,” Husker said.

Part of the reason earthquakes are difficult to study is proximity.

“Next time you’re on a plane and they say, ‘we’re at 30,000 feet (above sea level)’ or ‘at cruising altitude,’ that’s about the elevation above the surface of the earth,” Scharer explained. “But earthquakes happen (that same distance) below the earth.”

While it may be unnerving to not know when the next big one will happen, experts say residents should channel that anxiety into getting prepared.

The most important items are water, food and medicine, Scharer says.

The Centers for Disease Control recommends having an emergency supply of a gallon of water per person a day – as well as non-perishable food and extra medicine for at least three days. It may take some time for stores and pharmacies to reopen. The American Red Cross has guidelines on how to prepare for an earthquake as well.

They also suggest a shift in thinking: Don’t just prepare for yourself; prepare for and with your community. When all hell breaks loose, you’ll be in it together.

“I would recommend that (residents) think about how to help their friends and family and their local community if there is an earthquake,” Scharer advised. “Start with yourself and your family and then connect with your neighborhood and try and think about how you’re going to respond.”

A relatively new early warning system — ShakeAlert — covers 50 million people in California, Oregon and Washington. It detects ground motion as soon as shaking starts on the earth’s surface.

An estimated size and location are quickly calculated and become the basis for emergency alerts straight to cell phones, municipalities and schools.

The alert will tell you who is likely to feel the strongest shaking, giving people a precious few seconds of warning to get in a safe place before the earth begins to move.

“We want people to think of it as something that you can add to your arsenal of things that you can use to be ready before the earthquake, during the earthquake, and of course, after the earthquake,” said de Groot, adding that people should remember to practice “drop, cover and hold on” when they feel an earthquake – even if their phone doesn’t send them an alert.

October 17^th is the Great ShakeOut – a day for people around the world to practice earthquake safety drills.

“We need to be prepared, because we could have an earthquake that’s pretty big at any time,” Husker said. “It’s easy to get lax.”

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