A 4.4 magnitude earthquake rattled Los Angeles on Monday. Its epicenter was in the East LA neighborhood of El Sereno on the Puente Hills fault system. While it's not as famous as its sibling, the San Andreas fault, it could be more dangerous for Angelenos.
While some people strongly felt the quake, others didn’t. But that is not connected to magnitude, which seismologist Lucy Jones says indicates how much energy was released. The amount of energy that reaches you depends on your location and the surrounding soil condition.
“This earthquake was much closer to many people than the other ones we've been having recently. And proximity is 90% of the game. The other part of the game is your local soil conditions. So if you're sitting in a loose sediment, like most of us are here in the LA basin, the waves come in, and they slow down as they get into the soft material, but they still have to carry the same amount of energy, so the waves actually get bigger.”
She continues, “That, combined with being nearby, means that we felt it very strongly. There's also some reverb, essentially, that happens. You start getting reverberations going on within the basin as the waves bounce off of different structures, and that can make it feel like it's lasting for a longer time as well.”
The Puente Hills thrust fault
This fault system is buried and doesn’t rise up to the Earth’s surface, Jones explains, and it is covered by the hills that have grown on top of it. It spans across some of the oldest and most densely inhabited parts of the area, including Downtown LA and Hollywood. The buildings there are often older, weaker, and therefore can’t withstand earthquakes. It is also less active than the San Andreas fault, moving every few thousand years compared to 100-150 years.
“When we say more dangerous, well, the San Andreas is going to hit us so much more often, the cumulative amount of damage could be really bad. But … when that one earthquake happens in Puente Hills, because it's right under Downtown Los Angeles, the estimates are about an order of magnitude more damage than what we get from the San Andreas earthquake.”
While other earthquakes have formed on the same fault, Jones says they don’t indicate whether more are coming.
“I would rather think of it as a reminder that we've got this really bad, active fault right underneath us. Don't forget. We need to take the risk seriously. But it really doesn't allow us to say with any certainty what's coming,” Jones says.
Where to go during an earthquake
Leave the door jam in the dustbin of history, Jones stresses.
That piece of ancient advice, she explains, comes from a Red Cross volunteer who encountered a crumbled, adobe house in the aftermath of the 1952 Kern County earthquake. A wooden lintel — a beam found above doors or windows — was still standing.
“Unless you're in a 200-year-old adobe house, your door is not any stronger than the rest of your building,” Jones says.
Instead, she suggests dropping, covering, and holding on under a desk.
Planning is important
Jones says that during a significant quake, you’ll be depending on your neighbors, so know how you will communicate.
Also, check your water supplies. A pipe within Pasadena City Hall was broken during the recent 4.4 quake — a reminder of how fragile these parts are.
“Pipes are one of our weakest parts, and it's one of our really big problems in big earthquakes. So have some stored water,” she recommends.