Transcript: Berkeley seismologists capture earthquake’s rumble

This is Fiat Vox, a podcast that brings you news from, for and about UC Berkeley. I’m Anne Brice.

Toppling tchotchkes and whining dogs were only some of the sounds from last week’s 4.4-magnitude earthquake on the Hayward Fault. Underground at UC Berkeley, seismic sensors captured the quake’s deep rumble.

There are a bunch of sensors all over campus, but the sensors that captured this quake are in a seismic station in the Byerly Vault about 140 feet underground in the Berkeley hills behind the UC Botanical Garden.

This earthquake’s epicenter was near the Claremont Hotel in Berkeley, about 8 miles below the earth’s surface. The sound it produced took about two seconds to get to the Byerly Vault.

So what are we hearing?

Hellweg: “Hah, an earthquake.”

Peggy Hellweg and her colleagues at Berkeley’s Seismological Lab monitor the sensors.

Hellweg: “To a certain extent, earthquakes are like thunder. Something happens and sound waves go out.”

A quick earthquake 101 lesson: We, in the Bay Area, are on the edge of two tectonic plates. They’re mashed together and they want to break free. When they build up enough pressure, they push past each other and the fault ruptures. The greater the pressure, the bigger the quake.

Hellweg: “We imagine it’s like, bam! Like thunder. But in fact, earthquakes take time to happen. And the bigger the earthquake, the longer it takes the fault to rupture.”

This shift in the ground produces two kinds of sound waves — P waves and S waves. The low rumbling noise at the beginning is P waves and the S waves’ arrival is the big bang you hear.

Hellweg: “Earthquakes do produce sounds, and people do hear them. What I heard when the P wave came was a noise that was kind of like a freight train going by but not very loud… and then the S wave came that actually shook the house and you could hear the house shaking and stuff like that.”

So earthquakes produce sounds we can hear as well as infrasonic frequencies, below the range of human hearing. The sounds the seismic sensors recorded are infrasonic, so Hellweg speeded them up so we can hear them.

UC Berkeley has been recording earthquakes since the 1880s. The data are collected for several reasons. It provides earthquake information in real time. And it allows researchers to use data from the earthquakes to learn more about the structure of the earth.

Hellweg: “And the other is so we can share sound files with people like you.”

Recording earthquakes on the Hayward Fault has proven easier than predicting when the next big tremblor will shake the Bay Area.

Hellweg says the reason it’s so hard to predict is the same reason we don’t know when a rubber band will snap.

Hellweg: “So when you take a rubber band and you stretch it and stretch it. You stretch it a little bit and nothing happens and you keep stretching it, and — you never know when it’s going to happen — the rubber band breaks.”

Rubber bands — even ones from the same bag — have all different breaking points. It’s the same with the Hayward Fault, she says. In the past 2,000 years, there have been 12 big earthquakes spaced about 140 years apart, plus or minus 60 years. The last big one happened in 1868, so 150 years ago.

Hellweg: “So we think that the rubber band that is the Hayward Fault is very close to the point that it’s going to break. But we don’t know exactly how much more it can stretch before it breaks.”

Experts guess that there’s a 30 percent likelihood it will happen in the next 30 years. But we have to remember:

Hellweg: “It could happen anytime in between. It could happen tomorrow. None of that would surprise me. But it could also not happen for another 40 years or 50 years.”

When it does, UC Berkeley will be listening.

For Berkeley News, I’m Anne Brice.