Rose Knows

Q: What does a “5.8″ (or “5.6,” or “5.4″) really mean? And why did the number assigned to the July 29 Los Angeles-area earthquake keep changing? — A bunch of people

A: 5.8 (or 5.4) is either an individual judge’s score for a mediocre dive in international competition or a number describing the magnitude of an earthquake as measured by the Richter scale. As these questions were asked over a month after the U.S. Olympic Trials for diving, I’m going to assume people are asking about the quake.

You can read all about the Richter scale on the Wikipedia page or the US Geological Service page, if you’re so inclined. For our purposes, just remember that the Richter scale number describes the amount of energy released by a given earthquake. It’s a logarithmic scale. Yeah, my eyes glazed over at “logarithmic,” too, but it’s not that scary: It means that a 1.0 difference in magnitude on the scale is equivalent to 10 times the amount of energy released. So a 5.4 quake is 10 times stronger than a 4.4 quake, and 100 times stronger than a 3.4 quake. Not terribly intuitive for those of us who don’t care to dredge up memories of high school math, but it’s the scale that caught on. (Those of you who don’t mind so much are welcome to correct and/or clarify in the comments section.)

Right, so what do those numbers actually mean? The USGS categorizes anything from 5.0 to 5.9 as a moderate quake. To put that in context: You don’t usually feel anything under a 3.0. The 1989 SF Bay Area quake (which was actually centered in Santa Cruz County – downtown still looked like a war zone when I started college there a year later) was a 6.9. The 1994 L.A. quake was a 6.7.

A “moderate” quake doesn’t cause much damage here in earthquake country. Last Tuesday, every television station with a helicopter had aerial shots, and the only damage they could find was a busted water pipe. News web sites were plastered with photos of stuff that fell off store shelves. There were some shattered windows here and there, but the main consequence was that cell phone service was overloaded. Many residents have noted that this was likely due to out-of-state friends and relatives calling because they saw “Earthquake in California!!” in bright red letters on a web site.

Everything built here (legally, anyway) is designed to withstand a good-sized quake. We don’t have unreinforced brick buildings, for example. The building codes don’t guarantee that all buildings and roads will be safe, but they’re constantly revised as the people who study this sort of thing learn more. However, in areas of the world that don’t expect earthquakes, a relative low-magnitude temblor can cause significant damage. For example, a December, 2003 magnitude 6.6 quake in San Simeon, CA resulted in two fatalities, about 40 injuries, and serious damage to about 40 buildings. A few days later, the December 2003 earthquake near Bam, Iran, also magnitude 6.6, all but leveled the city and resulted in about 30,000 casualties.

The July 29 quake magnitude number bounced around from 5.6 to 5.8 to 5.4 because the number is computed from data coming in from various seismographs in the region. The seismographs are all reporting their experiences of the earthquake — what it felt like where each individual machine is located — and the final number for a given quake tends to jump around while the USGS is crunching the data.

No post on Southern California earthquakes would be complete without shout-outs to Drs. Kate Hutton and Lucy Jones, the seismologists who give the official word from the USGS lab at Caltech. They’re like folk heroes around here. Dr. Kate is widely known as “The Earthquake Lady”. She’s like your friend’s super-cool grandma who knows everything and doesn’t mind explaining it to you over and over again.. You can watch most of her July 29 press conference on YouTube. Dr. Lucy is the aunt who assures you that whatever Mommy and Daddy are fighting about, it’s not your fault. She’s the one more likely to show up on national newscasts. Locals remember the time in 1992 when she held a press conference while holding her sleeping kid. Earthquakes are serious business, but so is trying to keep a kid asleep.