Skepticblog

Damaged parking garage at Cal State Northridge

Twenty years ago today, January 17, was an event I shall never forget: the Northridge earthquake of 1994. Born and raised in Southern California, I’ve lived through the 1971 Sylmar quake, the 1987 Whittier quake, and many, many smaller events that most people have forgotten. The events were traumatic to anyone who experienced it, even at great distances, and for years afterwards I could remind my students about how it felt, and how it changed Los Angeles. Unfortunately, time marches on, and now that it is 20 years ago, most of my college students either were too young to remember, or were not even born yet!

It struck at 4:31 in the morning, in total pre-dawn darkness, on a Monday that was the Martin Luther King holiday. Luckily, it meant that most people were sleeping and fewer than usual were headed to work, so the casualties were much less than if it had hit during the peak of rush hour. Even though it had a moment magnitude (Mw) of “only” 6.7, its upward acceleration of the ground motion was 1.8 g, much stronger than many quakes of the same size, and the highest ever recorded in an urban area in North America. It seemed to last a long time, although it actually lasted only 10-20 seconds. There was another Mw = 6.0 aftershock an hour later, and another 11 hours later. It was felt strongly as far as Las Vegas, an unusually long distance for southern California quakes (which tend not to propagate very far because of the shattered, highly faulted blocks in our crustal rocks don’t transfer energy well). Contrary to the name, its epicenter was not Northridge, but in Reseda, a town to the south—and both were very close to where I now teach at Pierce College in Woodland Hills. The initial reports of the magnitude as 6.6 and the epicenter at Northridge proved to be wrong once better seismic data came in a day later.

I remember it vividly as if it were just yesterday. My family and I were sound asleep in our house in Sunland, California, when it shook the house strongly and then stopped. Since we were built on a bedrock ridge, the vibration was short and not very damaging. Just a few items fell off the walls and shelves, and some large mirrors leaning against the bedroom walls fell over. In the total darkness with no glasses on, I fumbled across our waterbed to the bed of my (then) 2-year-old son, which was covered with large shards of glass from a mirror that had toppled right over him. I grabbed him and dragged him to our bed, and luckily no one was cut. Once the aftershocks were over, we found flashlights and candles, and did what we could to assess damage and pick up the broken glass. My son, however, would not settle down and go back to sleep, and insisted on seeing his Sesame Street videos to comfort himself–but we didn’t get power restored for many hours. Eventually, we all fell asleep again, and awoke at dawn when we could walk around and see the damage better. Thanks to being on a bedrock ridge, the damage was very minimal, except for the broken glass mirrors. No signs of gas leaks, no cracks in our chimney or house or foundation, nothing that would be a problem. Nevertheless, all we could do was sit tight and wait for power to be restored, and avoid going out on the road that day. We got what info we could from our battery-powered radio, then when power and TV came on, we saw what had happened elsewhere.

Block diagram of the geologic setting, showing the faults

The worst damage was in the western San Fernando Valley near the epicenter, where a number of buildings had collapsed (particularly those with underground parking structures beneath heavy construction). A lot of chimneys fell, and a few concrete walls that had no steel rebar reinforcement, but nowhere near as many as you’d expect, since all masonry structures in California must be reinforced by steel rebar through them (ever since the 1933 Long Beach quake changed the state building codes). The loss of life is estimated between 60-70, depending on what you count as earthquake-related deaths. A high ramp at the Interstate 5-Hwy. 14 interchange fell (just as it had done during the 1971 quake), and a California Highway Patrol motorcycle officer Clarence Wayne Dean went flying off the broken end in the darkness to his death, unable to see that it was gone. (That interchange was rebuilt again, and is now named after Officer Dean). Total property damage was around $20 billion, one of the costliest earthquakes in American history, and almost as costly as Hurricanes Katrina and Sandy.

We learned a bunch of lessons from the quake. As has been long known, the shaking on structures built on hard bedrock was minimal, and they mostly did well. The worst shaking was on the loose sedimentary valley fill of the San Fernando Valley, where nearly all the damage was concentrated, but not on the Santa Monica Mountains to the south (hard bedrock). Loose valley fill tends to vibrate much longer, like a cube of jello that oscillates for a long time after the initial shaking. Then there was additional bad damage in Santa Monica and some of the beach communities, because they are build on soft valley fill saturated with seawater—even though they are much farther from the epicenter than the Santa Monica Mountains between them and the San Fernando Valley. This reinforces the fact that what substrate you build on is often much more important to building safety than how far you are from the epicenter. The fault movement itself was entirely underground on what is known as a “blind thrust”—a fault whose existence was unknown until after it moves. This is discouraging to people who try to map and predict quakes, because most of the recent quakes (Sylmar 1971 and Northridge 1994 being the largest) were on faults we did not know about.  Meanwhile, we have mapped hundreds of active faults all over Southern California that have not yet had significant quakes but are expected to move soon, or even overdue. Between all the active faults we know about (which geologists can map and study) and the realization that there are many more we do not know about (and won’t until they produce a quake), it’s virtually impossible to build anything in most parts of California that does NOT lie near a deadly fault. Thus, we know the region is a ticking time bomb, and we have the ability to map and predict just a few of the many bombs that must exist.

The broken overpass from which Officer Dean plunged to his death in the darkness after the quake

For me, the most memorable part of the experience after the shaking ended was the eerie sensation of a world with no lights, no electrical devices lighted or running, no normal city noises. It was whole region completely dark and still because nothing was running and no one could go anywhere until daylight. That doesn’t happen in urban or suburban areas any more, where the streetlights burn all night, there is traffic 24/7, emergency sirens every few hours, plus airplane and helicopter noise and many other parts of the “background” noise of suburb. The other part of it was the realization of how vulnerable we all are when the power is out, the telephones are down, the water and gas are shut off, the stores are all closed, and many streets are blocked by earthquake damage. The 1994 Northridge quake only cut off power and water and gas for a few hours, and only a few roads were so badly damaged they changed traffic patterns. (Most notable was the I-10 Santa Monica freeway which collapsed, forcing an emergency 24/7 construction on it in record time to restore this vital artery). When we get one of the really BIG quakes that we’ve predicted for so long, everyone will be without power, water, gas, phones, food, and ATMs for days to weeks, and there will be no simple ways to replace them. That’s why my family always keeps a complete emergency kit in a safe place, because we all know that day is coming. Unfortunately, one of the first things you hear on the news after any quake is reporters interviewing lots of people on the street, none of whom have an earthquake kit at home. It’s a classic case of human procrastination: once a quake has just happened, we all swear that we’ll get that kit in order—and then we forget. And for those who are not prepared for “the Big One,” such foolish procrastination will be a matter of life or death.