August 24 – The (Not So) Big One

Today’s factismal: A magnitude 6.0 earthquake just rocked the San Francisco bay area.


Seismograms of the event (Image courtesy IRIS REV)

Seismograms of the event
(Image courtesy IRIS REV)

At about 3:30 AM, or about the time most techies are putting their computers to sleep, a strong earthquake hit the Bay area. Though there has been plenty of damage and a few fires, so far there are only two major injuries reported along with about seventy smaller ones. Here’s what we know as of right now:

1. The earthquake was a magnitude 6.0, which makes it about 1/23th as strong as the 1989 Oakland temblor and 1/500th as strong as the 1906 event. (Remember that earthquakes are measured on a logarithmic scale, not a linear one.) This event was also 180 times stronger than the largest earthquake in the recent Oklahoma swarm.

Location of main event and aftershocks (Image courtesy USGS)

Location of main event and aftershocks
(Image courtesy USGS)

2. There have already been six aftershocks; we can expect to see another 1,000 or so before the series is done. Earthquakes happen because of strain. To understand this, take a strand of uncooked spaghetti by both ends and slowly move the ends together. This puts stress on the spaghetti which ends up deforming its shape; that’s the strain, building up. When enough strain has built up, the spaghetti will snap. The same thing happens in the Earth. As plates move against each other in places like the San Andreas fault, they cause strain to build up. When there is a major earthquake, it releases strain on one part of the fault but it adds strain to other parts. If one of those other parts gets enough strain, it will have an earthquake, too.

A week's worth of earthquakes in California (Image courtesy SCEC)

A week’s worth of earthquakes in California
(Image courtesy SCEC)

3. The reason that there were so few casualties is because California has a lot of earthquakes and a reasonably smart government. California has learned from previous earthquakes and has mandated that new buildings must be constructed so that they can resist the shaking that comes from an earthquake of this size. In addition, California requires that hot water heaters and other large household equipment be tied down so that it can’t be tipped over during an earthquake ; this reduces the number of fires and other damage considerably. If California buildings had been built with unreinforced masonry, then we’d be looking at a death toll in the thousands. Instead, we’ve got two serious injuries and a bunch of minor ones.

A view of the San Andreas fault near Parkfield, CA (My camera)

A view of the San Andreas fault near Parkfield, CA
(My camera)

4. The earthquake didn’t happen on the San Andreas fault but it did happen because of the San Andreas fault. California is located right where two different plates (the outermost parts of the solid Earth; think of them as giant bumper cars gliding on the top of the mantle) slide past each other. This creates a lot of strain and earthquakes. Thanks to the way the Earth works, that big fault also creates a lot of little secondary faults; in some cases, those faults have smaller faults of their own. And that’s where this event took place – not on the San Andreas or one of its main subfaults but on a subfault of a subfault.

5. The cost of this earthquake will be about $300 million dollars. The 1989 Loma Prieta earthquake caused nearly $6,000 million in damage but it took place in an area with a higher population density and was much stronger than this one.

Of course, there are lots of things that we don’t know about earthquakes. How does one earthquake trigger another? How does the strain get transmitted across the globe? (We used to think that this didn’t happen, but now we’ve got some pretty convincing evidence that in certain limited cases, it may.) How can we predict when an earthquake will happen? (We’re really, really good on where; it is when that is causing us consternation.) And what we need in order to answer those questions is your help. When you are in an earthquake, please go to the USGS Did You Feel It page and fill out a report. That will help us know just how far the effects of any given earthquake were felt which will help us do a better job of ensuring that the next earthquake is even less costly than this one was.


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