Monday, April 14, 2008

Well, I Was Gonna Talk About Earthquakes in Class Tomorrow Anyway...

New Study Shows Odds High for Big California Quakes (with commentaries at California Quakes: 99.7% Sure, and New Forecast for California Earthquakes) is a new calculation of earthquake probabilities in California, and there are some sobering numbers in the report: 99.7% chance of a 6.7 quake somewhere in the state in 30 years, with the highest likelihood in southern California. The chances of a 7.5 magnitude is 46%, and that of an 8.0 is 4%. And, the study didn't even count the probability of a tremor on the Cascadia subduction zone. The workgroup that produced the study was organized by U.S. Geological Survey, the California Geological Survey and the Southern California Earthquake Center.

Memories fade so easily. I was a new teacher at Modesto Junior College in 1989 at the time of the Loma Prieta quake, the 7.1 magnitude event that occurred at the worst possible moment of a most miraculous day (5:06 PM on a commuting day; but on the day of the one and only Bay Area World Series between the A's and the Giants...everyone was already home, and deaths were very much reduced by the coincidence). That quake, and effects that it had on our lives here in northern California, was profound; but now, I am one of the old teachers at MJC, and I am getting students who were babies, or not even born yet in 1989. It is no longer a part of our collective consciousness.

The problem of a social recognition of earthquake dangers is somewhat less in southern California, given the tragedy of the 1994 Northridge quake, but I still find that many of my students are unprepared for the eventuality of large quakes. They were apparently convinced that the Y2K computer meltdown would affect their lives more. Few have water, food, and first aid supplies set aside, and few really know what to do if a quake takes place. Many still intend to try and stand in a doorway if one happens.

The picture of the day above is part of the scarp formed by the Landers quake of 1991 (actually 1992, I was sloppy and writing too quickly, thanks, andrew ), a 7.5 magnitude event that produced a 50-mile long tear in the Mojave Desert, and killed two people. The new fault line offset pipelines, streets, fences, and houses, and only the remote location kept the death toll down. The picture was snapped in 2003; the dry environment has allowed the scarp to persist.

We got lots of little reminders that we live in earthquake country. The Alum Rock quake last November took place minutes after my night class took their earthquake quiz. They were standing outside, and most of them rushed into the building to check out the department seismometer (everyone else was running outside). We are currently experiencing some mysterious activity offshore of Oregon (Earthquake Swarm off Oregon Coast). And always, the little tremors continue all over the state (586 in the last week at magnitude 1 and above at; that's about twice as many as normal for the last few years).

Oh, and I wasn't sure I should even bring it up, but last night's episode of Eli Stone involved a crank prediction of a 6.8 quake that is going to bring down the Golden Gate Bridge...and then...a quake happens, and the Bridge collapses! Given the "story arc" style of the show, it will be interesting to see if the earthquake continues to be a factor in future plots. And the special effects of the bridge collapse were pretty good for television, at least on my little set.

Diagrams and maps of the California faults in this study can be found at including probabilities of particular faults ( and the overall probability map (


Silver Fox said...

Oooh, you got a good photo of that scarp, even if it is eroding somewhat.

The 1989 fault was impressive and ugly, and I think the overpasses were improved after that.

Weren't some people still in the stadium watching the game?

Garry Hayes said...

Some overpasses were rebuilt, but some freeways never were. The stadium was filled to capacity during the quake, but luckily it was not seriously damaged.

Julian said...

Awesome picture of the Landers scarp!
Which part of the rupture zone (which fault, which nearest city, however you want to classify it) is that? I recently made an unsuccessful attempt to find the Emerson Fault scarp nearish Barstow, and this photo makes it clear that I didn't miss the scarp for it being weathered into oblivion. Do you have any suggestions for finding this part of it? Thanks!

Andrew Alden, Oakland Geology blog said...

Landers: 1992. It used to be you could look at the California quake map, and every day there would be aftershocks from Landers. They finally stopped last year, maybe the year before. Today you see aftershocks from the Paso Robles quake.

Kim said...

Have you seen anything that discusses the methods they used to determine the probabilities? I'm curious whether they're making use of the work that Ross Stein and other people have done, on the stress changes that occur as a result of movement along faults.

In 1989, the initial word was "we expected that" - at least from the Geophysics department, a week after Loma Prieta, when the School of Earth Sciences gathered to talk about what happened, and whether we would ever be allowed back into our damaged building again.

Garry Hayes said...

Thanks Andrew for the correction. It was late, and I didn't click a few clicks to make sure I got the date right on the Landers quake.

Julian, I was on the Spring 2003 field trip of the National Association of Geoscience Teachers Far Western Section, sponsored by Chaffey College, just up the road from you at Riverside. The guide we used is available from the NAGT at I was a passenger when we took the dirt roads to the scarp, so my memories are pretty hazy about the particular turnoffs, but the guide should probably clear that up.

Kim, I've only seen the linked press release from the USGS, so I don't have an answer for your question. BTW, I will be in Durango for the Fort Lewis College graduation ceremony; hope to run into you!

Kim said...

I'll be there... though possibly not with the faculty. I don't have a cap & gown - I never got around to buying one until this year, but it hasn't arrived yet.

Andrew Alden, Oakland Geology blog said...

Kim, at the press conference Monday where the USGS revealed the new 30-year California quake forecast, Stein was there and yes, they are explicitly using stress shadows now. I asked whether they were using the latest info tying the Hayward, Calaveras and Rodgers Creek faults together, and he said very diplomatically that they looked forward to using that new knowledge in future forecasts.