The Western Geologist

Actually the title ought to be "creeping faults", but that's not nearly as catchy. This past weekend I went on a field trip to the creeping section of the San Andreas fault. The creeping section of the San Andreas extends from Parkfield to the south to San Juan Bautista to the north. That section of the fault is odd because instead of moving through a series of stops and jerks (i.e., earthquakes), it creeps. In other words instead of leaping ahead by many meters during a large earthquake it just inches along. The sections of the San Andreas to the north and south of the creeping section generate very large earthquakes. The section to the north generated the 1906 San Francisco earthquake. The section to the south generated an even larger quake in 1857 called the Fort Tejon quake.



The photo above shows the mission at San Juan Bautista. It's taken from a scarp along the San Andreas. This town marks the southern extent of the portion of the fault that ruptured during the 1906 earthquake, and as I said earlier it marks the northern part of the creeping section of the San Andreas fault.



The cracks in this freshly-paved road (Highway 25 south of Hollister, California) are caused by creep along the Calaveras fault (a fault related to the San Andreas).



The curb in this photo, located in the town of Hollister, has been offset by creep along the Calaveras fault. The offset is by the woman toward the top of the photo.



Photos of fences that were offset during earthquakes (like this one) are fairly common. However, that's not how this fence was offset (the offset occurs near the telephone pole in the middle of the picture). This fence, located just off of Highway 25 south of the Pinnacles, was offset by the creeping San Andreas.



The image above shows a stream channel that's been offset as it crosses the San Andreas fault. As the channel hits the fault it bends away from the man in the red jacket toward the man in the blue jacket. This channel is ~ 15 miles south of the town of Parkfield just off of Highway 46. This location is near the northernmost portion of the San Andreas the ruptured during the 1857 event (and that also marks the souther boundary of the creeping San Andreas).



The bridge in this photo leads to Parkfield, which despite its small population (the sign currently reads 18) is one of the better-known places along the San Andreas. It's been intensely studied for decades because the fault there generates earthquakes fairly regularly (you can read more about that here). The kink in the railing has been caused by creep along the fault as the Pacific plate moves north relative to the North American plate.



A much-anticipated earthquake occurred in the Parkfield area in 2004. The photo above shows a pylon supporting the Parkfield bridge that moved during that event (the offset is shown by the silver mechanical pencil). That motion didn't occur immediately during the earthquake; it occured after it as the fault crept much faster than normal. This sort of motion is unsuprisingly called "postseismic motion." It doesn't occur after every earthquake, but it is fairly common. In fact some fault have preseismic motion. Unfortunately that's not universal either, so it can't be used to predict earthquakes (although a warning system in Japan is based on that idea).

There's still a lot about the ways in which faults move that we don't understand. Of course, all that means is that it's a great time to study faults!

I came across this personality quiz from one of the blogs at ScienceBlogs. I have to admit that I get a kick out of interactive quizes like this, and this one is one of my favorites.

I was lucky enough to see a swarm of bees (am I the only one who thinks of U2 when they hear that?) at work the other day.





Apparently this is a fairly common occurrence around here this time of year. For some reason (the hive is too large, the current queen is getting old) a new queen is hatched, and when she flies off some of the drones follow her. The bees aren't usually aggressive when they're swarming, and these certainly weren't since I was able to get quite close to them without any trouble.

I could see the old hive in a tree above the swarm. At some point in the past a large branch had broken off the tree, and the bees built their hive in the resulting hole.

Today is the anniversary of the great earthquake that destroyed San Francisco in 1906.

Here are links to a few really useful sites with information about that earthquake, as well as earthquakes in general.

USGS Earthquake Hazards Program:The Great 1906 Earthquake, 100 years later

This link in particular is really cool: A Virtual Tour of the 1906 Earthquake in Google Earth. The picture below shows the location of the quake and the distribution of slip along the fault during the quake.



I think the most important message to learn from this Centennial is that the 1906 earthquake was not a unique event. It wasn't even an unusual event. Earthquakes like this have occurred many times in the past, and they'll continue to occur as long as the Pacific plate continues to move against the North American plate.



I do think that the Bay Area of 2006 is much better prepared than the Bay Area of 1906, although there is still work to do. I've linked to Putting Down Roots in Earthquake Country before, but it's worth repeating. There is a lot that you can do to make your home safer. Pay a visit to your local hardware store, they'll probably have a display of material that you can use to secure your water heater, pictures, and furniture, not to mention smaller items (I've been using a sort of putty to secure vases, candles, etc. to the shelves they're resting on).

I think it's appropriate to end this message with a remembrance for the people who died to the earthquake, as well as the people who helped to rebuild the city.

While looking up information on moonquakes, I came across this story about the 10th planet in our solar system.

NASA's Hubble Space Telescope has resolved the "tenth planet," nicknamed "Xena," for the first time and has found that it is only just a little larger than Pluto.

Previously, Xena was thought to be about 30% larger, based on its brightness.

Because Xena is smaller than earlier thought, but comparatively bright, it must be one of the most reflective objects in the solar system. The only object more reflective is Enceladus, a geologically active moon of Saturn whose surface is continuously recoated with highly reflective ice by active geysers.

Xena's bright reflectivity is possibly due to fresh methane frost overlying the surface. It is possible that Xena had an atmosphere when it was closer to the Sun, but "froze out" at its current large distance, and material settled on its surface as frost.

Another possibility is that Xena is also continuously leaking methane gas from its warmer interior. When this methane makes it to the cold surface it immediately freezes solid, covering craters and other features to make this Kuiper Belt object (KBO) uniformly bright to Hubble's telescopic eye.

Of course, the debate over whether Xena (or even Pluto) should be considered a planet is far from settled, since there is no good consensus on what exactly constitues a planet. It seems that if Pluto is considered a planet, then based on its size, Xena should be too.

With preliminary plans for a permanent base on the moon under way at NASA, one of the less obvious engineering challenges is moonquakes. The Apollo astronauts installed a total of 4 seismometers on the moon. The seismometers radioed their data back to earth until they were turned off in 1977. The moonquakes recorded fell into 4 broad categories, three of which are quite mild and generally harmless. However the fourth type, called shallow moonquakes, could pose a significant challenge to moonbase designers. According to Clive R. Neal, associate professor of civil engineering and geological sciences at the University of Notre Dame, of the 28 shallow moonquakes recorded between 1972 and 1977, there were some that "registered up to 5.5 on the Richter scale." That is powerful enough to rearrange your furniture and crack plaster. In addition, they all lasted longer than 10 minutes! (A typical earthquake will last about 1 minute or so.) As you can imagine, any moonbase would have to be built from flexible materials that can withstand prolonged and repeated shaking in order to prevent any cracks or leaks from developing in a moonquake. Turns out this space exploration thing isn't so simple. NASA press release about the story.

"At approximately 4:20 p.m. EST, the sun began to lower from its position in the sky in a westward trajectory, eventually disappearing below the horizon. Reports of this global emergency continued to file in from across the continent until 5:46 p.m. PST, when the entire North American mainland was officially declared dark."

I love The Onion.