Monday, March 30, 2009

Shaking in my Boots (er, my Keen's)


It's been kind of "late in the semester" busy, so blog posts have been limited, but it doesn't mean that nothing interesting is happening in the world. Volcanoes in Alaska (how's that "something called volcano monitoring" working out for you Gov. Jindal?), and now earthquakes in California. Nothing that is totally out of the ordinary, but enough to pay attention to.

It started a few days ago down in the Salton Sea area where the San Andreas fault stops. It just ends down in the sand and gravel beneath the Imperial Valley. What actually happens is that the zone of fault movement skips to the west a bit, and an area of stretching resulted in the formation of the deep fault valley (or graben) where we grow our winter crops around Brawley and Calexico. The quakes ranged as high as magnitude 4.8, and there have been 300-400 quakes in the immediate vicinity. The swarm has seismologists a bit concerned, as this is a part of the San Andreas fault that has not produced a major earthquake (7.5+ magnitude) in hundreds of years. They consider that this swarm has a 1-5% chance of being a foreshock to a major event.

The San Andreas is a complicated piece of work. It slices across California, through Thousand Palms, San Bernardino, Palmdale, the central Coast Ranges, Santa Cruz, Daly City, and Mendocino, and despite what the ridiculous TV movies tell you, we aren't going to fall into the sea. At least, not yet. Wait around for 20-30 million years, and Baja may be an island out in the Pacific. The land is moving northwest an average of 2 inches per year, but the actual motion is more like 10-20 feet during earthquakes every century or two.

Our big earthquakes occur on three discrete areas of the fault, in the north from San Juan Bautista to Mendocino (site of the 1906 event), from Parkfield to Cajon Pass (site of an equally large ~7.8 event in 1857), and from Cajon Pass to Salton Sea (an area that has not had a big event in maybe 300 years). Some overlap occurs sometimes, especially around Cajon Pass. An area from Parkfield to San Juan Bautista creeps a little bit every year without producing major quakes. And Parkfield has magnitude 6 quakes roughly every 23 years or so, although the last one (in 2004) came about 18 years late. Each major section of the fault has an average recurrence interval of around 150 years, but events cluster a bit with time gaps of 90 to 300 years (and I will gladly correct these numbers; they're a few years old and lots of research continues).

You may have heard of the great Shakeout earthquake drill that took place a few months ago in Southern California. They did it for a reason. Large earthquakes ARE coming, SOMEDAY, and if you choose to live in California you need to be ready. When one takes place, the power and water are going to be out, maybe for days or weeks. Make sure you have emergency supplies of water, food, and batteries. Know what to do during the quake; where to take shelter, how to do basic first aid, how to help others. Have a plan for contacting other family members, as local telephone service will be disrupted. Save the phones for 911 calls. Make a person outside the region a point of contact if you are separated from other family members and can't contact them directly.

It happens that I live in Northern California, where the San Andreas fault is considered slightly less of a threat for no good reason other than that it has been less time since it last rumbled. But lest I become complacent, there was a 4+ magnitude quake near San Jose this morning, near where a 5.6 quake hit last year, on the Hayward fault. The Hayward/Calaveras fault system is sort of a wild card in the earthquake game, less well-known, but capable of much mayhem. There was a "Great San Francisco Quake" years before the "Great San Francisco Quake" of 1906. It was in 1868, and occurred on the Hayward fault, killing two dozen people. Seismologists fear a large quake on the Hayward almost more than the San Andreas, as it passes through a more populated region, and will have more of a direct effect on the levees of the Sacramento Delta.

Oh, yes. The Delta. Hundreds of fatalities will be bad enough, but one of the greatest vulnerabilities in California is the system of century-old levees that keep the three dozen islands of the delta from flooding. They are below sea level today because of soil loss from winds, from soil compaction, and from oxidation of the organic material in the soil. Some lie 25 feet below sea level. It's not so much the flooding itself, but the fact that the flooding will be from salt water drawn out of San Francisco Bay. The California Water Project draws a massive amount of water from the delta, and if the islands flood, fresh water will be unavailable. This is the domestic water source for something like two-thirds of the state's population, and it may be cut-off for a year or more.

Make no mistake: I am not making any predictions in this post. At all. But mom, sis, dad, son, daughter, you do live next to various faults in central and southern California, so pay attention to whatever news comes out of these quakes in the next few weeks!

Quakes are a way of life in California, but in the end, I would rather live here rather than just about any place in the world. Earthquakes and faulting cause damage, it is true, but the spectacular scenery of our state is also the result of a long geologic history of fault activity. You can have your volcanoes, floods, hurricanes, tornadoes, and ice storms: I'll take the occasional quake any day!

Today's photo is a section of the San Andreas fault where it crosses I-5 at Fort Tejon at the top of the Grapevine. The fault runs up the center of the photo, and over the distant pass (the snowy flank of Frasier Mtn is on the left side). These straight valleys, eroded out of the weak rock in the fault zone are called linear valleys.
Post a Comment