This sleepy little central California town, which sits atop the San Andreas fault, has been hit with a magnitude 6 earthquake about every 22 years since 1857. The frequency and regularity of this quake action attracted many geologists and research projects to Parkfield in the 1970s and 80s, all hoping to collect data that would help us predict quakes both in Parkfield and elsewhere. Ironically, the Parkfield "22-year quake" chose not to occur at its regular interval in 1993, but scientists are still waiting—monitoring and measuring all the while. Duane Hamman, in fact, has been taking measurements for the USGS in Parkfield with a rare laser geodimeter (shown at right) for 14 years. A laser geodimeter is really just an ultra-precise ruler that can measure large distances with extreme accuracy. Here's how it works: A technician beams laser light at one or more reflectors up to 6 miles away. The amount of time it takes for the light to make a round trip to the reflector and back is a function of the distance between the laser and the reflector. Repeated measurements reveal whether the earth is changing shape. If the light returns just a tiny bit earlier than before, it means that the two points have moved closer together. If the light is a little late, then the two points have moved apart. This technique is extremely accurate; it can measure shifts in distance as small as a millimeter. In practice, measuring distance with lasers poses some problems. The path between the laser and the reflector must be completely clear of obstructions, and fog and rain can block the laser beam. Also, the speed of light traveling through air changes with the air's temperature and pressure. Accuracy demands that these slight changes be factored into the calculations.
Duane Hamann has been taking measurements using this laser geodimeter for 14 years.
Parkfield's golden rolling hills are riddled with bizarre-looking instrumentation, such as this global positioning system receiver.

On the Road with the Faultline Project
©1999 Exploratorium