Station FARB

Questions for Ingrid Johanson

How did you become a scientist/engineer?

I've always tried to do the things I thought were the most exciting and find out more about the things I thought were the most interesting. By doing that I managed to follow my nose into a career in science. My mother helped get me started on this path as my high school trigonometry teacher. Whether at home or at school, she always shared her enthusiasm for math. Watching her approach everyday problems using math gave me the confidence to do the same. I became interested in physics in college because it provided a physical meaning behind the math I'd learned, and ended up in Earth science when I discovered all the interesting problems in that field with the bonus of being able to spend time outside instead of in a lab.

What is your job like?

I am a Research Geodesist with the Berkeley Seismological Lab at UC Berkeley. As part of my job, I oversee the operations of the Bay Area Regional Deformation (BARD) network, which is a network of 32 continuous GPS stations (including FARB) throughout Northern California. Much of this work is focused on characterizing earthquake hazard in the San Francisco Bay Area and I am currently working on using GPS data to improve earthquake early warning. Most of my work these days is done in front of a computer. I use specialized programs and write my own algorithms for data processing, handling, and modeling in order to turn the GPS observations into estimates of strain accumulation or fault slip.

What are you hoping to learn from your research?

GPS is a unique instrument in that it allows us to track very slow motions, such as plate tectonics. California's faults contain a mix of behaviors: some experience fault creep, some have large earthquakes, and some have both. Fault creep has the effect of reducing the size of an earthquake needed to “catch up” with plate tectonic rates. Therefore understanding where and when fault creep occurs is important for having accurate estimates of earthquake hazard. I would like to better understand how strain accumulation rates and earthquake occurrence are affected by fault creep.

Pacific Plate Motion

In Northern California, the San Andreas Fault is located very close to the coast for much of its length. This means that most of the GPS data we have is from one side of the fault, which is only half the picture! The Farallon Islands are a chain of rocky islands made from the same granite (Salinian) as the Sierra Nevadas. The Farallons were separated from the Sierra Nevadas 10-20 million years ago and have been carried north up the coast with the Pacific Plate. Now a National Wildlife Refuge, the Farallon Islands give us our best estimate for current motion on the Pacific plate by providing a home for an offshore GPS station.

Station FARB is moving northwest at a rate of 46 mm/yr (relative to North America). The time series in Figure 1 is a very straight line, which indicates that FARB is moving slowly and steadily all the time, as is expected from steady tectonic plate motion. However, measurements from other Pacific plate sites, like Hawaii, show that the plate is moving at 50 mm/yr relative to North America. So why is FARB slower?

FARB is slower than other sites on the Pacific plate because it is being held back by the San Andreas Fault. The tectonic plates move past each other constantly, but faults only move during an earthquake. At other times they are stuck and the crust around them must warp to accommodate the motion of the tectonic plates. The slower motion of FARB tells us that it is located within this warped zone around the fault.

Understanding the extent and pattern of deformation in the warped zone tells us how deep the seismogenic, or earthquake-producing, portion of the fault goes and how much stress is loading the fault, which will later be relieved in an earthquake.

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