Station MASW

Questions for Kaj Johnson

How did you become a scientist/engineer?

All through middle school and high school I thought I wanted to be a teacher. I especially liked my math classes, so I went to college convinced I would become a high school math teacher. In college I realized that while I did like teaching, what I really loved more was math and physics and I looked for a career where I could apply math and physics to solve problems. I learned about geophysics from my father, who is a geologist, and realized that I really enjoyed studying the mechanics of how rock gets moved and distorted over time due to plate tectonics.

What is your job like?

I like my job because I get to do a lot of different things throughout the year. I get to travel to interesting places like Japan and Taiwan to do research on earthquakes. I often travel out to California for various meetings about earthquakes. On a daily basis, I teach classes at Indiana University to both undergraduate and graduate students. I help guide graduate students through classes and research projects. Most of my daily research is conducted on computers because my research is focused on numerical modeling of earthquakes and deformation of the earth. Occasionally I get to take my work outdoors and collect GPS measurements in the field or take students to the Appalachian Mountains to study structural geology in the field.

What are you hoping to learn from your research?

My primary research interest is in understanding how energy is stored up in the Earth's crust before a large earthquake and how this energy is released during and after the earthquake. The Earth builds up potential energy before it breaks in an earthquake much like a rubber band stretching before it breaks. We can use GPS measurements of motions of the ground to track the distortion of the Earth's crust (much like the stretching of the rubber band) before an earthquake. We can also measure the sudden lurching of the ground during an earthquake (much like the breaking of the rubber band) with GPS measurements. Site MASW is a great example of GPS measurements that record the build up of energy in the crust before an earthquake on the San Andreas Fault and the release of energy during and after the earthquake.

Parkfield Earthquake

MASW is located near Parkfield, California. This town gave its name to a very famous earthquake prediction experiment. Although scientists started getting ready for this earthquake in the 1980s, it didn't happen until September 28, 2004.

MASW is located only about 10 km away from the San Andreas Fault. The GPS site measures the ground surface during the build-up and release of energy associated with a moderate-sized earthquake on the San Andreas Fault.

Figure 2 shows the total displacement of the site over eight years. From late 2001 to the very beginning of 2004, MASW moved at a steady rate along the "pre-2004" trend line. This trend is interrupted by a sudden motion caused by the late 2003 San Simeon Earthquake (not on the San Andreas fault) and then again in early 2004 by the Parkfield earthquake on the San Andreas fault. Note that the site moved about 30 mm very suddenly during the 2004 earthquake and then continued to move rapidly (compared to the rate before the earthquake) for several more years. In fact, you can see from the trend line that the site did not return to the pre-2004 rate of motion until at least 2009.

The motion of MASW in 2004 was due to sudden sliding of the San Andreas Fault during the earthquake. The continued rapid motions of site MASW from 2004-2009 were due to continued slow sliding (slower than during the earthquake) on the San Andreas Fault after the earthquake in a phenomenon we call “afterslip."

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