Robert A. Baxter, Certified Consulting Meteorologist

There is a keen interest in seismic activity because of the frequent earthquakes and "rock and rolling" in the southern California area. Following the Landers earthquake in the early 1990's, I developed a motion sensor based on a simple principle of pendulum movement. The motion was measured by recording the light intensity of a beam that was broken by the pendulum. To record the data a computer interface was developed using the analog to digital converters available in a personal computer game port. The motion of the pendulum movement was scanned 20 times per second, and the maximum deflection within each minute was recorded. This system was very useful during, and subsequent to, the 1994 Northridge quake. Once the power was restored the system operated continuously providing a record of all aftershocks. The database collected was subsequently analyzed and compared to the recordings made by Cal Tech. The data has been used in science fair displays as well as motion surveys as part of a subway construction project in the Los Angeles area. More recent versions of the motion sensor and associated software allow the use on any PC compatible computer that has a serial port. The processor power requirements are an 8088 processor or above. The system has subsequently been interfaced to a Campbell 21X data logger, and it reports the earth motion data at 10 minute intervals.