Haicheng, China 1975

Seismologist use a variety of instruments to monitor earthquake prone areas. Seismometers measure the abrupt oscillations of the earth.
Creepmeters measure the relative displacement between two points on opposite sides of a fault line. The distance between the two reference points is typically 3 meters. Creep is a slow movement. For example, the relative displacement across the San Andreas Fault is 1.5 centimeters per year, as measured near Hollister, California. Reference: Bruce A. Bolt, Earthquakes, Freeman, New York, 1988.
Geodimeters are laser-based instruments which measure the relative displacement between mountain tops on opposite sides of a fault line.
Seismologist can sometimes predict the time and location of earthquakes based on measurements from these instruments.
Haicheng Earthquake
For example, seismologists noted increasing seismic activity in Liaoning Province, China in 1975. The activity occurred near the cities of Haicheng and Yingkow.
The seismologists sent out warnings days before the February 4, 1975, earthquake of magnitude 7.3. The people in nearby cities remained outdoors, despite the cold weather. As a result, many lives were saved. Reference: M. Levy and M. Salvadori, Why the Earth Quakes, Norton, London, 1995.
Nevertheless, there have been some "false alarms" over the years. Furthermore, many earthquakes occur without any notable precursor signs.
Please send comments and questions to Tom Irvine at: tomirvine@aol.com
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