Parkfield earthquake prediction experiment. Dr Richard Clymer operates a VIBROSEIS truck. It produces artificial S (shear) waves of a precise frequency which can be used to probe the underlying rock structures in a manner similar to sonar. The vibrations from the 26 tonne truck are transmitted to the ground via a baseplate (visible between the wheels). The returning signals are picked up by a nearby network of 10 seismometers. Seismologists have predicted that an earthquake of around magnitude 6 will occur on the San Andreas fault at Parkfield before 1993. Dozens of researchers have made the village the most heavily instrumented earthquake region in the world.