Tectonic plates creep silently past one another; glaciers flow sluggishly down mountains; ground level slowly rises and falls. The geologic forces that shape the surface of the earth usually act with such stealth that most people remain entirely unaware of them. But then the sudden break of a geologic fault or the explosive eruption of a volcano occurs in a populated area, and the devastation instantly makes thousands frighteningly aware that the solid earth is indeed prone to motion.

To better understand and, perhaps, forecast such catastrophic events, scientists have labored to measure the ongoing bending and stretching of the earth's crust. For this task, they have employed instruments of many types, from simple surveyor levels to sophisticated electronic positioning equipment. With all such methods, a person must travel to the site that is to be evaluated to set up some sort of apparatus and make observations. Yet this commonsensical requirement, as it turns out, is not an absolute prerequisite.

   
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