A variety of precursory signals are generated by the many processes that occur as molten rock (magma) forces its way up through miles of the Earth’s crust before eruption at the surface. Many of these signals are extremely subtle and complex, and consequently require expensive arrays of sensitive instruments to detect and scientists with years of experience to interpret. The sections in the rest of this chapter outline the different major techniques used by the USGS to detect, quantify, and interpret each type of signal. In one way or another, each technique tracks types, magnitudes, and locations of earthquakes; uplift and subsidence of the ground surface; or changes in heat, water, and gasses emitted by volcanoes. Because the techniques measure different processes that occur during magma ascent, effective monitoring requires applying many techniques simultaneously to assess near-real-time developments at a volcano.
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