A number of geothermal fields explored so far in the Circum-Pacific area occur along spreading ridges and subduction zones in areas of young tectonism and volcanism. A preliminary analysis by pattern recognition techniques, however, suggests that these geothermal fields are not situated along entire segments of plate boundaries but only at certain locations. These locations are (1) near the ends of plate boundary segments or (2) near sharp bends in the trench systems or (3) in transverse zones which divide plates into several blocks 62 to 620 mi (100 to 1,000 km) long. The locations of geothermal fields, therefore, appear to be influenced by plate geometry and correspond to lateral breaks in the continuity of the underthrusting plate. Shear heating in these transverse zon s, which separate different blocks in a plate boundary, may explain the excess heat flow which allows the development of geothermal fields in these zones. In divergent zones, the geothermal fields are situated near where the spreading systems are offset by transform faults. Exploration for geothermal fields is on a haphazard basis at present, guided primarily by the presence of hot springs and fumeroles. This study will help in the development of geothermal energy by identifying broad zones in which exploration should be carried out and by providing a hypothesis for their occurrence.

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