Geologic structure and volcanic history of the Yanaizu-Nishiyama (Okuaizu) geothermal field, Northeast Japan

The Yanaizu-Nishiyama geothermal ®eld, also known as Okuaizu, supports a 65 MWe geothermal power station. It is located in the western part of Fukushima Prefecture, northeast Japan. This ®eld is characterised by rhyolitic volcanism of about 0.3±0.2 Ma that formed Sunagohara volcano. Drillcore geology indicates that volcanism began with a caldera-forming eruption in the center of this ®eld, creating a 2-km-diameter funnel-shaped caldera. Subsequently, a fault-bounded block including this caldera subsided to form a 5- km-wide lake that accumulated lake sediments. Post-caldera volcanism formed lava domes and intrusions within the lake, and deposited ash-¯ow tu€s in and around the lake. The hydrothermal system of this ®eld is strongly controlled by subvertical faults that have no relation to the volcanism. The principal production zone occurs at a depth of 1.0±2.6 km within fractured Neogene formations along two northwest-trending faults to the southeast of the caldera. These faults also formed fracture zones in the lake sediments, but there was no apparent o€set of the sediments. Stratigraphic studies suggest that post-caldera activities of Sunagohara volcano have migrated southeastward to the present high-temperature zone. The source magma of Sunagohara volcano may contribute to the thermal potential of this ®eld. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.