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| | Abstract | | After the 2016 Kumamoto earthquake (Mw 7.0), certain hot springs in the Uchinomaki area became dormant, whereas others experienced increased flow compared with their pre-earthquake state. Here we report the mechanisms underlying the variations of the hot spring supply using satellite data, field observations, descriptions of the temporal variations of the hot springs, and data from a borehole camera. Direct borehole observations demonstrate the sliding along the horizontal geological formation at ~50 m depth, which is where the shallowest hydrothermal reservoir developed. Owing to >1 m northwest movement of the geologic block, as shown by satellite data analyses, extensional open fissures were generated at the southeastern edge of the horizontal sliding block, and compressional deformation and spontaneous fluid emission from wells were observed at the northwestern edge of the block. The temporal and spatial variation of the hot spring supply during the earthquake can be explained by the horizontal sliding of kilometre-scale hot spring area and borehole failures. Because there was no strain accumulation around the hot spring area prior to the earthquake and gravitational instability could be ignored, the horizontal sliding was likely caused by liquefaction (i.e., low-friction along the ~50m formation). |
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