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Hidden magma system causing intense earthquake swarm in the northeastern Noto Peninsula, Japan
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  • Keisuke Yoshida,
  • Masaoki Uno,
  • Toru Matsuzawa,
  • Yohei Yukutake,
  • Yusuke mukuhira,
  • Hiroshi Sato,
  • Takeyoshi Yoshida
Keisuke Yoshida
Tohoku University

Corresponding Author:[email protected]

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Masaoki Uno
Tohoku University
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Toru Matsuzawa
Tohoku University
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Yohei Yukutake
Earthquake Research Institute
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Yusuke mukuhira
Tohoku University
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Hiroshi Sato
Earthquake Research Institute, The University of Tokyo, Tokyo, Japan
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Takeyoshi Yoshida
Tohoku University - Graduate School of Science
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Abstract

An intense earthquake swarm is occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but the last volcanic activity in this area occurred in the middle Miocene (15.6 Ma), and no volcanic activity has occurred since then. In this study, we investigated the cause of this earthquake swarm using spatiotemporal variation of earthquake hypocenters and seismic reflectors. Hypocenter relocation revealed that earthquakes moved from deep to shallow areas via many planes, similar to earthquake swarms in volcanic regions. The strongest M5.4 earthquake initiated near the migration front of the hypocenters. Moreover, it ruptured the seismic gap between the two different clusters. The initiation of this earthquake swarm occurred at a locally deep depth (z = ~17 km), and we found a distinctive S-wave reflector, suggesting a fluid source in the immediate vicinity. The local hypocenter distribution revealed a characteristic ring-like structure similar to the ring dike that forms just above the magma reservoir and is associated with caldera collapse and/or magma intrusion. These observations suggest that the current seismic activity was impacted by fluids related to ancient or present hidden magmatic activity, although no volcanic activity was reported. Significant crustal deformation was observed during this earthquake swarm, which may also be related to fluid movement and contribute to earthquake occurrences. A seismic gap zone in the center of the swarm region may represent an area with aseismic deformation.