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Buried large block revealed by gravity anomalies in the Tonankai and Nankai earthquakes regions, southwestern Japan

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Abstract

We confirmed existence of a buried large block beneath seafloor off Cape Shionomisaki, Kii Peninsula, southwestern Japan, by improving marine gravity data. We found that the 1944 Tonankai and 1946 Nankai earthquakes occurred close to this block. The block is characterized by high gravity anomalies of 80 mGal larger than surrounding area with a diameter of about 40 km. Center of the anomaly is located 30 km off Cape Shionomisaki, southern end of the Kii Peninsula. This high gravity anomaly can be explained by existence of a higher density body with approximate thickness of 8 km just beneath the seafloor. This block coincides with the reported high velocity zone from recent seismic refraction survey in this area. Judging from seismic velocity, density, and geologic evidences, the block presumably consists of intruded acidic rocks, not seamount.

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Correspondence to Ryo Honda.

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Honda, R., Kono, Y. Buried large block revealed by gravity anomalies in the Tonankai and Nankai earthquakes regions, southwestern Japan. Earth Planet Sp 57, e1–e4 (2005) doi:10.1186/BF03351799

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Key words

  • Gravity anomaly
  • Cape Shionomisaki
  • Nankai earthquake
  • Tonankai earthquake