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Three-dimensional P wave velocity structure of shallow crust beneath the northern Awaji Island derived from refraction seismic explosion

Abstract

After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M7.2), the Research Group for Explosion Seismology (RGES) conducted a refraction seismic experiment in the northern part of the Awaji Island in order to investigate a detailed structure around the source region. In the surveyed area, 145 temporal observation stations were deployed two-dimensionally at 200–250 meters intervals, and 7 shots of 150 kg charges were detonated. Using the first arrival times, we derived a three-dimensional P wave velocity structure of the shallow crust in this region. A dominant feature was the low velocity in the northern part of the region as compared with the southern part. The averages of velocities in the northern area were 4.1 km/s (at a depth of 0.09 km) and 4.5 km/s (at 0.49 km). In the southern region, these were 4.9 and 5.2 km/s, respectively. The study region was mostly within the Cretaceous granitic rocks, and we cannot find a difference in geology that corresponds to the velocity variation. On the other hand, the Bouguer anomaly and the resistivity distributions are consistent with the lateral velocity difference.

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Correspondence to Tomotsugu Demachi.

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Demachi, T., Hasemi, A., Iwasaki, T. et al. Three-dimensional P wave velocity structure of shallow crust beneath the northern Awaji Island derived from refraction seismic explosion. Earth Planet Sp 56, 473–477 (2004). https://doi.org/10.1186/BF03352500

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Key words

  • Seismic refraction
  • Hyogo-ken Nanbu (Kobe) earthquake
  • 3-D velocity
  • granitic rocks