Volcanic events associated with an enigmatic submarine earthquake

• Published in: Geophysical journal international 2000-08, Vol.142 (2), p.361-370
• Main Authors: Sugioka, Hiroko, Fukao, Yoshio, Kanazawa, Toshihiko, Kanjo, Kenji
• Format: Article
• Language: English
• Subjects: CLVD (compensated linear vector dipole) earthquake; ocean bottom seismology; T phase; volcanic activity
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1046/j.1365-246x.2000.00153.x

On 1996 September 4, a submarine earthquake occurred underneath the Smith Caldera near Tori-Shima on the Izu-Bonin arc, Japan. Its mechanism was a CLVD with the principal tensile dipole in the vertical direction. The tsunami magnitude Mt of 7.5 was significantly larger than not only the body wave magnitude, mb, of 5.6 but also the moment magnitude, Mw, of 5.7, and thus slow faulting is not a major cause of the generation of large tsunamis. We have detected successive T-wave trains subsequent to the direct T-wave train from this earthquake on the records of the OBS submarine cable arrays off northeastern, central and southwestern Honshu. We have located these T-wave origins to be coincident with the epicentre of the CLVD earthquake within their solution errors. The T-wave events were repeated for about 35min, while their wave characteristics changed with time. The T-wave amplitude at a station about 700m deep increases with time more strongly than those at stations at greater depths. The T-wave duration is shortened progressively with time. The spectral shape of these T waves is similar to that for ordinary shallow earthquakes, indicating that the ultimate origins of these T waves are within the solid Earth rather than at or above the ocean bottom. Such temporal changes and the spectral shape suggest that the origins of these T waves are seismic shocks that migrated upwards well into the body of the Smith Volcano. The spectra of seismic waves from the CLVD source lack significantly high-frequency components, suggesting a lower than normal rupture velocity due presumably to a hotter source environment than in the surrounding region. We suggest that as a result of the CLVD earthquake, the magma-bearing source material was squeezed radially inwards and expelled vertically outwards to induce the magma ascent and associated upward migration of seismic events.