ALBERT

Description: Since 1978, a series of frequent earthquake swarm have occurred at the sea east of Izu Peninsula, while Japan Meteorological Agency (JMA) observed 12 swarms for the period of 12 years up to 1989. The 12th earthquake swarm began on June 30, 1989. Seismic foci of the swarm were plotted in the sea only a few km north/ east of downtown Ito City and its coastal area. At about 18h 33m (JST) of July 13 1989, a submarine volcanic eruption spontaneously occurred at sea about 4 km northeast of downtown Ito. Japan Hydrographic Department (JHD) Survey Vessel TAKUYO was just around the point of the eruption and was surveying the seismic active area. TAKUYO was hit by frequent very strong shocks of water waves of the eruption at distances of 800m to few km from the newly born volcano. Surveyors and crew members of the vessel took photo, video tape record and audio record of the shock noise caused by the very strong water waves, while the positioning was done by GPS. Water mass gushing phenomena (water dome and/ or water jet) occurred five times, and the top of the biggest one was measured 113 m high above sea. Estimated time of beginning of the water mass gushing phenomena were 18 h 35 m 57 s, 18 h 39 m 57 s, 18 h 40 m 40 s, 18 h 41 m 25 s and 18 h 43 m 37 s of July 13 1989. HDJ conducted sea bottom surveys of the area four times. On July 9, a echo sounder and a side scan sonar of S/V MEIYO confirmed sedimented flat bottom at and around the forthcoming eruption point. At 18 h 28 m of July 13, or only 5 minutes before the eruption, S/V TAKUYO found a knoll of 25 m relative height, of about 500 m diameter at the base. JMA observed very large amplitude tremor from July 11 through July 13 without any remarkable surface phenomena, which suggest shallow magmatic intrusion beneath the knoll. On July 15, or two days after the eruption, unmanned automated survey craft MANBOU surveyed the knoll and found an about 200 m diameter crater in the center of the knoll, which reduced height of the knoll to only 10 m. MANBOU’s echo sounder recorded a big vertical cloud of volcanic gas welling up from the crater. On October 16 and 17, S/V TENYO surveyed the details of the newly born volcano (the knoll) and the vicinity with her narrow multi beam echo sounder (Hydrochart). The result shows shallowest water depth 81 m of the knoll on its southeast part of the crater rim, and deepest water depth 122 m at the southern part of the crater.

Description: Published

Description: Volcanic tremor accompanied by sound was recorded on July 11 and 13, 1989, by two strong-motion accelerographs located at the east coast of Ito City in the Izu Peninsula, Japan. The tremor on July 13 was resulted from successive explosion earthquakes during a submarine volcanic eruption off Ito. Through a comparison of the tremor episodes on July 11 and 13, we found the same types of sources. Therefore, it is reasonable to postulate that volcanic explosions, small-scaled compared with those on July 13, occurred during the late evening of July 11, though no surface evidence was reported. The tremor accompanied by sound on July 11 and 13 was characterized by both high-frequency (3–30 Hz) and low-frequency (0.5–1 Hz) motion. The high-frequency motion corresponded to water waves and low-frequency one to both Rayleigh and Love waves. Several types of volcanic earthquakes were observed by temporary measurements during the late evening of July 11 and the early morning of July 12. These observations suggest that volcanic activity became intensified during this period and was strongly related to the creation of the Teisi Knoll. © 1991, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Propagation characteristics of the intermediate-period (1-10 s) surface waves predominated in strong motion records are analyzed on the basis of an array observation deployed in metropolitan Tokyo area, Japan. Seismic energy source is the main shock of the 1984 western Nagano earthquake (M 6.8) with a focal depth of 2 km; epicentral distances (A) for the stations are about 157-205 km. Phase analyses using the sub-array at KOG (Δ = 175 km) indicate that dominant phases in the period range 1-10 s are Love and Rayleigh waves with clear dispersion. Apparent velocities of their phases are lying in the range of 800-2,100 m/s. They are matched with the fundamental mode of Love waves for transverse motion but are stretching over the two modes of Rayleigh waves (M11 and M21) for radial component; the crustal model, on which the normal mode solution is based, includes thick (~2,000 m) sedimentary layers lying on the Pre-Tertiary bedrock. The above results in conjunction with direction of wave propagation and particle orbit of the ground motion suggest that the intermediate-period surface waves are perturbed by other phases such as diffracted surface waves which are owed to lateral heterogeneities in the uppermost crust. Correspondence of major phases in the strong motion records of the long-span array gives rise to “inter-station group velocities” of the surface waves of the intermediate-period. Thus derived group velocities enable us to interpret that the well-dispersed later phases are on a branch near the Airy phase of which propagation velocity is 300-800 m/s. © 1992, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: In order to understand the propagation characteristics of earthquake strong motion in a sedimentary basin, combined analyses were made using running spectra, polarization and frequency-wavenumber spectra for array data obtained in Ashigara Valley, Japan. The data from three moderately large events (M= 5.5-6.5) which occurred in Sagami Bay, south of the Ashigara Valley, were analyzed. The long-period (5-10s) Love waves were predominant during the two shallow events (〈10km), while short-period waves less than 2 s dominated in the event of intermediate depth (20km). The predominant waves propagated from the azimuths of about 10°-20° west of the sources. Not only the sedimentary basin at the Ashigara Valley, but also the underground structure beneath Izu Peninsula may affect the propagation path from source to station. A significant variation in the Love wave propagation direction was found during the earthquake of Feb. 20, 1990, the shallow largest event. Three distinct arrivals of Love waves were identified in the event. Two wave groups were identified as Love waves propagated from the azimuth of 15° west of the source. The other one group propagated from the azimuth of 30° west of the source. Its propagation path turned towards a more easterly direction. The arrival was identified as Love waves from both the polarization and propagation direction. This identification was only possible by combining the results of polarization and frequency-wavenumber spectral analyses. The significance of the effects of three-dimensional heterogeneity in the sedimentary basin was also suggested. © 1992, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Frequent sea bottom surveying in an area of earthquake swarms in Sagami Bay about 2 to 3 km from the east coast of Izu Peninsula clearly disclosed the birth of a new submarine volcano and processes of its formation. Bathymetric soundings, taken on July 9, 1989, showed that the seafloor, where the new volcano was to be born, was characterized by undeformed sedimented bottom in depths of about 100 m. Large-amplitude tremors, beginning on the evening of July 11, were associated with the formation of a new volcanic knoll (cryptodome). This was confirmed by the survey vessel “Takuyo” on July 13 at 18 h 28 m when it was found that the knoll had risen 25 m above the surrounding seafloor. Five minutes later, after the existence of this newly formed bulge was established, a submarine eruption from the knoll began. While escaping from the hazardous area, Takuyo took photographs and made video tape recordings of the effects of the eruption on the sea surface and in the atmosphere. In addition, audio records of a series of frightening acoustic shocks of the volcanic activity were made. These shocks suggest frequent discharges from the seafloor of high pressure gasses associated with sub-bottom phreatomagmatic eruptions. On July 15, an unmanned automated survey craft, the Manbou, recorded a column of gas bubbles rising from the 200 m diameter crater in the center of the knoll. The eruption of July 13 had lowered the height of the dome from 25 to 10 m. Three months later a detailed bathymetric survey, using a highly accurate multibeam sounding system, Hydrochart, confirmed the maar-like topography of the volcano and also recorded the weakened emission of gasses. © 1991, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.