Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non‐volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the source area of tremors. In each episode, the source of the slow slip event and tremor migrate simultaneously. The spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate.

No abstract

No abstract

On July 13, 1989, the Ito‐oki submarine volcano was created and erupted after an earthquake swarm and volcanic tremor. At the final stage of the swarm activity, on July 9, 1989, a mainshock (M5.5) occurred. This earthquake consisted of two events separated 45 seconds from each other. The second event included high stress drop faulting (asperity). In this paper, the M5.5 earthquake was analyzed focusing on this asperity. The results are that 1) the radius and dynamic stress drop of the asperity were approximately 300m and 50MPa, respectively; 2) the epicenter of the asperity was estimated at 5km west of the erupted volcano and its depth was 6km; and 3) without this strong asperity, the waveforms of the two events were very similar, suggesting a similar source process except for the asperity.

Shear wave attenuation in deep soil deposits down to 2.3km in depth was directly and systematically measured at one of the deep-borehole observatories which were constructed for geophysical observations in the Tokyo metropolitan area.Introducing the newly designed SH-wave generator for an S-wave source, the down-hole measurements at 12 different depths were conducted down to 2.3km at intervals of 0.1-0.3km. Attenuation factors determined by the spectral ratio method were linearly inverted to QS-values under an assumption of frequency-independent QS.bottom of deep soil deposits, respectively, and larger than 150 in the substratum, for the frequency range of 3.5-20Hz.The QS for about 20 of the upper-most soil layer is faily consistent with known values which were estimated in shallower wells at various places.The substratum QS-value of 150 or over is not inconsistent with the crustal QS.The present measurement in such deep soil deposits is a new contribution to seismological and earthquake engineering studies.A significant effect of the attenuation factors upon the seismic waves is present by computing the amplification characteristics of deep soil deposits. At the present experimental site, the attenuation factor has a rather drastic effect upon the S wave in the period range shorter than 3sec.

VIBROSEIS reflection explorations profiled the crustal structure under the east coast of Izu Peninsula where a seabed volcano erupted on July 13, 1989. Profiles along two exploration lines show a common feature that consists of three layers. The uppermost layer is floored with relatively intense reflections from shallow depths within 0.5s of two-way travel time, the next one is rather transparent layer, and the last is the basement ceiled with weaker reflections around 1.0s. The shallower reflectors are seen to be broken here and there. The depth of deeper reflector correctly coinsides with the upper limit of the earthquake swarm which is concentrated to the depth range of 2-4km. It is the most interesting that narrow spot-like high velocity bodies are found in the shallowest crust near the seismic swarm area. We suppose that those bodies may be interpreted as aged magma intrusions, and may create an asperity of the major earthquake of M5.5 prior to the submarine volcanic eruption.

No abstract

No abstract