Ken'ichiro Yamashina

Induced earthquakes in the Izu peninsula by the Izu-Hanto-Oki earthquake of 1974, Japan

Abstract Recently small earthquakes in the Izu Peninsula, central Japan, occurred in a region where differential strain, or shear strain on the nodal planes, may have been enhanced by the Izu-Hanto-oki earthquake of 1974 ( M = 6.9 after JMA). It is suggested that the seismic ctivity was induced by the redistribution of strain accompanying the Izu-Hanto-oki earthquake. The activity from August, 1975, may have also been affected by an abnormal uplift in the northeastern part of the peninsula. Based on plausible models, the uplift caused the accumulation of differential strain in the focal region of the subsequent earthquakes. Quantitatively, this change of crustal strain was of the order of 10 −6 ; it is ten times as much as the average annual accumulation. Consequently, the sudden or rapid change of strain was likely to have played an essential role in the subsequent seismic activity. This effect could be one of the factors which trigger a shallow intra-plate earthquake.

Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan

Abstract Tilt observations at FG1 station located about 680 m west of the central crater of Unzen volcano, southwestern Japan, revealed details about the process of magma ascent during the mid-May 1991 crisis, just before the appearance of a lava dome. Crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike, modeled by a vertical line source and a planer tensile dislocation in an elastic half space. Marked change in tilt was observed at first in the EW direction at FG1 during May 11–14, suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater. After the interruption of the rise of the magma column, magma began to intrude laterally as a dike on May 15 toward approximately the N80°W direction. This resulted in a large tilt change essentially in the NS component at FG1. The horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m. On May 17, the magma column started to rise again, probably because the impediment to magma ascent was fractured by the growth of the dike. The top of the magma column finally reached to the crater bottom probably on May 19, and a new lava dome was actually observed on May 20. After which, the remarkable upward tilt to the south at FG1 due to the dike intrusion decreased rapidly. The deformation after May 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of May 1991. Although large errors are inevitable in estimated values because of a lack of sufficient data, the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions.

Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy

Abstract Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.

A possible factor which triggers shallow intra-plate earthquakes

Abstract Taking the focal mechanisms into consideration, strain perturbations caused by preceding earthquakes are computed at the foci of 24 shallow intra-plate earthquakes (M ⩾ 6) which occurred in the Japan area in the last 30 years. Significant accumulation of differential strain (namely, shear strain on the fault plane or earthquake-generating stress) preceded the occurrence of these major shocks in many cases (16 out of 24). The duration of strain accumulation amounted to several or ten years. Quantitatively, these effects were about 10−8-10−6 in strain and 1–100 mbar in stress. It is questionable whether small disturbances of this magnitude always constitute a definitive trigger. In some critical cases, however, a fractional change in strain (and stress) will probably play an essential role in initiating the precursory process which results in the occurrence of earthquakes.

Correlations between tectonic earthquakes and volcanic activity of izu-oshima volcano, japan

Abstract The correlation between eruptions of Izu-Oshima volcano and preceding tectonic earthquakes has been studied for the period from 1921 to 1975, on the basis of earthquake-caused strain changes calculated from fault models. Results are: 1. |(1) Long-term effect of squeeze-up and drain-back of magma due to volumetric strain is evident for the period of 1923–1950, but not for the period of 1951–1975. 2. |(2) Among short-term responses, volcanic events have a positive correlation with changes in differential and probably tensile strains caused by earthquakes. Calculated values of these changes are on the order of 10 -8 to 10 -7 in strain. This indicates that such small changes in strain may induce activity when a volcano has been under critical conditions, and/or that actual changes in strain are much larger than those calculated, due to mechanical weakness of the volcano. 3. |(3) In short-term responses, the effect of dynamic and volumetric strains is neither simple nor obvious.

Ground temperature change observed at Unzen Volcano associated with the 1990–1995 eruption

Abstract Ground temperature associated with eruptive activity of Unzen Volcano, southwestern Japan, was observed in a cave about 680 m west of the central vent. Based on the data during 1991–1996, eliminating the effects of seasonal change, the temperature rose to the highest level probably around the first half of 1992. Hereafter the temperature decreased gradually year by year at least up to the autumn of 1996, beyond the level recovered at the beginning of the lava extrusion in May 1991. Although it is not necessarily conclusive, the present observation suggests a possible rise in ground temperature prior to the commencement of the first phreatic eruption in November 1990. If so, a careful observation may help to predict future eruptions by detecting a precursory rise of the ground temperature in this volcano.