ALBERT

Description: Macro-anomalies preceding the 1984 Naganoken Seibu earthquake of magnitude 6.8 are analyzed in relation to maximum detectable distance, Dmax, from the epicenter and to precursor time, T, between anomaly appearance and main shock occurrence. These anomalies are anomalous animal behavior, strange detonation, change in underground water and the like. Making use of the previously-obtained relation between main shock magnitude, M, and Dmax, the probable epicentral area is assessed from the area for which all the Dmax circles, that are drawn centering at each observation spot, overlap. When M= 6.8 is assumed, the area includes the actual source fault and epicenter. The areas determined are too wide or narrow if M =7.0 or 6.6 are assumed, respectively. When an anomaly appears, it is possible to evaluate the probability for an earthquake occurring within a specified time-span as shown in the previous study. Such an evaluation is based on the frequency distribution of precursor time. It is proved for the Naganoken Seibu earthquake that the synthetic probability increases as time goes on and multiple macro-anomalies appear one by one reaching a value close to 1 immediately prior to the main shock occurrence. © 1993, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: That some volume of magma could be squeezed up from the magma reservoir triggering a volcanic eruption when compressional tectonic stress is strengthened by plate motion has been proposed as a possible explanation of successive occurrences of volcanic eruption and large earthquake. This idea, called the “flask model,” has been put forward in attempting to account for the relationship between the activities of Izu-Oshima volcano and the occurrences of great inter-plate earthquake associated with the Sagami trough off the Pacific coast of Central Japan. It is intended in this paper to examine whether such a model really works or not by taking the detailed configuration of magma reservoir into account. When a combination of parameter values usually accepted is assumed, it turns out that the volume of magma squeezed up by compressional tectonic stress may not be enough for triggering an eruption. It is therefore doubtful that a flask-model mechanism is actually working. Occasional eruptive activities of the volcano may rather be due to an increase in internal gas pressure in association with development of chemical reaction in the magma reservoir. © 1989, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Steady thermal field associated with magma mass intrusion is studied in two-dimensional (2D) and three-dimensional (3D) spaces. It is proved that the latter model leads to surface heat flow substantially smaller than that for the former. A numerical approach based on a relaxation technique for solving the Laplace equation is made use of. Transient heat conduction is solved when a lava dome makes an appearance on the earth's surface. Converting the differential equation that governs the problem into a difference equation, 2D and 3D solutions are obtained. It becomes apparent that the 2D analysis results in a cooling rate considerably smaller than that for the 3D analysis. A similar transient problem related to the filling of a reservoir with high-temperature magma is also studied. Even in this case, a 2D analysis leads to cooling appreciably slower than that for a 3D analysis. The mode of heat flow anomaly development at the earth's surface for the 2D analysis is noticeably different from that for the 3D analysis. In short, 3D analyses of the geothermal problems are important for obtaining realistic results although 2D analyses have so far been customary in many cases. © 1995, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: A resistivity variometer of unusually high sensitivity has been in operation at a crustal movement observatory about 60 km south of Tokyo since 1968. The variometer records changes in ground resistivity caused by crustal strain in association with tidal loading. When a large earthquake occurs, the variometer often records a step-wise change. Steps are observed for earthquakes having a magnitude of 8 or thereabout even if the epicentral distance exceeds 1,000 km. It is also the case for earthquakes of magnitude about 5 or more in the Tokyo area. Detailed investigations indicate that the variometer seems likely to detect extremely small coseismic strains in the form of resistivity steps. The resistivity steps are often preceded by a precursory change having a precursor time of a few hours. A Weibull distribution analysis of 21 examples brings out that the characteristic parameter controlling the distribution of precursor time is more or less the same as that for a group of short-term precursors such as land deformation, tilt and strain, and underground water. This kind of resistivity change, that may probably reflect an occurrence of premonitory rupture in the focal region, seems to have an important bearing on short-term earthquake prediction. © 1977, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Close examination of the resistivity record of the Yamazaki resistivity variometer in operation at Aburatsubo brought out a precursor-like change starting a few days prior to the Izu-Hanto-Oki earthquake of magnitude 6.9 on May 9, 1974. This is different from an imminent precursor having a precursor time of 4 hours as already reported for the same earthquake. The precursor, should it be a precursor at all, is of the order of 10-4 in rate of resistivity change and 10-8 in linear strain. Similarity between the resistivity change, change in the maximum amplitude of volcanic tremor on Volcano Mihara, Oshima Island and the tidal G factor observed at the southern Izu Peninsula is discussed. © 1979, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Nature of macro-anomaly precursory to an earthquake such as anomalous animal behaviour, changes inunderground water and hot springs and so on is studied based on an extensive data set related to anumber of large earthquakes in Japan. It turns out that the larger the magnitude of the main shock, the wider the area over which macro-anomalies are observed. Underground water and hot spring anomalies appear in an area where a marked crustal deformation can be expected to take place. The area for anomalous animal behaviour is slightly wider than that. No dependence of precursor time of macro-anomaly on main shock magnitude is found for an individual earthquake. However, there is a certain regularity of precursor time distribution. The maximum frequency of precursor appearance occursaround one day before the main shock. The mean precursor time takes on a value amounting to 0.42 day. It is likely that animals of smaller size such as rat, squirrel and so on become tumultuous earlier than those of larger size such as dog, pig, cow, and so on. Much macro-anomaly precursor data is spurious. Changes in synthetic probability of the main shock occurrence with time when multiple signals appear are evaluated in two ways: one is assuming the reliability, r, which is defined by the ratio of genuine data to the total (genuine and spurious) data, for all the actual data. The other is obtained from a computer simulation of random appearance. Comparing the two evaluations, it seems appropriate to assume that r amounts to 0.05 or thereabouts. © 1994, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Seismic energies released from areas covered by 18 map sheets of 1:200,000 scale in Japan during the years 679–1990 and 1601–1990 are evaluated. They are denoted by E0 and E1, respectively. Meanwhile, the potential seismic energies Ef that could be expected from possible movement of active faults located in respective sheets are also evaluated. The seismic activities considered are mostly due to intraplate earthquakes as the locations of map sheets indicate. As E1 is considerably larger than E0–E1 for most areas, it is highly likely that earthquake records in old days are missing. The relation between E1 and Ef indicates that the seismicity in the area where we have many active faults is high. On the assumption that overall earthquake occurrence or fault movement is governed by a Poisson process, the repeat time of fault movement is estimated as 500–2,000 years on the average. There are a few areas for which we have Ef≫E0. It is suspected that a large earthquake might occur there in the future. © 1991, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.