Rikio Miyajima

Ground uplift detected by precise leveling in the Ontake earthquake swarm area, central Japan in 2002-2004

Repeated precise leveling in the earthquake swarm area of Ontake, central Japan has revealed uplift of 3–6 mm in proximity to the epicentral region of the most active earthquake cluster in 2002–2004. Although the uplift is small, the vertical displacement is significant even considering leveling error. This uplift is associated with increases in 3He/4He ratios and CO2 δ13C values at a mineral spring in the region, indicating an upper mantle contribution. A region of low resistivity at a depth of 2 km beneath the uplift area has also been inferred, suggesting that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarm area.

Mass budget of the magma flow in the 2000 volcano-seismic activity at Izu-islands, Japan

Eruptive and caldera-forming activity at Miyakejima volcano island, Japan, commenced on 26 June 2000 was accompanied by more than 40 day of seismic swarms and significant crustal deformation in the nearby islands and sea region besides those at Miyakejima itself. The migration of the hypocenters in the early stage suggests that they were triggered by magma intrusion from Miyakejima. However, it remains uncertain whether the long-lasting seismic swarms and ground displacements in the northern Izu-islands were totally maintained by the magma flow from Miyakejima, because another magma source nearby Kozushima was suggested previously, which is 40 km north-west of Miyakejima, based upon anomalous ground displacements. Here we report the detection of associated changes with the 2000 activity in both absolute gravity and elevation at Kozushima as well as those at Miyakejima. Combining these data with horizontal GPS displacements, we extend the analysis of Nishimura et al. (2001) and construct an optimum source model, so that we can account for the observed changes in geodetic data and determine the mass budget of the magma flow. The total mass of the newly intruded dike offshore of Miyakejima and nearby Kozushima turned out to be 130% or greater than the lost mass at Miyakejima. As long as there are no other source elements, another magma reservoir near Kozushima is required and is suggested to have been activated, causing the seismic swarms and crustal deformation. We may speculate as a phenomenology that the rapid lateral magma flow from Miyakejima in the very beginning of the unrest awakened a dormant reservoir offshore of Miyakejima and Kozushima.

Hydrological gravity response detection using a gPhone below- and aboveground

We used a gPhone (serial number 90), the newest spring-type gravimeter manufactured by Micro-g LaCoste Inc., to acquire high-quality, continuous gravity records, both below- and aboveground. At a depth of 100 m, when the gPhone was situated under an unconfined aquifer, the standard deviations of the residual gravity based on first- and second-order curve fittings are 4.2 and 2.7 µGal, respectively. Some gravity decreases caused by rainfall were clearly observed, and unknown gravity variations may also have occurred. Alternatively, when the gPhone was placed aboveground on the flank of a high mountain, the standard deviation of the residual gravity was 1.7 µGal for both the first- and second-order curve fittings. The rainfall amount and snow depth can explain most of the residual gravity. On the basis of these results, we propose to detect and correct hydrological gravity responses using multiple gravimeters to study gravity signals from deep within the earth.

Repeatable Measurements of Baseline Length by Global Positioning System in Central Japan

An accurate determination of baseline length is of basic importance in the interpretation of crustal motions in earth’s interior for the study of earthquake prediction. A complete understanding of the crustal motion of a particular region requires a dense set of geodetic measurements in space and time. During the past decade, Global positioning System (GPS) at the United State National Geodetic Service has been developed chiefly for instantaneous positioning and dynamic navigation. This system has developed further with the capability of measuring baseline length on the order of 1 to 2 ppm accuracy for distances of several ten to several hundred kilometers in length, by using phase interferometric method with the GPS carrier wave length (e.g,, Hothem and Fronczek, 1983).

Gamma Ray Intensity Surrounding the Fukouzu Fault

Surveys of y-ray intensity around the Fukouzu fault were carried out by automatic measuring systems with a GM-survey meter and microcomputer. The Fukouzu fault, and its secondary fault were formed at the time of the Mikawa earthquake in 1945. These are reverse faults. We selected observation sites along these faults where the crustal deformation resulting from the event is well preserved. The survey was conducted by several observation systems of 7-ray intensity on a line across the faults at intervals of 5 to 280m. The results of the survey show the following for almost all of the sites: (1) the intensity of y-rays at the upheaval sites is greater than that of others; (2) peak y-ray intensity is seen at the upheaval sites; and (3) as thickness of alluvion increases, peak value of intensity decreases. We concluded that radioactive elements are condensed at the upheaval sites along the faults where many fissures are produced.