The source for the last stage is under A crater at a depth of 200 m below the caldera floor. The direction of the magnetic moments responsible for the variations.
J. Geomag.
Geomagnetic
Variations
Observed
after
of Izu-Oshima
Geoelectr.,
the
42, 319-335,
1990
1986 Eruption
Volcano
Y. HAMANO1,H. UTADA1,T. SHIMOMURA1, Y. TANAKA2,Y. SASAI1,I. NAKAGAWA1, Y. YOKOYAMA1, M. OHNO1,T. YOsHINo1,S. KOYAMA1,T. YUKUTAKE1, and H. WATANABE1 1 Earthquake ResearchInstitute, Universityof Tokyo,Tokyo 113,Japan 2Aso VolcanoObservatory , Faculty of Science,Kyoto University,Kumamoto869-14,Japan (ReceivedJune 6, 1989;RevisedDecember5, 1989)
After installed
the
1986 eruption
at various
March,
1987.
During
1987 gas explosion observed three
could
from
of the volcano,
March-June,
of the variations stage
the period
at the sites located
stages,
of Izu-Oshima
sites in the island.
March
to November
large
magnetic
on the caldera
can be attributed
floor.
magnetometers
sites started
to the
by
15, the day before
variations
in total
The variations of the
were
measurements intensity
into
The first stage
1986 eruption,
1987 gas explosion.
the were
can be divided
and September-November.
to after-effects
change
proton
of the
July-August,
be a precursory
Volcano,
Most
The
and the last
location
of the
source region for each stage of the variation was estimated based on the spatial distribution of the rate of the variation. The center of the source region for the first stage crater. caldera
was estimated The
floor.
is anti-parallel remanence
1.
source
to be located
about
for the last stage
The direction
A crater
of the magnetic
to the geomagnetic of the rocks
400 m below
is under
is a possible
field.
moments
Hence,
cause
the caldera at a depth responsible
the thermal
floor
under
the pit
of 200 m below
the
for the variations
demagnetization
of the
of the variation.
Introduction
On the foundation of the long history of magnetic observations at Izu-Oshima Volcano (see RIKITAKE,1951a, b; YOKOYAMA,1969), continuous observations of the geomagnetic variations at Izu-Oshima Volcano were started after the establishment of the Izu-Oshima Magnetic Observatory at Nomashi on the west coast of Izu-Oshima Island in 1959 (YUKUTAKEet al., 1978). In addition to the observations at Nomashi, several magnetic stations were operating on the island before the 1986 eruption of Izu-Oshima Volcano. One of the stations at the southern foot of Miharayama, the central cone of the caldera, showed a distinctly anomalous decrease in the total magnetic intensity relative to the standard Nomashi station since 1981, and the rate of the decrease accelerated several months before the eruption. An abrupt decrease of the intensity associated with the onset of the eruption was also observed at this site. As for the variations related to the fissure eruption on November 21, most of the operating sites including the Nomashi observatory indicated rapid changes in the magnetic field. Descriptions and explanations on these observations are given in separate papers (SAsAI et al., 1990; YUKUTAKEet al.,1990). 319
320
Y. HAMANO et al.
Unfortunately, the magnetic stations operating before and during the 1986 eruption were very few. Hence, any detailed investigations into the cause of the magnetic variations based solely on the magnetic data are quite difficult (see SASAI et al., 1990; YUKUTAKE et al., 1990). Moreover, the stations on the caldera floor , which indicated dominant variations associated with the eruption, were damaged during the initial phase of the present activity. After the initial activity ceased , we started construction of new magnetic stations on the island. By March 1987 , most of the new stations were operating and continuous data sets have been accumulating since then. On November 16, 1987, one year and one day after the onset of the eruption, a gas explosion occurred from the center of Miharayama, and the lava filling the pit crater drained back, accompanying some explosive activities on November 18. During the period from March to November, large variations of the magnetic total intensity were observed, mainly at the observational sites in the caldera area. The variations seem to be some after-effects of the 1986 eruption and
Fig,
1. after
Magnetic the eruption
observational
sites
are both
shown.
in Izu-Oshima Sites
Volcano.
in the shaded
area
The
sites
are given
before
the
in Fig. 2.
1986 eruption
and
Geomagnetic
Variations
Observed
after
the
1986 Eruption
321
precursory changes related to the 1987 gas explosion. In the present paper, we describe the variations and give a possible explanation. 2.
Magnetic Observations at Izu-Oshima Volcano
Magnetic observational sites within Izu-Oshima Island are shown in Fig. 1, while Fig. 2 indicates the sites in the caldera area. Izu-Oshima magnetic observatory (NOM, now a part of the Izu-Oshima Volcano Observatory, Earthquake Research Institute) started continuous measurements of the magnetic field in 1959. Before the 1986 eruption, WST and FUT were working outside the caldera rim. On the caldera floor, MI*, the site at the southern foot of Miharayama, had been working since 1978. Because of the observed anomalous variation at this site, sites MI2*, MNW and MNE were installed several months before the eruption. During the initial phase of the 1986 eruption, the proton magnetometers at MI* and MI2* were destroyed by the ejecta of the eruption, the one at MNE was covered by the lava flow from B fissures during the fissure eruption on November 21, 1986, and the magnetometer at MNW was retrieved in front of the lava flow overflowing from the
Fig.
2.
Observational
terminated
during
sites the
inside
the caldera
1986 eruption.
Other
rim.
Observations
sites were
newly
at MNW,
MNE,
constructed
after
MI*,
and
the eruption.
MI2*
were
322
Y. HAMANo
et al.
flat rim of the pit crater. Hence, at the time of the fissure eruption on November 21, 1986, no magnetometer was working on the caldera floor. However, the magnetic variation associated with the fissure eruption was so large that the magnetometers outside the caldera rim (NOM, FUT and WST) recorded rapid changes of the magnetic field. SAsAI et al. (1990) and YUKUTAKEet al. (1990) describe these magnetic variations in more detail. After the cessation of the initial phase of the eruption, we started to reconstruct a new magnetic observation system on Izu-Oshima Island. Because of the potential danger of re-eruption, we first installed the magnetometers outside the caldera area. Before the end of 1986, four sites, NCR, FNO, DMR and APT, were occupied, and sites SHR, ONS and MBS were installed with proton magnetometers on the first and second day of January, 1987. SHR, at the southeastern edge of the caldera floor, is the first observational site inside the caldera rim (see Fig. 2). By the first week of March, new caldera floor sites, Mb, MI1, and M12, were constructed at the southern foot of Miharayama, of which MI0 is close to the previous MI* and MI1 is a few tens of meters northeast of MI2*. Site OMT, located northwest of Miharayama, was also installed in March. Outside the caldera area, SKR, YOR and HAB were installed during this period. Measurements of the first few weeks at the southern caldera stations (Mb, MI1, and MI2) indicated large decreases of the total magnetic intensity. In order to clarify the cause of the variations, a new station, MIE, was constructed at the eastern foot of Miharayama, and the second site, MSE, was added at the southeastern part of the caldera floor in April. Observations at some sites outside the caldera floor were temporarily stopped, but all the caldera sites have been operating until the present. Table 1 lists the observational period for all the newly constructed sites on Izu-Oshima Island. Two types of proton magnetometers are employed at the observational sites. The first type which had been used at all the previous sites, is made by Kokusai Electric Co. This type of magnetometer is used at MI1, MI2, OMT, YOR and HAB. The magnetometers are powered by 24-V car batteries which are continuously charged by solar batteries. The total intensity is sampled each minute, and the data Table 1. Observational periodof the newlyconstructedmagneticstationson Izu-Oshima Island.
Geomagnetic
Variations
Observed
after
the
1986 Eruption
323
is transferred to the Izu-Oshima Volcano Observatory at Motomachi by radio signals and telephone cables. The data is also stored in a digital cassette recorder in the magnetometer for backup purposes. The other type of magnetometer was recently developed. This proton magnetometer has extremely low current consumption (70 mA in average) and is easy to handle because of its small size and light weight. All the other stations are installed with this type of magnetometer. The data is stored in EPROM (Erasable and Programmable Read Only Memory), and the memory capacity is 64 kbyte. Three weeks' data with one-minute sampling can be stored in each chip. In both magnetometers, resolution of the measurements is 0.1 nT. 3.
Magnetic Variations Observed on the Caldera Floor
In order to obtain the magnetic variations related to crustal activities, the total intensity values observed at Nomashi observatory (NOM) are subtracted from the raw data at each station, and the data is averaged for 24 hours to obtain daily mean values. We will discuss the variation of the magnetic field based on this daily mean data in the following. Because this simple method is not accurate enough to reduce the effects of the local disturbances of the external field to less than 1 nT, magnetic variations associated with the volcanic activity may not be identified from the daily mean data of the stations outside the caldera area where the variations are expected to be small. On the other hand, on the caldera floor, magnetic variations arising from the volcanic activities are large enough to be observed on the data sets. Therefore, we will mainly describe the magnetic variations of the caldera floor sites in the present paper. Among the sites, site OMT is very close to the new lava flow, and the magnetic effect due to the cooling of the lava is too large to extract any deeper information. Hence, we discuss the variations observed at six sites on the caldera floor (MI0, MI1, MI2, SHR, MSE and MIE. See Fig. 2). The daily mean variation of the magnetic total intensity relative to NOM at these sites is shown in Figs. 3(a)-3(f) for the period from March to November, whereas Fig. 3(g) shows a variation of NOM relative to the data at Yatsugatake Magnetic Observatory (YAT), about 160 km northwest of the Izu-Oshima Island. The last data point in each figure is on November 15, the day before the 1987 gas explosion. After the explosion, the magnetic variation on the caldera floor became much smaller. As for the overall change during the present period, the variation at site MI0, which is the site closest to Miharayama, shows the largest change among the present sites. The total amount of the decrease during the 9-month period is about 130 nT. The amount of the decrease falls off southward. Total changes at sites MI1 and MI2 during the same period are about 45 nT and 30 nT, respectively. At the two southeastern sites, SHR and MSE, the variations are much smaller than that at site MI2, although the distances from the center of Miharayama are comparable in these sites. All the above five sites, MI0, MI1, MI2, SHR and MSE, show a decrease of the total intensity during the entire period. Contrary to this, the total intensity at site MIE, which is located nearly directly east of the center of the pit crater of Miharayama, increased during this period; the amount of the total increase is as much as 20 nT. These observations can be useful constraints to estimate the location
324
Y. HAMANO
et al.
of the source responsible for the magnetic variation. As is evident from Fig. 3, temporal variations of the total intensity are not monotonous in these caldera sites. Figure 3(a) indicates that the rate of the decrease at site MI0 continuously decelerates during the period from March to June. This tendency is favorable for the interpretation that the variation is related to the 1986 eruption. Some types of after-effects of the eruption, such as the heating of ambient rocks due to the injected magma, could be a cause of the magnetic variation. On the
Fig. 3. Variations of the magnetic total force intensity relative to NOM observed at caldera floor sites during the period from March to November, 1987. (a) MI0-NOM, (b) MI1-NOM, (c) MI2-NOM, (d) SHR-NOM, (e) MSE-NOM, (f) MIE-NOM and (g) NOM-YAT. Vertical scale is in nT.
Geomagnetic
Variations
Observed
after
Fig. 3. (continued).
the
1986 Eruption
325
326
Y. HAMANO
et al.
other hand, during the period between September and November, the rate of the decrease increased with time towards the gas explosion on November 16. Therefore, it is not unreasonable to assume that the variation is related to the explosion. Increases of the gas pressure and the degassing rate prior to the explosion could be causes of the variation. In July and August, the rate of the variation of the magnetic field is small compared to the above two periods. The pattern of the variation at site MI1 is very similar to that of site MI0, though the amount of the change is much smaller than at the latter site. The division of the three stages during the entire period is more evident at this site, since the data is more continuous than that of site MI0, where the September data is missing. The amounts of the decreases in the first and the last stages are both about 20 nT, and the field is nearly constant in the second stage. At site MI2, the observed magnetic fields in April and May are not reliable because a tilt meter was being constructed at a location very close to this site. Comparison of the magnetic fields in March and June, however, indicates the decrease of the total intensity during the first stage. The variation of the magnetic field during the last stage at site MI2 is very similar to that of sites MI0 and MI1, where the acceleration of the decreasing rate is more conspicuous. The total intensity seems to be slightly increased during the second stage. At site SHR, a gradual decrease of the total intensity relative to NOM can be seen, but the distinction between the three stages is not apparent. A small bump of about the same size at around the end of May is observed in all the present sites, and is also observed in NOM-YAT data (Fig. 3(g)) with an opposite direction. Hence, the variation can be attributed to the local external disturbance at NOM station. At site MSE, the overall variation of the magnetic field is very small and is almost constant. However, the decreasing in the first stage and the increasing tendency at the last stage can be observed. The variations at site MIE are quite different from the other sites. The magnetic total intensity is increasing over the entire period. The rate of the increase in the later stage is higher than the earlier stage. An accelerating tendency in the rate of the increase at the last stage can also be seen from Fig. 3(f). 4.
Location of the Source of the Observed Magnetic Variations
In the previous section, temporal variations of the magnetic field observed at six caldera sites were described. Although the number of the observational sites is much more abundant than prior to the 1986 eruption, it is still not possible to obtain a unique two- or three-dimensional distribution of the crustal magnetization responsible for the variations by a direct inversion technique. Hence, we need to employ some intuitive methods for the interpretation of the observed data. We first try to estimate the locations of the source region for each stage of the variation. During the first and the last stages, spatial variations of the amount of the decrease of the total intensity among the present sites are rather systematic, where the amount is closely related to the position (distance and direction) of the observational sites relative to Miharayama. The amount of the decrease during the two stages falls off sharply to the south (MIO to MI2); the amount is very small at the southeastern sites (SHR and MSE) in spite of their comparable distances from site MI2; and the sign of the variation is different at the eastern site (MIE). These observations indicate that the location of the source region responsible for the
Geomagnetic
Variations
Observed
after
the
1986
Eruption
327
variation is confined within the central cone area, and that the center of the source region is below the caldera floor, i.e., the altitude of the observational sites. The spatial pattern of the magnetic variation clearly indicates that the direction of the magnetic moment responsible for the variation is anti-parallel to the ambient geomagnetic field. For a quantitative specification of the source location, some data manipulation is necessary. As long as the location of the source region responsible for the magnetic variations in multiple sites is stationary, the ratio of the rate of the variation between any two sites remains constant. Hence, if we plot the magnetic fields at two sites along the horizontal and the vertical axis, the variation should be linear on the plot, and the change of the gradient indicates the difference of the location of the source region. In order to do this analysis, the observed variations of the total intensity for sites MI1, MI2, MSE, SHR and MIE are plotted along the vertical axis against the total intensity variation observed at MI0 along the horizontal axis in Figs. 4(a)-4(e). As is evident from the figures, the variations during the first stage (March to June) and the last stage (August to November) can be well approximated by straight lines, which confirms that the magnetic variations during the two stages can be explained by single sources. The difference of the gradients of the fitted lines in each plot indicates that the locations of the sources are different between the two stages. As for the variation in the second stage, a linear dependency is not evident. The offsets of the two straight lines fitted to the first and the last stages indicate that the total intensity increased during the second stage and that the amount of the increase is approximately constant for most of the sites. In order to specify the locations of the sources in more detail, we employ two pairs of sites, MI0-MI1 and MI0-MIE. These two pairs were selected since the estimate of the gradient for MI0-MI1 is most reliable because of the large size of the variation, and the variation at MIE is crucial to determine the location of the source. For the former pair (see Fig. 4(a)), the least squares fit of straight lines gives the ratio MI1 / MI0 of 0.53 and 0.33 for the first and the last stages, respectively. The lower ratio for the last stage suggests that the source region in this stage is closer to site MI0 than that in the first stage. The corresponding ratios for MI0-MIE pair (Fig. 4(e)) are -0.13 and -0.12, respectively. They are negative and comparable in both stages. For the model calculation of the ratios, we assume two cases in which the horizontal position of the center of the source region is the center of A crater (point A in Fig. 2) and is the center of the pit crater (point P). By assuming that the observational sites are outside the source region, which is not a severely irregular shape, we can apply a dipole source model to the model calculation. The change of the total intensity of the magnetic field at position (x, y, 0) caused by a dipole source at position (0, 0, z) can be expressed as (1) where
328
Y. HAMANO
et al.
Fig. 4. Comparison of the magnetic variations observed at MI1 (a), MI2 (b), SHR (c), MSE (d), and MIE (e) with that observed at MI0. Straight lines fitted for the periods of March-June and September-November were obtained by the least squares method.
Geomagnetic
Variations
Observed
after
the
1986 Eruption
329
Fig. 4. (continued).
and M is the intensity of the magnetic moment, I is the inclination of the ambient geomagnetic field, and x, y and z directions are in magnetic north, magnetic east and vertical downward, respectively. In Eq. (1), the direction of the dipole moment is assumed to be anti-parallel to the direction of the geomagnetic field. The distribution
of
△F
in
the
xy
plane
is symmetric
around
the
magnetic
north-south
axis(x
axis), where the value in the northern half plane is mostly positive and that in the southern half is mostly negative. The nodal line is a hyperbola which cuts the x-axis at x=3z and approaches the southeast and the southwest directions further south, when
the
inclination
is
45°
and
the
source
depth
is
z.
Hence,
the
pattern
is
qualitatively consistent with the observed spatial variations on the caldera floor. If we assume that the horizontal locations of the center of the source are points A and P in Fig. 2, ratios of the variations of the total intensity for any pair of observational sites can be calculated from Eq. (1), since the positions of the sites are known. Figure 5 shows the results of the calculation for the ratios MI1 / MI0 and MIE/ MI0 as a function of the source depth (z), where the observed ratios for stage 1 and stage 3 are also indicated. For the first stage, the observed ratio of MI1/ MI0 is 0.53, which can be explained if the depth of the source is about 350 m for both cases, as shown in Fig. 5(a). For MIE/ MI0, the estimated depth is about 200 m for A crater and about 400 m for the pit crater. In order to explain the variations by a single source, the location of the center of the source is under the pit crater with a depth of about 350-400 m. On the other hand, the low MI1/ MI0 ratio of 0.33 for stage 3 cannot be explained if the source is under the pit crater. The source should be
330
Y. HAMANO
Fig. 5.
The calculated
variations
et al.
of the ratio of the rate of the geomagnetic
change for pairs MI1-MI0
(a) and MIE-M10 (b) as a function of the source depth. Observed ratios for the first stage (solid line) and the last stage (dotted line) are also shown.
very close to site MI0. If we assume that the source is under A crater, the estimated depth is about 200 m. This position of the source can also explain the MIE/ MI0 ratio, as shown in Fig. 5(b). Location of the source for the magnetic variations of the second stage (July-August) is not clear. However, the increase of the total intensity at all the sites, including MIE, suggests that the source is situated higher than the caldera floor, and that the direction of the dipole moment is antiparallel to the ambient field. The center of the source should be at the north side of Miharayama, because the spatial variation of the amount of the intensity increase is not apparent in stage 2. An increase in vapor activities around the rim of the pit crater observed in July may be responsible for the thermal demagnetization. 5.
Discussion
Large variations of the magnetic total intensity were observed on the caldera floor of Izu-Oshima Volcano during the period from March to November, 1987. In
Geomagnetic
Variations
Observed
after
the
1986 Eruption
331
the long history of magnetic observations at the Izu-Oshima Island, such a large variation had not been detected. At site MI*, which is very close to the newly constructed site MI0, the total intensity decreased by 12 nT during the five months just prior to the 1986 eruption. This rate of decrease is about an order of magnitude smaller than that observed after the eruption at site MI0. It is also to be noted that, after the 1987 gas explosion, the rates of variation of the magnetic field at these caldera sites are much smaller than before the explosion. Hence, it seems that the cause of the variation was introduced during the 1986 eruption, and disappeared after the gas explosion on November, 1987. In the previous section, the locations of the source region for the first and last stages were determined based on the spatial distribution of the rate of variation. For the first stage, the source is located under the pit crater at a depth of about 400 m below the caldera floor, and the position of the source for the last stage is 200 m below the caldera floor under the center of A crater. Although the measurement of the variations was initiated in March, it is reasonable to assume that this stage of the variation started right after the 1986 eruption on November. The data at SHR shows the decreasing tendency since January, 1987. The amount of the decrease for March to June at MI0 is about 60 nT. Considering the decelerating nature of the variation during this period, the total amount of the decrease must be larger than 100 nT. Then, the magnetic moment responsible for the variation is estimated to be 1.7×108 nT
and
Amt the
from estimated
Eq.(1). dipole
For
the
moment
last
stage,
is 2.5×107
the
total
variation
at MI0
is about
60
Am2.
Generally speaking, several physical processes can be considered as a cause of the magnetic variations associated with volcanic activities. These are (1) the piezomagnetic effect, (2) the electrokinetic effect and (3) thermal demagnetization of the magnetized body. However, the former two processes are not appropriate to explain the present observations. During the present activity, the magnetic total intensity at the southern caldera sites always decreased before the 1986 eruption, associated with the summit eruption, and after the 1986 eruption. This type of irreversible change of the magnetic field is difficult to explain by the stress effect of the magnetized body associated with the eruption. In addition to this difficulty, the piezomagnetic effect is mainly caused by a deviatric stress; however, the possible deviatric stress after the eruption should not exceed the lithostatic pressure. In the present case, the center of the source region is shallow. Hence, the deviatric stress must be much smaller than 100 bars, which is a probable lithostatic pressure at a 400 m depth. Assuming a normal stress sensitivity of 1%/ 100 bars and the maximum possible stress of 100 bars, we need a stressed region with a volume 100 times larger than the volume required from the thermal demagnetization model (see below). If such a large region were continuously stressed after the eruption, a large deformation is expected. However, after the 1986 eruption, no appreciable crustal deformation was observed from the tilt meters on the caldera floor or other sites outside the caldera area. Moreover, no reasonable mechanism for the continuous accumulation of stress after the eruption can be invented. The electrokinetic effect (MIzuTANI and ISHIDO,1976) is also inappropriate for the present case. As a first-order model, an increase of the pressure at the source region causes an axially symmetric flow of fluid, which causes a troidal magnetic field, not observable outside the source region. Hence, a second-order anisotropic
332
Y. HAMANO
et al.
flow should be considered for the explanation. However, the electrokinetic effect due to fluid motion has recently been considered to be smaller than previous estimates (MURAKAMI,1989). Hence, the assumption of a very artificial situation is required to explain the present observation by the electrokinetic effect. In the following, we only consider the third possibility, the thermal effect. The intensities of the remanent magnetizations of the volcanic rocks from Izu-Oshima Volcano were first measured by NAGATA(1943). Recently OHNO(1988) sampled the historical lava flows and measured the remanent magnetization and its thermal properties. These measurements suggest that the average intensity of the remanence
is higher
than
10
A/m
and
that
the
Curie
temperature
is about
500℃
on
average. Assuming a remanence intensity of 10 A/m and total demagnetization of the rocks, the radius of the source spheres for the first and last stages are estimated to be about 160 m and 80 m, respectively. The north-south cross section of the caldera area (Fig. 6) indicates the relative configuration of the surface topography, the observational sites and the positions of the source regions. In the following, we will give a simple scenario of the volcanic activity after the 1986 eruption based on present magnetic observations.
Fig. 6. North-south cross section of Miharayama. Three southern caldera sites (MI0, MI1 and M12) and the estimated source regions for the magnetic variations are shown.
After the 1986 eruption, no evidence for the rising up of magma has been reported. Hence, the heat source for the demagnetization of the source regions must have been introduced at the time of the eruption. Then, it is natural to assume that a vapor phase caused by the heat source is responsible for the heating of the ambient rocks. ELF-VLF measurements at Izu-Oshima revealed a conductor which exists all over the island at a depth of approximately sea level (UTADA and SHIMOMURA, 1990). They concluded that the conductor has arisen due to sea water which has penetrated
under the island through
the porous body. This could be a source of the
Geomagnetic
Variations
Observed
after
the
1986 Eruption
333
vapor. At the first stage of the magnetic variation, the bottom of the source region nearly coincided with the sea level. The introduction of the magma as a heat source during the 1986 eruption caused the heating of sea water and increased the vapor pressure. The vapor continuously flows out through the cracks around the source and causes the heating of the rocks. This is a possible explanation for the first stage of the magnetic variation. If the fluid permeability of the surrounding rocks is not sufficient to enable all the generated vapor to escape, the vapor pressure may increase and cause an increase of cracks and fractures. In July or August, the observed surface vapor activity increased (Fujii, private communication). This phenomena was most dominant around the rim of the pit crater. This may suggest that the cracks reached the surface and the bottom of the pit crater started to collapse. The increase of vapor around the summit area caused the demagnetization of the rocks making up Miharayama, which could explain the magnetic variations in the second stage. Around late August or early September, the further increase of gas pressure caused the introduction of vapor through the vent to A crater, which was used during the 1986 eruption, and the vapor heated the rocks surrounding the vent. It is possible that the heating and the subsequent demagnetization of the rocks caused the magnetic variations observed at the last stage. The accelerating nature of the magnetic variation observed in the last stage could be explained by the increase of the crack surface area in the source region due to the increase of gas pressure. An increase in seismic activity under Miharayama around August, 1987, was actually observed (WATANABE,1988). Other parts of Miharayama seem to have been affected by this increase of vapor activity. But, the vent area is closest to the observational sites on the caldera floor. This may be the reason why the source location in the last stage was determined to be under A crater. On November 16, the gas caused a single explosion, which triggered the collapse of the bottom of the pit crater and the drain-back of the magma which had filled the pit crater on November 18. During this activity, the heat source responsible for the variations before the explosion also seems to have returned to a deeper region, which could explain the termination of the large variation after the explosion. As mentioned before, the decreasing rate of the total intensity observed on the caldera floor sites (MI* and MI2*) prior to the 1986 eruption is much smaller than the presently observed variations at sites MI0 and MI1. In order to explain this difference, a change of the efficiency of the heat transfer should be considered. It is not unreasonable to assume an increase in the crack or fracture density, and therefore, an increase in the effective surface area, through which heat is transferred from the heat source to the ambient rocks, during the initial phase of the volcanic activity. We first estimate the crack density after the 1986 eruption. The magnetic variations observed after the 1986 eruption suggest that the characteristic time-scale of the variation is on the order of about three months for both the first and last stages. Then, the characteristic length for the diffusion of heat in each stage is calculated to be about 3 meters by assuming a thermal diffusivity of 10-6m2/ s. If we assume a total demagnetization of a source region in each stage, an average crack separation of 6 meters is required. This means that the average block size of the volcanic body is about 6 m. Considering the thickness of the basalt flow in the Izu-Oshima Islands, this value is not unreasonable for the possible block size of the
334
Y. HAMANO
volcanic last
body.
stages
are
In other words, 2.7×106
ml
and
et al.
the area of crack surface 3.3×105m2,
respectively.
required Most
of
for the first and the these
crack
surfaces
must have been introduced during the 1986 eruption, since the magnetic variations before the eruption are an order of magnitude smaller than those after the eruption. However, it is not appropriate to assume that these cracks were newly formed during the eruption, because intensive earthquake activities were not observed during the eruption, although some seismic activity was observed under Miharayama after the eruption. Probably, the volcanic activity during the 1986 eruption caused an increase in the connectivity of the pre-existing cracks and fractures. 6.
Conclusion
Large magnetic variations have been observed by proton magnetometers situated on the caldera floor of Izu-Oshima Volcano during the period between March and November, 1987. The variations can be attributed to the thermal demagnetization of rocks under the central cone, Miharayama. The spatial distribution of the amount of magnetic variations observed on the caldera floor indicates that the source region is under the pit crater at a depth of about 400 m below the caldera floor for the first stage of the variation, and it is situated under A crater at 200 m depth for the last stage. The demagnetization of the source regions is supposed to have occurred due to high-temperature vapor produced by the heating of groundwater by a magma source introduced during the 1986 eruption. The observed very low level of magnetic variations after the 1987 eruption suggests that the magma source drained back to deeper regions associated with the retreat of the magma filling the pit crater, which is considered to have occurred on November 18, 1987.
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after the 1986 Eruption
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