Journal of Volcanology and Geothermal Research 176 (2008) 610–616

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Short communication

The role of thermal viscous remanent magnetisation (TVRM) in magnetic changesassociated with volcanic eruptions: Insights from the 2000 eruption of Mt Usu, Japan

T. Hashimoto a,⁎, T. Hurst a,b, A. Suzuki a, T. Mogi a, Y. Yamaya a, M. Tamura c

a Institute of Seismology and Volcanology, Hokkaido University, N10W8, Kita-ku, Sapporo, 060-0810, Japanb GNS Science, Ltd., 1 Fairway Drive, Avalon, PO Box 30368, Lower Hutt 5040, New Zealandc Geological Survey of Hokkaido, N19W12, Kita-ku, Sapporo, 060-0819, Japan

⁎ Corresponding author.E-mail address: hasimoto@uvo.sci.hokudai.ac.jp (T. H

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.05.009

A B S T R A C T

A R T I C L E I N F O

Article history:

Volcanic eruptions can pro Received 23 January 2008Accepted 8 May 2008Available online 25 May 2008

Keywords:Usu volcanomagnetic fieldthermal viscous remanent magnetisationVRM

duce large magnetic field changes by thermomagnetic effects, especially whenmagma cools from high temperatures and acquires a permanent magnetisation from the Earth's magneticfield. After the 2000 eruption of Mt Usu, Japan, significant magnetic field changes were observed not only inthe vicinity of the magmatic intrusion but also in an area some distance away that was unlikely to be at atemperature near the Curie Point.The magnetic changes in the latter area appear to be caused by thermal viscous remanent magnetisation(TVRM), in which the elevated subsurface temperatures have accelerated the acquisition of magnetisationparallel to the existing Earth's field in the ground material. The unusually large changes (up to 40 nT/yr) areprobably because of underlying reversely magnetised Pleistocene rocks, which are being normallyremagnetised by exposure to temperatures of over 200 °C. We made an order of magnitude estimate ofthe likely effects of TVRM in this case, based on some previous laboratory studies, which confirms that TVRMis a plausible mechanism for the observed magnetic changes. This paper provides probably the first fieldexample in which a natural TVRM process associated with ongoing volcanic activity has been observed. Thisindicates that even when temperatures are well below the Curie Point, significant magnetic changes canoccur on active volcanoes.In many cases, areas of recent volcanism are dominated by normal magnetisation and thus the TVRM onlyplays a minor role in volcanomagnetic changes. It will, however, be a significant effect when materials withreversed or randomly-oriented magnetisation are moderately reheated through volcanic activity. Weemphasize that separation of thermal viscous magnetisation from thermal magnetisation will sometimes benecessary for properly interpreting magnetic changes around volcanoes, especially in relation to ongoingvolcanic activities.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Volcanomagnetic effects and reversed magnetisation

Changes in magnetic field due to volcanic activities, the so-calledvolcanomagnetic effects, have been frequently reported. Most of thempostulate the thermomagnetic effect as the most dominant process(Hurst and Christoffel, 1973; Tanaka, 1993; Del Negro and Ferrucci,1998), though a few others suggest the piezomagnetic (Del Negro andCurrenti, 2003) or electrokinetic effects (Zlotnicki and Le Mouel, 1988;Zlotnicki et al., 1993). Modelling of such volcanomagnetic effectsgenerally assumes the magnetisation is in the same direction as thepresent Earth's magnetic field. This is, in most cases, a reasonableassumption sincemonitoring is normally targeted on recent volcanoesin the present normal magnetic period. However, in the case of flank

ashimoto).

l rights reserved.

eruptions or the formation of parasitic volcanoes, we should be awareof the possible existence of reversed magnetisation adjacent to aneruptive zone. This paper describes the magnetic field changes afterthe 2000 eruption of Usu volcano and discusses the Thermal ViscousRemanent Magnetisation (TVRM) of reversely magnetised rocks as apossible cause of the observed magnetic changes.

1.2. The 2000 eruption of Mt Usu

The 2000 eruption of Mt Usu volcano began after several days ofprecursory seismicity and ground deformation, with the first seismicevent noticed on 27 March 2000. The first new crater opened in theWest-Nishiyama area on 31 March 2000, followed by the activity inthe Kompira area on 1 April (Fig. 1 for locality map). The strong surfaceactivity lasted for several months, and gradually declined, with thenew vents having reached a fumarolic state by mid 2001.

The surface activity was spectacular, but it actually was only asmall part of the magma-driven activity. Miura and Niida (2002)

Fig. 1. Location map of Japan showing Usu Volcano, and location map of Usu volcano showing West-Nishiyama (N) & Kompira (K) craters from 2000, and the peaks of major lavadomes and crypto-domes in the past: Ko-Usu(KU), Oo-Usu (OU), Meiji-Shinzan (MS), Nishiyama (NY) and Showa-Shinzan (SS). MTY is the reference magnetic station site. In thedetailed map of the West-Nishiyama craters, grey contours show the increase in height in metres in approximately the first month of the eruption (after Jousset et al., 2003; originaldata from Public Works Research Institute), R–R′ is the profile of the shallow resistivity survey. Open squares mark magnetic field survey points. Crater NB is marked by concentriccircles.

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estimated a total volume increase of 0.15 km3, while the fracture zonemodel of Jousset et al. (2003) estimated the intruded volume as0.22 km3, whereas the actual volume erupted was about 10% of this.Note also that both these deformation models had the intrudedmagma reaching depths as shallow as 50–140 m and 400 mrespectively, below the ground surface.

A wide area of Usu volcano showed deformation, but the largestchanges were near the West-Nishiyama craters, where the groundsurface moved upwards by tens of metres, with a maximum elevationincrease of 75–80 m (Mori and Ui, 2001; Jousset et al., 2003). (Fig. 1shows the deformation that occurred by 26 April 2000, about 90% ofthe final amount).

It was noted at the time of the eruption by Nakada (2001) that theactivity showed a strong influence of water on the explosions, i.e. that

these were phreatic or phreato-magmatic explosions. Some of thiswater would have been from the snow that was lying on the ground atthe time of the eruption, but a plentiful supply of groundwater musthave been present to produce the explosions shown in Ui et al. (2002).Yokoo et al. (2002) discussed these phreatic explosions, andconcluded that most were sourced between the surface and a depthof a few tens of metres. Neither direct observation nor thedeformation modelling suggests that the actual hot magma was thisclose to the surface, so it seems that the actual explosions weremainlydriven by steam coming up from narrow fissures. These fissures mayhave been intermittently blocked by water and ash, allowing steampressure to build up to produce explosions.

Another important feature of the latest eruption is its location.AlthoughMt Usu has a cone-shape edifice with a caldera in its summit

Fig. 2. Total magnetic field changes from 2003 through 2007. Upper graph showsstations in the south–east area, lower graph stations in the north–west area. Note thatvertical scales are different in the two panels. Most sites in the upper plot started inNovember, 2003. At some sites measurements were initiated at a later date. Note thatthe traces with dashed lines do not have records from 2003 and are shifted for clarity onthe basis of linearity to cross the zero point.

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area, the eruptive activities in historical times have not always beenconcentrated at the central vent. The recent eruptions are dominatedby dacitic magmatism along the NW–SE trending lava domes andcrypto-domes in the northern half of the volcano, which can becontrasted with the effusive production of basalt to basaltic–andesitelava (Somma Lava) in the initial stages that formed the edifice of thevolcano at about 10–20 ka. The latest eruption, which took place at theNW end of this linear alignment, is also just one of the sequence ofhistorical dacitic activity. It is noteworthy that this area is also aboundary between Mt Usu and older geological formations. It isknown from the geologic mapping (Soya et al., 2007) and interpreta-tion of drilled logs nearby (Oshima and Matsushima, 1999) that aPleistocene andesite formation lies to the western side of theboundary. The reverse magnetisation of this Pleistocene andesitebecame apparent in the recent analysis of an aeromagnetic survey(Okuma et al., 2002), and was then confirmed from actual samples(S. Okuma, 2007, pers. comm.). The Somma Lava, which covers theeastern side of the boundary, has a strong normal magnetisation.Interestingly, the 2000 eruption of Mt Usu took place almost on thevery boundary of the two magnetically opposite domains.

2. Magnetic changes associated with the eruption

2.1. Measurements and modelling

Several recording total field magnetometers were running at thetime of the 2000 eruption. From their data Hashimoto et al. (2007)showed that the main effect observed was an increase in total field inan area near the vents that was being pushed upwards, andcommented that this was most easily explained if the underlyingPleistocene rocks were reversely magnetised.

These measurements ceased by August 2000 due to difficulties inaccessing the eruptive zone, but a further set of measurements werecommenced in November 2003, with repeated total field measure-ments at sites in the active area north–west of Nishiyama, includingaround the NB crater (concentric circle in Fig. 1), the only one of thecraters formed in 2000 that is still steaming in 2007. These werecarried out at sites marked by permanent wooden and plasticbenchmarks located on the ground. On occasions, benchmarks wereso obscured by vegetation that they were not able to be used inparticular surveys. A local reference station (MTY in Fig. 1) was used toremove diurnal and other time variations of ionospheric or magneto-spheric origins.

Fig. 2 shows the changes in differential total magnetic field at mostof the survey points. During the 2003–2007 period, the changesare approximately linear with time. First of all, it was surprising to theauthors that the magnetic field was still changing steadily seven yearsafter the eruption since most of the surface manifestation of theeruption had long since finished. Foreshadowing the two-sourcemodel that we will develop below, we display the magnetic tracesin two separate panels in Fig. 2. It should be emphasized that the rateand cumulative amount of these magnetic changes are large (up to40 nT/yr, 100 nT, respectively). These magnetic changes suggest, atfirst glance, that two separate locations under the ground are be-coming more magnetised with time in the direction parallel to thepresent geomagnetic field.

Because the points were not all measured over the same periods,we modelled the average rate of change in the total field at each point.In this model two magnetic dipoles were introduced to fit the spatialpattern of the time change in the total magnetic anomaly. Orientationsof the dipoles were assumed to be parallel to the present geomagneticfield in this region (declination=−9°, inclination=56°). The averagefield was assumed to be 50,000 nT. The least squares fit betweenmodelled and observed total field changes was applied to search thecombination of the source locations and magnetic moments. Sincewe deal with the temporal variation we did not consider the surface

topography but just took the altitude difference between sites intoaccount in the calculation. Equal weights were assumed for all sites inevaluating the fitness. Prior to the two-source model we applied asingle dipolemodel. As was expected, a single dipole compromises theseparate anomalies with different wavelengths, resulting in a largemisfit (3.7 nT/100 days in average). As the second simplest candidate,the two-source model improves the fitness to reduce the averagedmisfit to 0.9 nT/100 days.

Fig. 3 shows the fit between the observed rate of change, and thetwo-source model, together with the field pattern produced by thismodel. There is a broad but low-amplitude positive anomaly in thesouth–east area, near the NB crater (hereafter, the source A). A highamplitude but smaller positive anomaly, with its negative imageanomaly to the north, is in an area to the north–west (the source B).From a viewpoint of the model fitting, the latter source B is well-defined by the intensive anomalies with both polarities. As long as wetrust the measurements under a condition that the source should notbe above the ground surface, source B cannot be shifted more than50 m neither horizontally nor vertically. although there still remainsan uncertainty arising from the small number of measurement points,which is due to the difficulties in establishing survey points in steepterrain. On the other hand, the depth and intensity of source A aresomewhat doubtful since the northern half of this anomaly is masked

Fig. 3. Comparison of observed (black) with the two-source model (grey) total magnetic field changes (in nT per 100 days) at the stations shown in Fig. 1 in theWest-Nishiyama area.Coloured contours in the right panel displays the total field anomaly change per 100 days from the two-source model at the level of the crater NB. Red dots and a cross indicatemagnetic stations and the position of the crater NB, respectively. Positions of the estimated sources A and B are indicated with solid squares.

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by the stronger anomaly B in the north. Because of a trade-off bet-ween the source depth and intensity, these parameters are not wellconstrained. For instance, allowance of an additional 0.05 nT/100 days

Fig. 4. (I). Ground temperature (1m depth). (II). DC Resistivity in December 2000. (III). DC Resiare after Saba et al., 2007).

misfit yields a depth range of 400–700 m and moment range of5.6×106 to 2.9×107 Am2/yr. Meanwhile, the horizontal location ofsource A moves less than 50 m regardless of its depth.

stivity inMarch 2002. (IV) DC Resistivity in November 2006, all along profile R–R′ (I to III

Fig. 5. A schematic illustration explaining the magnetic field changes, their sources andthe relevant mechanisms.

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2.2. Interpretation of magnetic anomalies

In ourmodel the broadmagnetic anomaly A is centred at a depth of320 m below sea level (about 500 m below the current level of theuplifted ground), which is consistent with coolingmagma or overlyinghot rocks on it becoming magnetised in the Earth's field. Thecorresponding magnetic moment change is 1.3×107 Am2/yr. Theshallow anomaly B is modelled by a source approximately 100 mabove sea level, about 100 m below the ground surface, with amoment change of 8.5×105 Am2/yr. The latter anomaly is located atsome distance to the north–west of the centre of deformation, so it isunlikely there is magma at very shallow depths here. This area becamesteaming ground soon after the major activity of the 2000 eruptionhad ceased. Although this may have been a result of heat propagationalong the NW–SE trending fissures, it does not mean a localizedmigration of magma itself. As the maximal ground temperature of thisarea has been close to the boiling point, subsurface boiling of groundwater is suggested. If magma has reached a very shallow depth, suchground water would probably have dried out within a short period,resulting in much higher temperatures at the ground surface. Assupported by the measurements of ground temperature and resistiv-ity shown in the following section, the temperature regime since theestablishment of the NW steaming ground has been fairly stable,hence one would not expect these magnetic field changes to be due toThermal Remanent Magnetisation (TRM).

The most likely reason for the north–west source B is thermalviscous remanent magnetisation (TVRM), with the elevated tempera-tures accelerating the magnetisation of the rock material to match thecurrent Earth's field. The magnetisation of common magneticminerals such as magnetite is not completely stable, and viscousremanent magnetisation is a generic term for magnetisation acquiredover a long time period. Elevated temperatures dramatically hastenthis process, and it is termed as thermal viscous remanent mag-netisation. Thismagnetisation is proportional to the logarithm of time,and increases rapidly with temperature (e.g., Dunlop, 1983). It shouldbe noted that such a TVRM process has mainly been a concern inthe research fields of rock magnetism and paleo-magnetism andhas never been actually discussed in the context of using magneticchanges to monitor volcanic activity.

Since the study of Dunlop (1983) is based on laboratory experi-ments using pure magnetite, let us make an order of magnitudeestimation to make sure whether the TVRM is a process that can alsobe observed in a natural volcanic environment. (The next several linesare in cgs units, to be consistent with the original papers.) Dunlop(1983) comments that a 20 minute room temperature exposure ofgrains of 0.037 μm magnetite to a magnetic field, produces only 1–2%of the TRM obtained in the same field. This can be seen in the graphs ofDunlop (1983) where the TVRM obtained by 0.037 μm magnetite in20 min at room temperature in a 1 Oe field would be only 0.1 emu/cc,compared to a TRM of 6 emu/cc for the same material, bothnormalised to 100% magnetite (Dunlop, 1973). However, increasingthe temperature to 300 °C would increase the TVRM by a factor of 5.Increasing the time at high temperature to 3 years would increase theTVRM by a further factor of 3. This is based on Log10 (3 yr/5 s),compared to the Log10 (20 min/5 s) of the laboratory experiments.In other words, rather than 1/60 of the TRM, in these favourableconditions, the TVRM can get up to 1/4 of the TRM. This gives a TVRMof up to 1.5 emu/cc for that sample in a 1 Oe field. Now relating this tomore normal conditions, with the Earth's field of 50,000 nT (0.5 Oe), atypical volcanic rock with 0.1–1% magnetite, and larger magnetiteparticles with lower magnetisation, we can still obtain a TVRM of theorder of 3×10−4–3×10−3 emu/cc (0.3–3 A/m), which is not muchsmaller than the NRM of typical andesitic–dacitic rocks.

Normally in recent volcanism, the TVRM gives rise to onlyminor alterations to the dominant process of TRM, and cannot bedistinguished from it. However, if reversely magnetised rocks are

heated to temperatures of the order of 200–300 °C then TVRMwill bethe dominant process, as the magnetisation will tend to change fromreversed to normal. This will occur even though the temperature ismuch lower than the Curie Point at which the NRM, whether it isreversed or not, would be completely removed. So one case in whichTVRM produces noticeable effects is when an area of reverselymagnetised rocks is heated. Another possible case in which a TVRMcan be observed is when the volcanic material is randomly aligned inrelation to the magnetic field, such as tephra deposits and thesecondarily deposited landslide or rockfall materials. The surfacematerial in the anomaly of Usu is fine volcanic tephra deposits, whichprobably cooled before landing, and so are randomly magnetised,although this may not have contributed much as the ejecta is mostlyaltered and not very magnetic.

As mentioned in the introduction we have reversed magnetisationto the west of the eruptive zone. Although at normal temperature thismagnetisation has stayed reversed for several million years, at themuch higher temperatures now present under this north–west hotzone, we are getting substantial changes in the magnetisation on atime scale of years. In the present case, the relevant time is the timesince the ground temperature reached its elevated values, which wasprobably some months after March 2000. This means that forobservations made from 2003 onwards, although the rate of magneticchange with time will decrease in a logarithmic manner, it is difficultto distinguish this from a linear trend.

Returning to the observed anomalies, a TVRM can reasonablyexplain the shallower anomaly B, in an area where the temperaturedoes not seem to have approached the effective temperature range forthe ordinary thermal de/remagnetisation. For example, the momentchange of 2.6×106 Am2 in three years for this anomaly, correspondingto 8.5×105 Am2/yr, can be explained by a spherewith a radius of about85 m at 1 A/m of TVRM acquisition. On the other hand, the deeperanomaly A appears to be the result of decreasing temperature in aformerly very hot area allowing remagnetisation. One of the reasonshere we do not consider the TVRM for the deeper anomaly is thethickness of the reversely magnetised layer. Although the Pleistoceneandesite seems to extend eastward, underlying the shallow part of theupheaval centre, it cannot reach a depth of 500 m, where the deepermagnetic anomaly is estimated to be. This is based on the geologicalcross-section of the eruptive area by Yahata (2002) from some nearbywell logs and precise investigation of the clay minerals ejectedthrough a series of explosions in the early stage of the 2000 eruption.He estimated the thickness of the Pleistocene andesite is at most a fewhundred metres. Thus, it is likely that there is an older layer withnormal magnetisation beneath it. Alternatively, if the intrusion hasreached within 500 m of the surface as suggested by some ground

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deformation studies such as Jousset et al. (2003), the magnetisingbody could be the intruded dacitic magma itself, which was initiallynon-magnetised, and again could be thermally remagnetised.

It should be noted that this thermal process, if the relevant coolingbody has substantial volume, may also take several decades to reachthe completely magnetised state. Thus, the corresponding magneticfield change should appear to be linear with time in the initial years.The magnetic moment change of 1.3×107 Am2/yr for the anomaly Acould be modelled for instance by the cooling process of a sphere-shaped dacitic magma with a radius of about 300 m at 0.1 A/m/yr ofthermal remagnetisation. A NRM of 2–4 A/m, if fully magnetised, isexpected for the dacitic magma of Usu (Nemoto et al., 1957). Then ittakes 20–40 years to reach complete magnetisation. Further estima-tion for a cooling rate of the intruded magma is possible if we makeassumptions about the magnetisation–temperature relationship.Given that the magnetisation increases linearly in a temperaturerange of 600–400 °C to thefinalNRMof 2–4A/m in the samemanner asthe laboratory experiment of Nemoto et al. (1957), the correspondingcooling rate is 60–120 MW, which roughly requires a century to cooldown the intruded magma body of 2.2×108 m3 (Jousset et al., 2003).

3. Supporting evidence: temperature information

The interpretation of the magnetic results above should be exam-ined with information on subsurface temperature. This is based onsurface temperature measurements, and measurements of electricalresistivity down to a few hundred metres deep.

Saba et al. (2007) started measuring ground temperatures at 1 mdepth in the West-Nishiyama area in October–November 2000,finding temperatures up to 100 °C in the area north–west of NBcrater. This area was to the west of the zone of greatest uplift, andalong the line of the numerous new NW–SE faults that ran from thearea of NB crater where activity by this time was concentrated. Therewas another fairly hot area south of NB crater, but by March 2002 thetemperature in this area was much lower, whereas it was still hot inthe north–west area (Fig. 4-I). In 2006–2007 the Geological Survey ofHokkaidomade further ground temperaturemeasurements across thenorth–west hot area, and found that the area with 1 m temperaturesclose to boiling was as large as previously measured, in fact it hadpossibly extended slightly (Tamura et al. in prep.).

In conjunction with these temperature measurements, Saba et al.(2007) also carried out resistivity measurements. This includedresistivity profiles along a 400 m SW–NE traverse over the north–west hot area. They used multi-electrode Dipole–Dipole and Wennersurveys, with electrode spacing from 5 to 80 m. A similar survey wascarried out in 2006 by Geological Survey of Hokkaido and HokkaidoUniversity with electrode spacing from 10 to 160 m. Fig. 4 (II, III & IV)shows a comparison of resistivity models derived from these surveys(Different parameters were used for inverting the 2006 survey,which seems to have produced a less smooth resistivity model).The characteristic feature was that from about 140 to 250 m alongthe profile, which corresponded with the high ground temperaturearea, the resistivity in the top few metres was about 3 Ωm, with theresistivity increasing at a depth of about 20 m.

In the very hot area north–west of NB crater, the increase inresistivity below a depth of about 20 m is most likely due to atemperature effect. One possible cause is the change from awater to asteam environment, with an increase in resistivity. Saba et al. (2007)explained the high resistivity below about 20 m depth near the majorfault (about 220 m along the profile) as being a superheated steamzone, whereas lower resistivity above and to the side was the result ofcondensation. It is unlikely that the pressure at depth in the upliftedregion exceeds hydrostatic, given that the surfacematerial was mainlyash deposits, and has been thoroughly faulted and deformed. Thissuggests a pressure of about 3×105 Pa at 20 m, at which pressure thewater/steam transition takes place at 134 °C. Another possible change,

which would occur in months to years at a temperature of about250 °C, is the change in clay mineralogy from montmorillonite(smectite) to illite (Pytte and Reynolds, 1989; Wersin et al., 2007),which also causes an increase of resistivity with temperature(Takakura, 1995). If these are the processes which are causing theresistivity pattern, then the average temperature gradient for the top20 m is of the order of 5–10 K/m. A new geothermal area has beeneffectively produced by the steam injection along the fissure zonesopened up by the volcanic deformation (Saba et al., 2007). Note thatthe 1 m depth temperatures are as high as 100 °C, but this highgradient in the top 1 m is the consequence of dry conditions requiringall the heat to be transported by conduction and the temperaturegradient will be much lower below this. Accordingly, it is unlikely thatthe temperature at 100 m deep is close to the Curie Point.

Both the surface temperature and resistivity results indicate thatthe shallow temperature regime was fairly stable, which supports ouridea that the source B is not getting magnetised by a thermal process.Meanwhile, we do not have reliable information on temperature forthe deeper source A. A recent magnetotelluric survey across theeruptive centre of the 2000 event may have given an indication of this,and will be discussed in a separate paper.

4. Conclusions

To summarize the magnetic results discussed above, we present aschematic cross-section illustrating the relevant sources and processes(Fig. 5). In the 2000 eruption area of Usu volcano we inferred twomagnetising sources (A and B) from repeat magnetic measurementsfrom2003 through 2007. The deeper source A is located at about 500mdeep beneath the intrusion centre, while the shallower source B isestimated at about 100mdeepunder the north–west steaming groundat some distance from the intrusion centre. While the source Aprobably represents the cooling remagnetisation (Thermal RemanentMagnetisation; TRM) of Tertiary rocks or the intruded magma itself,anomaly B corresponds to the reversely magnetised Pleistoceneandesite, gradually acquiring Thermal Viscous Remanent Magnetisa-tion (TVRM) due to the moderately elevated temperature. Thisshallower anomaly is located almost at the south–east edge of thenegative region on themagnetic anomaly map by Okuma et al. (2002).On the other hand, this anomaly map suggests that the TVRM is not adominant process in the shallow part just over the anomaly A.

This example of magnetic field anomalies following a volcaniceruption shows that the most obvious anomaly does not necessarilycome from the obvious effects of cooling below the Curie Pointallowing remagnetisation. Instead, in this case a more obviousanomaly seems to be the result of quite secondary processes, as aresult of reversely magnetised old rocks becoming heated due tofissuring and the passage of steam. This means that the separation ofTVRM from TRM will sometimes be necessary for properly interpret-ing magnetic changes around volcanoes. In this case, we are looking atchanges after an eruption, but TVRM could also be an accompanyingfactor as activity builds up, and may complicate the process ofassessing the risk of an eruption.

Acknowledgements

The authors thank Muzue Saba for providing the resistivity imagesfrom the original paper. Hidetoshi Akama and the undergraduates ofHokkaido Univ. are acknowledged for their assistance in the fieldwork.We are grateful to G. Currenti and an anonymous reviewer for con-structive criticism that greatly improved the manuscript.

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