,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $...

10
ῒΐ /* (,**/) / -11-20 腈腃腂腆 ,**. 3 + 腊腌腂腉腅腀腇腎腍腁 臜臀 臒膏膲臺 腌腌 臒臣膞膛 腌腌腌 臉臇臧膛 腌腌腌 臹腽 腌腌 膮臶 腌腌 膶膩腺 腌腌 ,**/ . ++ 臨膰臵,**/ 2 -* 臨膰Gravity Change Preceding the ,**. Eruption of Asama Volcano, Central Japan Sadato UEKI , Shuhei OKUBO ῍῍ , Hiromitsu OSHIMA ῍῍῍ , Tokumitsu MAEKAWA ῍῍῍ , Wenke SUN ῍῍ , Shigeo MATSUMOTO ῍῍ and Etsurou KOYAMA ῍῍ Asama volcano, one of the most active andesitic volcanoes in central Japan, started a series of eruptions on September +, ,**. and the eruptive activity lasted about three months. We have carried out microgravity measurements at the volcano three times ; one year before, immediately after the eruption and one and a half months later. “Hybrid measurement”, combining relative measurements over an area and an absolute measure- ment at a reference station, was employed to observe temporal changes in absolute gravity value. The data obtained before and after the eruption shows that the gravity changes preceding the eruption are from 0 to 3 microgals, which are of the nearly same value as that of the accuracy of observations. The gravity changes predicted from the tensile fault models and Mogi models proposed for the ground deformations are always less than + microgal at any gravity station. The observational fact that gravity changes did not exceed +* microgals gives some constraints on the magma accumulation in the conduit. A numerical examination suggests that the volume of the magma accumulated in one year preceding the eruption may be less than ,+* 1 m - . Key words : microgravity, hybrid measurement, Asama volcano, volcanic eruption, magma accumulation +ῐ῎ῑ῏ 臈膉腾膩腞腣,**. 3 + 臨腡腦臛膖臂 cm 腢腾膩腱臲腤腘 臘膍腢臬臭臟臸腾腒臭臃腖腛 膌膹臙,**/腆腁 臸腾膆臥腣腚腢膚腪膗臍腖3 +. +2 臨腡腣膶臸腾腢臕臍臟臭臃腒自腩腬腯腛腧腑3 ,- ,3 ++ +. 臨腢臘膍臬臭腱腣腗腩腟腖腝,**. +, 3 臨腨腞腢膋膉腡臬臭臟臸腾腒膔腭腖腝 臭臃腖腛腔腢膉,**. 3 +0 臨腡腣膦膀腢膆臥腡 腟腪腠腜腝膾腖腏膊腒腾膜臞腡膳腖腾膜臞膢臡腒 +** m 膺膷腖腝膁臮 ,,.** m 臉膚腡臔腖腛腔腟腒膃自 腕腯腝腏腮 膤臢臗,**/腆腁 ,**. 臫腢臸腾腣膝臰腷腡腾膩膨膓臷腱膠腬腙腮臸腾 腟腖腝腣+32- . 膘腢臸腾腶,+ 臫臯腥腭腢臸腾腞 腎腮腔腢膉 ,*** 臫膥腨腞腾膩膆臥腣臱膄臟臅腼腠膼 臏腡腎腜腛腒,*** 臫腶膠膕臭臗膿腢臭臃腟臸腻腢臋 臒腒膈臌腕腯腮腫腐腡腠腜腛,**- 臫腡腣, . 腾膜膱臼腡腾膩腱臲腤腘至臸腾腒 . 膀臭臃腖腛臈膉腾膩腣臨腱臐致腘腮腵膩膊膯腾膩腞腎腭膭臐腡臩腜腝腑腬腪臒臸腾腡腫腭臁臒腠膧膂腱腸腓膎腔 腖腛膆臥臟腠腾膩腞腎腮腨腛腰腒膤腞膴腩腝臝膻臟 腌腌 32*2/12 臆臑膫臄膒膟膇膬臄臒膅臒膅腹膅膙膐腽臗膿腂臸腾臖膙膐膈臌 腲腴腳腃 Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 32*2/12, Japan. ++-**-, 臤膑臠臹膑膒+++ 臤膑臒膅臗膿膙膐膵 Earthquake Research Institute, University of Tokyo, Tokyo ++-**-,, Japan. 腌腌腌 */,*+*0 膁臦臊+., 膁臦臒膅臒膅腹膅膙膐腽臗膿腾膩膙膐膈臌腲腴 腳腃 Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sobetu */,*+*0, Japan. Corresponding author : Sadato Ueki e-mail : [email protected]

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Page 1: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

� �� ��� � /*� (,**/)� /� -11�-20�

���� ,**.� 3� +����� ��

� � � ����� �� � � ����� � � ����� � ���� � � ���� � � ���

�,**/� .� ++ !"# ,**/� 2� -* !�$

Gravity Change Preceding the ,**. Eruption of Asama Volcano, Central Japan

Sadato UEKI�, Shuhei OKUBO��, Hiromitsu OSHIMA���, Tokumitsu MAEKAWA���,

Wenke SUN��, Shigeo MATSUMOTO��and Etsurou KOYAMA��

Asama volcano, one of the most active andesitic volcanoes in central Japan, started a series of eruptions on

September +, ,**. and the eruptive activity lasted about three months. We have carried out microgravity

measurements at the volcano three times ; one year before, immediately after the eruption and one and a half

months later. “Hybrid measurement”, combining relative measurements over an area and an absolute measure-

ment at a reference station, was employed to observe temporal changes in absolute gravity value. The data

obtained before and after the eruption shows that the gravity changes preceding the eruption are from%0 to&3microgals, which are of the nearly same value as that of the accuracy of observations. The gravity changes

predicted from the tensile fault models and Mogi models proposed for the ground deformations are always less

than + microgal at any gravity station. The observational fact that gravity changes did not exceed +* microgalsgives some constraints on the magma accumulation in the conduit. A numerical examination suggests that the

volume of the magma accumulated in one year preceding the eruption may be less than ,'+*1m-.

Key words : microgravity, hybrid measurement, Asama volcano, volcanic eruption, magma accumulation

+� � � ()*�+,# ,**.� 3� + -# ��./01 cm2*��3456 789�: 2;<=>*?<@AB�CDE# ,**/$F >*GH,I2JKLMA# 3� +. N+2 -,�>*2OM=<@?PQRSBTU# 3�,- # ,3 # ++� +. 289�;<3,VQWAX#,**.� +,� 3 Y+2Z)-;<=>*?[\AX<@ABF]2)# ,**.� 3� +0 -,#^_2GH-WK`aXbAcd?*ef-�gA# *efhi?� +**mjkAXlm ,,.**m�J-nAB]W?oP

pSXcq �rst�u# ,**/$F,**.�2>*,#vwx-*�yz{3|R}q>*WAX,# +32-� .�2>*~ # ,+���\2>*+�qF ]2) ,***��Y+*�GH,��=��`��-�aB?# ,***�~|�<t�2<@W>��2�?��pSq��-`aBF ,**-�-,# ,�N.�-# *e��-*��3456�>*? ._<@ABF()*�, �3��6q��d�*�+�\# ����-�aXURK>*-�\�`��3�� ]ABGH=`*�+�qF YB# ¡?r+¢QX£¤=

��

¥32*�2/12 ¦§¨©�ª«�¬©�­�®®u�®¯°�t�>*�±¯°��²³´µResearch Center for Prediction of Earthquakes and

Volcanic Eruptions, Graduate School of Science,

Tohoku University, Sendai 32*�2/12, Japan.¥++-�**-, ­¶·�¶ª�@ +�+�+­¶®t�¯°¸Earthquake Research Institute, University of Tokyo,

Tokyo ++-�**-,, Japan.

���¥*/,�*+*0�l¹º�»�¼ +.,�l¹®®u�®¯°�t�*�¯°��²³´µInstitute of Seismology and Volcanology, Graduate

School of Science, Hokkaido University, Sobetu */,�*+*0, Japan.

Corresponding author : Sadato Ueki

e-mail : [email protected]

Page 2: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

����������� ���� ����������������������� �!"�Minakami (+30*)#$ ��� %�&�����'��(�+3,+' +3,,����� +3-0�(�) �� %��*+&���� ���,�%��*�-.�/���&01��� �2� +313� P. ,.2#$ �3���4��'�56%��/�78&��'���9�:;��5��������(� <=>�?@AB��<�� LaCoste & RombergCG=%�+������D���6�5��3D��$ G=%�+���%�����(� ��:� (+312)���� +310 2 E�FG�����$ H�,� +311 ++ E� +32+ +*

E� +32, 2 E� +32. +* E� +33, 3�+* E� ,**-3 E���&I�J�+ 1 KLM��� N�:�(,**.)OP#$ �3�� +310QR� ,**-S�T.� !��U�5��� +32- . E� + KVW�X� ;������YX"���%�#�$�Z5�['����� [�S�'[\�4��]%��%��/�����^&(�����5�$����(� ,**- 3 E�%�����&LM���

�['3D� ,**. 3 E +'����(_��� ��),� 3 E)`' + aE*,� +* Eb`�c��&LM��$ <&(� (dG� ,**-' ,**.�%����ef&g�� ��� +.�%��/&+D3�h�$ i�� H��jk������lml�n,����^&&�6$

,� �������,�+ ����������(� %��/���3D�4��'�56

�b-.�/op�&78h�['&��'���,**- 3 E ,/'q ,1'� ,**. 3 E 3'q ++'',**. +* E ,,'q,.'� - K���X%�����&LM��$ [�D - K���(� ���0�r)&1��G�� js#�%��/&��h��G�2(��'3�5%�45&6th��G�7(��&�1I�J� uvwx@y?%����z 8{9� ,**+# &|���$ vwx@y?%����(� js#�}��%��/�!d���~:;����I�� ��0������&<��G�=�5�����$ ��(� >T.��?���h���������5���#3D� 7(%����js#&@�8������-.����A�BC�� Fig. + � AVO-FG/#$ �3��D�����@E . km��Ch�['3D��4�����%��/�!d�~:;�F�h�$ [��65��b�G��%��/���&<��G�

(� js#�}W���0�2(%����H~���$2(%���(� -.�����I9���}�

�� 7(��T.�I�J�� JK� E -'.�����LM��$ ���(� @�8�������=��KLb��2(%�+Micro-g SolutionsC FG/ (#+*3)&����$ =���K$(� H���� E +,,*** K��$ ��M3D�N��O�� �PO�5D��� ����%��/&*�¡�� ¢£QR&M��,�S¤h�['��X�¥33D��� +.-*m�}W�2(%���#�%�M&<�$ ¢£QR¦$�H§��&¨Th�'2(%�M��0( + mgal (©+*ª2 m/s,)U0'V����$ ¥3)���7(��js# (AVO-

FG/)�2(%�M(� %�«)¬W�L�M (ª-++

mgal/m)&���+­��X®G�$ [������ h¯��%���#�}��2(%�M&<�['��� H�-.�/&^&h�['�~:�5��$7(��(� 2(%�+)b�%�# AVO-FG/ &j

s#'��� Fig. + �°±gh +1#5D��-.����²A��s# BM1***A�+ +2#LM��$�V�� [�6³� BM0*-� BM0*0� BM0*1� ASM- 'ASM. ( ,**.�X´����#��$���( La-

Coste & RombergCG=%�+&Y$Z���$ ,**-

' ,**. 3 E�(� G-+� G/2+� G02, 'G1*/ � .

Z� ,**. +* E�([��G-1/ &´!�+ /Z�

Fig. +. Map showing the distribution of gravity

stations on and around Asama volcano, central

Japan. Solid circles denote the gravity stations.

N�[µ:8{9¶S:8�·¸:�\]¸:^ _¹:º»`:<¼a378

Page 3: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

�� ����������� ��� �������������������� � !���"���# $%&�'���(�)�� *��'�%+�,-./��'�� 01 2&�3��4 56&�'�� 7�8 9:� ; $%&�<=�'�>?�@�ABC� +321 ; Valliant, +33+D� EFGH��I�,-./��'��JKL/�� +MN� O&� � i���� PQ����� Goi+� R����Gi , JKL/� Di , 0� 2&�����3��4 qi ST'� ei�����

GoiUGiVADiVBccosqiVBssinqi

V��WCjcos@,pDi�TjDVSjsin@,pDi�TjDXVei

YQ�� ��>� A+,-./��'�Z�� Bc Bs+3��456#�Z�� Tj+<=�'��<=� Cj

Sj+[�'�Z�>?�� ����( ���'�Z�������\ Table + ]&� �^��Z�+� _ABC (+330)`a�BC (,**.) bc�d��^���e��� f��'���(�g� �� �h�( "i�j�k�( @�k� dU+.,D �)��������� ���\ EFGH� !��� PQ�lm� 2&�n2����o�� !��� PQ�pq�7n2���+� &r����� bc���^�n2����st�u bc�d���

,�, ��������v2�����d��lm� (AVO-FG/) PQ����w+� ,**-� 3x ,/y,1� ,**.� 3x 3y++

��� +�z>+{*.**0mgal (U+*{/m/s,)� ,**.� 3

x 3y++� +*x ,,y,.�� .-�z>+V*.*+0mgal

>?��7P�g|����w +� ,**.� +*x � !}�"~��# ,,� �# ,-� �7���@+*x -�y,+����� PQ���$�� -.+

mmD ��������� *��^� @��%BC�

,**/D�,**-� 3x ,/�y,1�� ,**.� 3x 3�y++�

+*x ,,�y,.��!�=z �)�n2������ � lm� (AVO-FG/) PQ�v2������������ bc�� [^�^��=z>����� PQ�v2����d��� [����Table ,ayc ]&� !��� ���� v2���� &m'�� 7�8 � �^���s&�� �������� b�����]�?�� ��� ���+ 313,***.***mgal����� +**mgalf���������]�� ���+� &r����� b�����u b�d�� &m'�+�^ 2&����' ��¡�� �'�����+ *.**0mgal (0

mgal)(�>?��Table , ]������� �� bc��

&r������ PQ����w$�� ¢��O�7 d��� �>��� ����£ � � [^����^����w$� Table - ]&� �� ����w$�¤z')� Fig. ,a, b ]�� 7P� BM/.3 BM+*.+0+� ¥"$�¦�§*�}� 4������ � �+�¨©,����� ��^���+���ª� *��^��[�>��� ,� ���+�f��«-�2¬����� &��.­ +�_� ,**-� 3x��.­/g� ,**.� 3x£>�=z ��^����w��0 �f����?®�^�� +)���w$+{0 mgalyV3 mgal>� �¯^� +* mgalb�°��$>?�� ,)v2�������� �^���w$+± ]��'� 1(�>?�� �'������²�2³7���w+���^��7��´�� .­µ¶g� ,**.� 3x�· +*x�·�z���� ���+� +) +/ mgalf��� �'���¸ �²���¹º�����>���^� (AVO-FG/�

Table +. Coe$cients of various kinds of error for the employed G-type gravity meters.

»z­¼ ,**.� 3x +�.­_g����w 379

Page 4: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

Table ,a. Gravity values obtained by the measurements in the period of ,/�,1 September ,**-.

Table ,b. Gravity values obtained in *3�++ September ,**..

AVO-FG/ is the station where an absolute measurement was carried out and the reference

station for relative measurements. To obtain absolute gravity values, 313***.*** should be

added to the numerals. Unit is mgal (�+*�/m/s,) for all the numerals.

���������� ��������� ������������380

Page 5: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

Pt**-� BM1*0*� BM1+**� BM0*-� BM0*1)� ����Pt**-� BM0*1� BM1*0*� BM1+**��� ,* mgal��� ������� ,)��� +/ mgal���������������� �������� � !�"!�#"!$�%"!��&�'��(�)*+,��� +*-�./��� 0�12�34�� 5�67��� 8,9:;�� -**mm<=>�?@�� � ABC�DE��F�38,����,����$� G�H�IJ� ,**/K� L�.��MN� +/ mgal

<=>��� ����OP�� Q�?@�3�'R@����� � S�TU�V�W��XY� ���Z[����

-� � ��R��� /\�]+����.^_4`� �V/abac97d<�e+,��f g��� ,**-

� 3-h,**.� 3-���i�j�,k�l��DE���.mn�oE�.��2,L������p� BM1+**�MN�q��F�3��.r�s��tuv����<w�l��� Fig. , (a)�]��,���V/x +�y�MN����izy��������������� � �����{|}u '

~��<+,��34���>�� +$+� �i������ +* mgal�Rv� ��� � � 0 mgaldn��.����.����tl��� �� '~������+���� ���� V���������� ���<�w�+,� ���izy��$�����<��������l����+�� ����&8,��R��� �V/x������'��p'�W$��.��,��aba����MN�abac9�j�,� ��������i�XY<k�l��

-�+ ������� �������V�0�+�'���$��yV�R���aba���j�,�� ��+,� GPS��p������ U�3 �>���'������,� ��j$ �¡�¢£��,�� GTable .¤� Fig. -K� ¥ IJ (,**/)�� ,**.� 1-h,**/� --GPS���������,�V����� 0 km¦@�R -

km�(<�§�+,S¨©ª «¬­ G®9 0.2¯+*0

m-K �°!+��+,��� ±"� ²#'$³ (,**.)

�� ²#'$³GPS������,� ,**.� .-�´$�'�µ¶�·¸ � 1-R¹����º� ���&8��+� V����� 2 km¦@�R -./

km�(�%��"!+�»��¼&¶ G®9 ,..¯

Table ,c. Gravity values obtained in ,,�,. October ,**..

�yV� ,**.� 3- +'�V/x���i 381

Page 6: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

Table -. Summary of the gravity measurements in September, ,**- (a), September, ,**.(b) and October, ,**. (c), and the di#erences between the observations in Sep-tember, ,**- and September, ,**. (d), and in September and October, ,**. (e).Unit is mgal for all the numerals.

Fig. ,. Distribution of gravity changes (a) obtained from the observations on ,/�,1 September ,**- and *3�++September ,**., and (b) those on *3�++ September and ,,�,. October ,**.. Unit is microgal (�+*�2m/s,).

���������� ��������� ������������382

Page 7: ,**. 3 / -11 -20,**. 3 + ˘ˇ ˆ˙˝ ˛,**/ ˚. ˜++ !"#,**/ ˚2 ˜-* ! $ GravityChangePrecedingthe,**. EruptionofAsamaVolcano,CentralJapan SadatoUEKI,ShuheiOKUBO

+*0 m-� �� ����� / km��� ,./ km ����� ��� *..�+*0 m-� ������� ,������� !"#$ %&'� ()*+,-. (,**.)/� 012"GPS34�567834�9:�;<=>#?'�@A���B���� ,** m� ,.*�+*/ m-��C,,,** m� 3�+*. m-�DEFG��HI �JK�����LM#$NO�PQ���'R�%S#TUTV&WX%S>YZ�[�\]'^3%S#YZ�[��_`'a�bcd=>$ e�fg���'�hG�YZ�['a�b/ Okubo (+33,)�i'j1� DEkl� �JK�������'j>YZ�['a�b/mn (+311)�i'jobpq"#$ #r"� st�#?'� uvwx'a�b lym�zw"� {|/}7~�h���� �TUT� hD"� ,,.** kg�m-�"#$ �����#i/� ��Sh� ���~���'�����1^3����'�>��'�"b�&S#h���>$ ���� �����%�3w���V&p3=>��'job� 3w��������L�"#$ #r"� C ,,,** m�DEkl����'�1� ���3w������V&���������%�"#$O �������V&pq%S#¡YZ�'¢£>YZ�[4� Table .�¤ (dGcalc)'¥=$ ��S��

Table .. Various source models proposed for geodetic observations (top) and gravity

values calculated from the models (bottom: dGcala). For the source models,

locations measured from the crater, depths below sea level of the centers and

volume changes (dV) are presented. Gravity values are in microgal.

Fig. -. Source models for ground deformations and

the assumed conduit (shaded area) projected

onto a E-W vertical cross section running

through the summit. Gravity changes caused

by Dike + (Aoki et al., ,**/), Dike , and Sill

(GSI, ,**.), Mogi sources + and , (MRI et al.,

,**.) are evaluated. The distribution of mag-

ma accumulated in the conduit without sig-

nificant gravity changes are also examined.

Mogi sources - and . are after Miyazaki

(+33*).

¦§�� ,**.¨ 3© +ª«�¬­�YZ�[ 383

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��%�0����� 1217 8�� *( 9�� � -£¤¥¦§¦ O�� i¨i"?©ªA«,1}�� ¬­ £¤¥¦§¦ 5�� S��:� 8®� 5(%�@�$%,A.�# ��� 1217 8�%� ?©ªA«������ ��(AB8�?©ªA«�����# Fig. .�� 1217 *+ � (Zb)�����( � (Zt)(=+�¯ª � ��(AB8�8�%� ?©ªA« :�(L8# $$��°�������, +* mgal���C±� / mgal�� ?©ªA«(D8# Fig. .a�*((¢���fg(�/ km%��¯ª� Fig. .b�1217 �(,fE ,..

km Ce*0��F����¯ª���# �F� ¯

Fig. .. Pairs of radius and height of magma columns which cause gravity changes less than *.*+*mgal (dot) or*.**/mgal (open circle) at all the gravity stations. The magna column is assumed to represent the magma

accumulated in the conduit within the one-year period preceding the eruption. Zb and Zt are the elevations

of the bottom and top of a magma column, respectively. (a) Zb is assumed to be �/.* km, (b) Zt to be ,..km and (c) Zt to be *.0 km.

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����������� ������� -*m����� ��� -*m����� �������� .

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m-��#���uA�$3�E1���$���#js� �%�N�& �����E������ ,�+*0 m-�@A�]��% *'(M&� ,**/,� � +*)JB���#A�$3�E��#�N���#A��E��>s���vw�q�E���� jdN��\%-�*�nop�+,��� -#�� Fig. .b�� +.2 km, �� +/*m����)���� N�./. +.0 km�\��0�@�&�0-*��� +0* mgal!��"������ 1������ -* m)� 1 mgal��2���3��4��� DE���� N�����56�����#E���� 0�@�0-*$�js$N��\%�1>�-�nop�!+,���

.� � � �2�2��� ,**.� 3= +�V7N��N�5:D�� 7� 3= 3�(++��� 1N�&��c%�!���-��b DE� ���� +�t ,**-� 3=,/�(,1�-����8¡�1��j��� �Nt� +�7�¢�E!��"e756�£�E� ¤¥P¦§¨©-�ª �b D� Z5-��«9DE¬!�h./�:�DE��� ¬-�*�c%�-�./ 0

mgal;����45!��67�"�2]3��1����E��Nt� +�7�!��"�� .�<�=�<�>�� =.�<��&�56�D�%js��@#� D3D� �?��%­�� +* mgalJK�� -�./� �`�2]$%���1�3]� ��$&�56��®�D�E%�¯°&@�?3]£�]��%� �N�±²�N�&�V�³���´µM#A�¶�·n¸�x%�� 0-��E!��"�¶¹�ºADE� %­�&@�?·n¸�x%��� ·n¸3]����¬!�*�c»!��"��� 0-�jd +¼l�%

Fig. /. Frequency distribution of the volumes of

magma columns which cause gravity changes

less than *.**/ (solid line) or *.*+*mgal (thin

line) at any station.

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� � � �

KLMde-./0eN�fOegP QeRGSX(,**/) ,**.�,**/�� ��� ��TOUh������ /* ]ij�_�

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33x���� jklyzm� rbc�{nUo| (,**.)� ��� K�&'&opq�i 33x���� jklyzm� rbc�{nUo| (,**/)}~�p���e�������I�� ���q�/���� i +**x���� jklyzm� rbc��Pr�esLtue-./0e,�v�dehG��

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(,**/)� � ,**.������������I� ���� ��� /*� -*-�-+-.�PXOe�j�Xe�v��e����Oe+ ��e� ��e� P�e� +� (+321)z�{����� (I)�� ¡¢��£ ]G�_ �I�������¤��� �UDy¥� --� +2/�,*..

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