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TRANSCRIPT
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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
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¥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]
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Fig. +. Map showing the distribution of gravity
stations on and around Asama volcano, central
Japan. Solid circles denote the gravity stations.
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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++
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x 3y++� +*x ,,y,.�� .-�z>+V*.*+0mgal
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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
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
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
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~��<+,��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 -
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km�(�%��"!+�»��¼&¶ G®9 ,..¯
Table ,c. Gravity values obtained in ,,�,. October ,**..
�yV� ,**.� 3- +'�V/x���i 381
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
+*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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-�, �������������(%�"0121 3�456��78������� 9:�;�<0�� &'<� +=>?! *�@�)�$%,A.�# ��� +320�B����CD ,***�E���C��� �"����(%�"AF�C���121,G� ���� 121 H��<.�C���I�,JK����$%,� ���L ���� (Furuya et al., ,**- ; Watanabe et al.,
+332)# M$�� :��� NOP ,**.��C�QR������(%�"A"�121 56,C����.�%S��� 121 TUD��V���� WX�? <0"Y!,Z"�������[#8�#NOCP C������ \$4� (,**/)�� P 4� (,**/)�<.�]%�^ �� ,**.� 3_`,**/
� +_ ab&'�cd)� Ce�fg(� +,***m
)hi�i.8j��kl���� M *+�m� nop��",.���%8�q+(,r��# ��� st (+33*)�� +3-*�-� +32+�i��uv ��gw� x.(y/��� Ce0� fgw)h%� M m�! , km fg(� / km)h ,z{�|�b 1}(~��# $ 2� |�b�� 121��3�<.�W�����%q+���#
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mO���/.* km`,., km, �7 � ( ,** mO��*.*`1., km :���+� ����� ;�, ,,.,*m(�+"�%8��� 1217 <��<���� �����(���# �7�<���� �= � �� Singh (+311) ¡(>��# 121 %6�$$��,,.** kg�m-%S��#� WX�� ¢������, +*���� / mgal
��%�0����� 1217 8�� *( 9�� � -£¤¥¦§¦ O�� i¨i"?©ªA«,1}�� ¬ £¤¥¦§¦ 5�� S��:� 8®� 5(%�@�$%,A.�# ��� 1217 8�%� ?©ªA«������ ��(AB8�?©ªA«�����# Fig. .�� 1217 *+ � (Zb)�����( � (Zt)(=+�¯ª � ��(AB8�8�%� ?©ªA« :�(L8# $$��°�������, +* 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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km������ � !��"�� / mgal��#$%�&� ' *.0 km(,.. km�������)� *Fig..b�Height++.2 km��, ������ +/*m�����-�./��#!��"�0-��$%1��23�
Okubo et al. (,**/)�� 45!��67�"� �895:�;<%�� ,**.� 3= +*�1>����?�'� 0** m�@A���%� ����BC(Zt)�' 0** m�DE�������FG�2H� Fig. .c�I�� 1)���� �������-* mJK����� L�M& (,**/)N�MOPQR��STU (+33*)V%W�XY��Z��'[�K +.2 km�\3]^_E�� ,.. km������!��"�� / mgal��#1��$`� 0-ab�����$c� Okubo et al. (,**/)�� ����?@A�d�c%�� �� +/* m, ef� --�N����,,01* kg�m-�����EDE�g�D�%� 1���2�2�hi������� 3*m�����Z��� Fig. .c�j����� 3*m)���0-ab����������� .** mJK *' ,**�0**
mkE� .**�0**m,�$�D3D�N���ef��V�� �`Y��l�%1��m#]��� 2�2�nopG3]������S���q��1��r����JBjs�� 8�!�-�nop3]�� �Nt�uADE���vwS� ��� ?��x%�y��
#1��r���� D3D� zA�x%���d!��{]�� Fig. /��� � !��"�� +*$%D / mgalJK�$|}~opoFG�2H�x%�� ����zA�8"�� D��� ������� !��"� +* mgalJK� / mgalJK)����I�� / mgalJK)��� ����zA +*�+*0 m-JK��d� +* mgal)��� ,+�+*0 m-JK���JBhi��3]� 0-nop3]��Nt +�7�N���#A��E���vwS?��x%���oQ$��!]�$%�� 17 +* mgal��#!��"�0-��$3�E0-ab�� N��� ,�+*1
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$N��\%�1>�-�nop�!+,���
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mgal;����45!��67�"�2]3��1����E��Nt� +�7�!��"�� .�<�=�<�>�� =.�<��&�56�D�%js��@#� D3D� �?��%�� +* mgalJK�� -�./� �`�2]$%���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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