Ground Deformation Associated with BL-type Earthquake Swarms at Sakurajima Volcano

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1 / (,**3) +1/ +20 BL,**3 - +0,**3 1,1 Ground Deformation Associated with BL-type Earthquake Swarms at Sakurajima Volcano Yuki T ATEO and Masato I GUCHI Sakurajima volcano experiences, in addition to repeated vulcanian eruptions, intermittent small eruptions similar in style to strombolian eruptions These strombolian-like eruptions are associated with swarms of BL-type earthquakes dominated by low frequency components ( +- Hz) Ground deformation associated with BL-type earthquake swarms was detected by water-tube tiltmeters and extensometers in an underground tunnel Tilt and strain records were corrected by BAYTAP-G to account for the tidal e# ect Gradual tilt change of crater-side-up (,* -,* nrad) and extension of the ground ( 2 +1* nstrain) continued for --* hours before the BL swarms The inflation was temporarily suspended for */ +3hours, and was then followed by deflation associated with BL swarms The degree of tilt and strain change was in the same order as that for vulcanian eruptions; however the duration of inflation processes is longer than that of a vulcanian explosion (by several minutes to several hours) The inflation rates (,,2 nrad/h,,+0 nstrain/h) prior to BL swarms are smaller than those prior to vulcanian eruptions (,* 3* nrad/h, +* /* nstrain/h) In the deflation process, tilt change of crater-side-down (* -** nrad) and contraction of the ground (,* +0* nstrain) continued for + 0 / hours and was accompanied by BL swarms The deflation rates ( nrad/h, +, /1 nstrain/h) accompanying BL swarms are small The deflation rates of +- / of the vulcanian eruptions exceeded the upper limit of the deflation rate that accompanied the BL swarms The depth of the source of pressure that is thought to induce the ground deformation associated with BL swarms is estimated to be - km for both inflation and deflation processes (assuming the Mogi source to be horizontally located at the center of crater) No di# erence in depth is detected for vulcanian eruptions Volcanic gases were emitted in the inflation process prior to BL swarms, however volcanic gas emission stopped in the inflation process prior to vulcanian eruptions It is inferred that prior to vulcanian eruptions, the top of the conduit is plugged by a lava dome derived from cooled and degassed magma, and the internal pressure rapidly increases In contrast, prior to BL swarms, the upper conduit is loosely choked and the internal pressure gradually increases due to the intrusion of new magma from a deeper source The di# erence in inflation rates may be caused by the degree of choking of the upper conduit BH-type earthquakes dominated by high frequency components (/2 Hz) occurred alongside inflation prior to a BL swarm The inflation rate almost reached the maximum rate prior to BL swarms that occurred without pre-bh-type earthquakes It is inferred that a high inflation rate due to choking of the conduit is the cause of BH-type earthquakes Key words : ground deformation, BL-type earthquake swarms, Sakurajima volcano /, m (Lipman et al, +32+ ) +32* / +2,** 3 + -/, ,, +3 Sakurajima Volcano Research Center of DPRI, Kyoto Univ, Sakurajima-Yokoyama, Kagoshima, Japan Corresponding author: Masato Iguchi iguchi@svodprikyoto-uacjp

2 176 (**- *++ mrad) BL,/ km +323 ( + - Hz) +323 BL,** -** m,**0 *, km --* (Iguchi, (Iguchi et al,,**2) +33 ) BL BL +** +** m +323 (Iguchi BL et al,,**2) +* (Kaneshima ( +323) BL et al, +330) BL +3// ( / 2 Hz) BH BL BH :,1 km +323 BL BH +* mm +* Pa BL,** BL /** Pa +33*,*** - km ,* * BL ( +322) - km **+ *- mrad 0 * *,/ +*,, 0km,1 km (HAR; Fig + ) (Ishihara, +33* ),/ m /* m (Kamo and Ishihara, +323) --* m -, m

3 BL 177 Fig + Map showing the location of tiltmeters and extensometers in the underground tunnel at Harutameters (HIK : open circle), a tide gauge (HAK : yama (HAR : solid circle) Also shown are Seismoreversed triangle) and the video camera equipped main observatory (SVO : diamond) Fig, Tilt and strain records associated with (a), (WT-R, WT-T) vulcanian eruptions, and (b) BL-type earthquake - (EX- + (R), EX-, (T), EX- -) swarms Seismicity is represented by amounts of,2 m seismic energy released in +* -m intervals N+++ E ++ (Ishihara, +33* ; Kamo and Ishihara, +323 ) R T (Fig, a) OriginLab Corporation +322, ,1 Origin Pro 1J BL WT-R Digitizer, (EX-R, EX-T) +** nstrain (Fig, b) BL /,* +* Fig, BL Fig, b +1 km (HIK ; Fig + ) +* / /* nrad -+ Fig, nstrain * nstrain Ishihara ( +33* ) (HAK ; Fig + )

4 178 Fig Tilt and strain records associated with BL-type earthquake swarms (after correction for tidal e# ects by using BAYTAP-G) Seismic energy released in +* -minute intervals is also shown Periods of Vulcanian eruptions are denoted by expl BAYTAP-G BAYTAP-G (EX-T) (WT-R) Fig - Representative tidal correction (a) Original -, tilt and strain records and tidal level at station , + +0 HAK Dashed lines show times of low tide (b) +* Fig Tilt and strain records after correction for tidal +, + + e# ects by using BAYTAP-G (heavy lines) and by using the tidal level at station HAK (thin lines) +3 3* nrad,, nstrain,*,+ BL, (WT-R : * nrad) (EX-R : +* nstrain, EX-T: 3 nstrain) BL BAYTAP-G +32 ; Tamura +,- et al, +33+,3 BAYTAP-G +0 WT-R +* nrad EX-R nstrain EX-T Fig , 0 1 * nstrain + - / 0 BL + nrad -nstrain BAYTAP-G 2 BL,* BAYTAP-G +, (WT-R :,* nrad) (EX-R : 02 nstrain, EX-T : 03 nstrain) BL +, + - *, +* +/ mkine s + +,+ +, +3 Fig -, -mkine s

5 ,* nrad 2 BL Fig BL +* +* BL / nrad / nrad BL,* nrad BL +* - -* / nrad - / nrad BL 179 Fig / Relationship between the amount of inflation - / nrad ground deformation and the duration of time prenrad / ceding BL-type earthquake swarms and vulcanian - / nrad eruptions (a) Tilt change of crater-side-up (b) / nrad Areal dilation BL,* 3* nradh +* /* nstrainh - BL / nrad / nrad Fig Fig 0 2, Fig / BL */ +3 BL +*,* -,* nrad 2 +1* nstrain,* -* nrad BL +3* nstrain * -** nrad,* +0* BL nstrain +* --*,* +,3* nrad - +/* nstrain +* 2 (Fig 1) BL + 0 / BL,,2 nradh, +*, BL, +0 nstrainh nradh

6 180 Fig 0 Examples of records of tilt Suspension of processes is indicated by bracket lines Dates referr to the start of upward tilt +, /1 nstrainh +,3nradh 2,,* nstrainh BL 0,,- (-1) BL ++- nradh Ishihara ( +33* ) (Mogi, +3/2) Fig 1 Relationship between amounts of deflation BL ground deformation and duration of time associated with BL-type earthquake swarms and vulcanian eruptions (a) Tilt change of crater- side-down (b) Areal dilatation BL,0 km - - +* 1 +* m - +* m (Fig 2b) BL -,+ * km +* / - +* m (Fig 2a) BL - +* m Ishihara ( +33* ) Ishihara ( +33* ), 0 km, km * - km BL Ishihara ( +33* ),0 / km * 1 +* m -+,+, km - - +* 3

7 BL 181 BL BL */ +3 ( +32-), (Ishihara, +33* ), ( +32-),* -/ ms BL Fig 2 Relationship between depth and volume change of the pressure source for (a) inflation process and (b) deflation process Solid and open circles indicate BL-type earthquake swarms and vulcanian / +* ms BL eruptions, respectively BL Ishihara ( +33* ) BL Fig 3 + BL / 1 -* BL ( + nrad/h) ( - nstrain/h), +* - km Ishihara ( +33* ), * 1 mkine s + km BL BL ++,* +,,,

182 Fig 3 Relationship between ground deformation and surface phenomena (a) Tilt and strain records on April +-, +322 Ground deformation records are divided into 1 time intervals : + : h * m1h -*

8 182 Fig 3 Relationship between ground deformation and surface phenomena (a) Tilt and strain records on April +-, +322 Ground deformation records are divided into 1 time intervals : + : h * m1h -* m;,: 1h -* m++ h,* m; -: ++ h,* m +, h,, m; : +, h,, m+, h * m; / : +, h * m+0 h ** m; 0 : +0 h ** m+1 h,* m; and 1 : +1 h,* m+2 h,* m Seismic energy for +* -minute intervals is also shown Bracketed numbers with arrows indicate the time of capture of photographs (b) of surface phenomena above the summit crater

9 BL 183-1* nradh -* nstrainh BL - (Iguchi, +33 ) A (Ishihara, +32/ ), A Fig, a + - km A, - A B +,,, A +, * B : +* nradh, : 2 Ishihara ( +32/ ) nstrainh /,, *3 mkine s +, A +0 +1,* 0 BL - A 1, BL - BL A / B, - * 0 BL */, mkine s, +3 BL Fig BL +/ / Fig +* BL + Fig +* A / B

184 Fig +* Schematic illustration of the internal change of the conduit inferred from ground deformation and surface phenomena Illustrations +1 correspond to time intervals in Fig 3 Vinin indicates

10 184 Fig +* Schematic illustration of the internal change of the conduit inferred from ground deformation and surface phenomena Illustrations +1 correspond to time intervals in Fig 3 Vinin indicates the volume of magma emitted as volcanic ash and gas ; Voutout indicates the volume of magma supplied from the magma reservoir Symbols A and B represent magma, which supplied into the conduit in di# erent stage A schematic of the ground deformation of tilt and strain changes is shown 0 / Nogami et al (,**0) BL B BL 0 B BL B B A BL (Cashman et al,,***) B A BL B +, +,- +/,, 1 (Fig ) - A BL A - - A / 0 1 Fig / BL / -** nrad

11 BL 185 Fig +* km BL BL (Iguchi, +33 ) BH BH BL Figs 3 +* / 0 BL + ) BL --* */ +3 BL,) BL BL - km Nogami et al (,**0) -) BL BL +* 2 BL,* -,* nrad 2 +1* nstrain,,2 nradh, +0nstrainh BL BH +323 BL +323 BAYTAP-G BH,0 nradh, BL + nstrainh BL (,,2 nrad/h,,+0 nstrain/h) BL BL BH BH BH + - km BL BH +,/ km,-/

12 186 Transport and Storage (Ryan, MP ed), John Wiley & Sons, --/ -/0 ( +323) () + -, B ( +322) --,03,1+ ( +32-),0 B Kamo, K and Ishihara, K ( +323) A preliminary experiment on automated judgment of the the stages of eruptive activity using tiltmeter records at Sakurajima, Japan In Volcanic Hazards (Latter, JH ed) SpringerVerlag, /2/ /32 Kaneshima, S, Kawakatsu, H, Matsubayashi, H, Sudo, Y, Tsutsui, T, Ohminato, T, Ito, H, Uhira, K, Yamasato, H, Oikawa, J, Takeo, M and Iidaka, T ( +330) Mechnism of phreatic eruptions at Aso Volcano inferred from near-field broadband seismic observations Science,,1-, 0, 0/ Lipman PW, Moore JG and Swanson D A ( +32+ ) Building of the north flank before the May +2 eruption Geodetic data US Geological Survey Professional Paper, +,/*, +- +// Mogi, K ( +3/2) Relation between the eruptions of various volcanoes and the deformation of the ground surface around them Bull Earthq Res Inst, Tokyo Univ, -0, Nogami, K, Iguchi, M, Ishihara, K, Hirabayashi, J and Miki, D (,**0) Behavior of fluorine and chlorine in volcanic ash of Sakurajima volcano, Japan in the sequence of its eruptive activity Earth Planets Space, /2, /3/ 0** Tamura, Y, Sato, T, Ooe, M and Ishiguro, M ( +33+ ) A procedure for tidal analysis with a Bayesian information criterion Geophys J Int, +*, /*1 /+0 (,**0),** /+ 3+ +*+ Cashman, KV, Sturtevant, B, Papale, P and Navon, O (,***) Magmatic fragmentation In Encyclopedia of Volcanoes (Sigurdsson, H ed), Academic Press, San Diego,,+ -* ( +323) BLBH -, B-+ +-,, Iguchi, M ( +33 ) A vertical expansion source model for the mechanisms of earthquakes originated in the magma couduit of an andecitic volcano: Sakurajima, Japan Bull Volcanol Soc Japan, -3, 301 (,***),, No / -+/ -,- ( +33* ) -- B , ( +32-),0 B- + 3,+ Iguchi, M, Yakiwara, H, Tameguri, T, Hendrasto, M and Hirabayashi, J (,**2) Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima, Suwanosejima and Semeru volcanoes J Volcanol Geotherm Res, +12, +3 ( +32 ) BAYTAP -, 1+ 2/ Ishihara, K ( +32/ ) Dynamical analysis of volcanic explosion J Geodyn, -, -,1-3 ( +322) -+ B- + /31- Ishihara, K ( +33* ) Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano : Sakurajima volcano, Japan In Magma

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