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9 Ground Deformation at Hachijo Island, Japan on 13–22 August 2002 Observed by GPS Measurements and Estimated Dike Intrusion Model Fumiaki Kimata*, Meilano Irwan* and Keita Fukano** An earthquake swarm was observed near Hachijo Island, Japan, between 13 August and September 2002. Continuous GPS measurements at four sites on Hachijo Island detected the displacements accompanying the earthquake swarm.  Eastward displacements of 2–6 cm were detected during the period of 13–16 August, after which the displacements tended toward the northwest.  A dike intrusion model is proposed based on the observed ground deformation.  The putative intrusion is located at 3.3 km depth under Hachijo–Fuji, and is a 2.2-m wide tensile dike of depth 3.3 km and is extended to 4.5 km deep.  The dike intrusion model estimated from the ground deformation is consistent with a location and a dimension of the model based on the low-frequency earthquake mechanism proposed by Kumagai et al. (2003).  We discuss three models of a dike intrusion, a point deflation, and a dike deflation from the eastward or southeastward displacements detected in the period of 16–22 August.  From the ground deformation it is difficult to decide the priority of the models.  According to the depth and volume changes of the estimated pressure source of three models, it is s suggested that the deeper part of the first inflated dike turns to deflation on August 16. Key words: ground deformation, Hachijo Volcano, GPS measurements, dike intrusion model 1. GPS GPS , GPS 1989み. GPS (Shimada et al., 1990).  Kilauea (Hawaii) (GEONET) Segall et al. (2001) GPS – *  〒464–8602 Research Center for Seismology, Volcanology and Disaster Mitigation, Graduate School of Environ- mental Studies, Nagoya University, Furo, Chikusa, Nagoya 464–8602, Japan. ** 〒104-0045 5–3–1 2000 27 cm ( Kaidzu et al. , 2000). GPS 2000 6 Hydrographic and Oceanographic Department, Japan Coast Guard, 5–3–1 Tsukiji, Chuo, Tokyo 104–0045, Japan. Corresponding author: Fumiaki Kimata e-mail: kimata@seis.nagoya-u.ac.jp (Meilano et al. , 2003; Ueda et al. , 2003). 26〜 14 . , 2002). , 2002 8 l ( 4.1 (8 1989 . , 1994 , 3 cm/yr (Kimata et al., 1994). – , GPS 1～2 GPS 15 ), 1 1825            1825 252 (8     15     8     ) 2 7000 ( . 4 GPS GPS 2. GPS (  15 17 . 1698 (2002) . (     ) 10 km . 8 15 . 10 Fig. 1 3 km 3 × 10 7 m³ 2 × 10 7 m³ . (1996) s 9 2002). ( , NGY1 NGY2 Zumberge, 1993) GPS GIPSY-OASISII PPP 4 GPS (Webb and . 8 12–22 GSI・JCG Fig. 2 40 km . Fig. 2 960602） GPS sites on Hachijo Island and daily counts of earthquakes around Hachijo Island by JMA during the period 12–22 August, 2002. Time is shown as UT time. Daily station solutions are processed using with GIPSY software and referred to the GIS GPS station on Aogashima Island, in 40 km of south from Hachijo Island. Earthquake number is counted by Japan Meteorological Agency (JMA, 2003). Solid circles and open squares mean the numbers of volcano-tectonic earthquakes (VT EQ) and long-period earthquakes (LP EQ), respectively. Fig.2 , 16 Fig. 3. Ground deformations and epicenter distribu- tion observed on Hachijo Island in the period 13–16 August, 2002 and deformation calculated from the dike intrusion model. The epicenters determined by JMA are shown as the open circles in 13–15 August, 2002 and the grey solid circles in 16–22 August. A dike intrusion model is estimated from the ground deformation and epicenter distribution. The locations of GPS sites tiltmeter site, and volcanoes of Hachijo-Fuji and Hachijo-Mihara are shown as the closed circles, open square and open triangles, respectively. a: Observed horizontal displacement vectors at the GPS sites. The displacements are referred toAogashima GPS site. Epicenters are shown as open circles and grey solid circles in the period 2 cm, , 16 2002). JCG cm 3. 13 1990 , 2002). of 13–15 and 16–22 August, 2002 respectively. b: The location of the dike intrusion model estimated from GPS measurements and calcu- lated horizontal displacement vectors from the model at the GPS sites. c: The vertical move- ments observed and estimated. The observed vertical movements are shown as the size scale of the  circles. The  calculated  vertical move- ments are shown as the contours of every 1 cm. Ground deformations and epicenter distribution observed on Hachijo Island in the period 16–22 August, Fig. 4. 2002 and deformations calculated from the magma models. The notes are same as in Fig. 3. Dikes and a point pressure estimated from the ground deformation and epicenter distribution in the period, which are shown as the rectangles and circle. Three models of a dike intrusion (model 1), an inflation point source (model 2), and an deflation dike (model 3) are estimated. Open and grey solid means the inflated and deflated pressure sources respectively. The location of the epicenter of very long-period tremors determined by Kumagai et al. (2003) is shown as an open star. Epicenters are shown as open circles and grey solid circles in the period of 16–22 and 13–15 August, 2002, respectively. a: Observed horizontal displacement vectors at GPS sites. b: The location of the dike intrusion (model 1) and the calculated horizontal displacement vectors from the model. c: The location of the deflated point pressure (model 2) and the calculated horizontal displacement vectors from the model. d: The location of the deflated dike (model 3) and the calculated horizontal displacement vectors from the model. e: The vertical movements observed and estimated from the model 1. f: The vertical movements observed and estimated from the model 2. g: The vertical movements observed and estimated from the model 3.
2
13〜16 - , GPS Fig. 5. first dike and model 1. f: The vertical movements observed and estimated from the first dike and model 2. g: The vertical movements observed and estimated from the first dike and model 3.
inferred sources and epicenters. The epicenters determined by JMA are shown as the solid circles in the period of 3–14 August, open circles in 15-17 August, and grey solid circles in 18–23 August, 2002. The open rectangles, solid rectangle and solid large circle mean an open crack, a closed crack and a deflation point source, respectively. Dike intrusion model from very long-period estimated tremors (Kumagai et al. , 2003) is shown as a square of a dashed line. , 90 28 μ radian , 2002). ( GSI NGY2 GSI . Fig. 6 km 3～8 km 2000 1 1999 al., 2001; Cervelli et al., 2002). 31 GPS ( 2～3 25 μ radian 3 μ radian 9 2 , 2002; NGY1 JCG JCG 6 26～27 (Meilano et al. , 2003). Hawaii Kilauca 1997 (Segall et 2000 km . 2 , 2002). JCG 1） 12 37 20 3–2   8     16〜22 , 16～22 16 2 cm 1 cm . 16 mm 4 cm . Fig. 4c, 4f     Fig. 6 1  （ km 12 1 ( (
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