˘ˇˆ˙˝merfc.chu.jp/briefcase/paper/p267.pdfrepresentative sections of the seismogenic zone, (a),...

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The Shape and Seismic Characteristics of the Philippine Sea

Plate around the Iyo-nada, Shikoku, Japan, Judging from the

Hypocenter Distribution

Shinichi M6IHJH6@> and Yuki O=CD

Shikoku Electric Power Co., Inc., Marunouchi 2�5, Takamatsu 760�8573, Japan

Michiharu I@:96

Shikoku Research Institute, Inc., Yashimanishi-machi 2109�8, Takamatsu 761�0192, Japan

Yoshimitsu FJ@JH=>B6

Ohsaki Research Institute, Inc., Uchisaiwai-cho 2�2�2, Chiyoda-ku, Tokyo 100�0011, Japan

(Received November 29, 2002; Accepted November 5, 2003)

Using hypocentral data of JMA from 1997 to 2000, we determined upper surface shape of thePhilippine Sea plate around Iyo-nada.

The contour lines indicate that the Philippine Sea plate is spreading northward from Iyo-nada toAki-nada in west Setonai-kai area. From 3-D bird’s eye view, slight dent of the plate-upper surface,possibly due to buckling, can be seen at bound of the plate subducting around Iyo-nada area.

It is considered that in addition to the plate bending, change of local tectonics from buckling tospreading may contribute to seismicity of the Iyo-nada area events. As well as this change maycontribute to the source mechanism of small events and cyclic Geiyo earthquake.

Key words : Philippine Sea plate, hypocenter distribution, plate-upper surface, 2001 Geiyo earth-quake

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Fig. 1. Hypocentral distribution map of JMA data from October 1997 to December 2000. (a) all eventsof depth from 0 km to 200 km, (b) slab events of depth from 20 km to 200 km. (c) and (d) are bird’seye view of slab events from N30�W and N85�W.

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Fig. 2. Relation between magnitude andfrequency.

Fig. 3. Number of events per month in thisarea. Magnitude is larger than or equal2.0. (a) for all events, (b) for slab events.

Fig. 4. Divided into 30 and 28 sections for EWand NS. Upper surface of seismogenic zonewas estimated at individual sections.

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Fig. 5. Representative sections of the seismogenic zone, (a), (b), (c) and (d) for EW15, EW16, EW20 andNS15, respectively in Fig. 4. Circles and stars denote hypocenter and the upper surface.

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Fig. 6. 3-D view of the upper surface. (a) and (b) are viewing from S15�W, and (b) is lower view than(a). (c) and (d) are viewing from N75�W and N30�W.

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Fig. 7. Upper surface of the seismogenic zone is indicated by blue contour lines. Red contour linesindicate distribution of number of slab events per grid in one year [Mj�2.0]. Green broken lineindicates truncated area due to quiet seismicity. Violet broken line indicates where theseismogenic zone is deformed greatly in the perpendicular direction. Black line is the seismicsurvey line and solid circles are observation points after Takahashi (2002). Black broken lineindicates the fault of slow thrust slip in 1997 after Hirose et al. (1999). 3 stars indicate majorevents for the period from October 1997 to December 2001. From north to south, 2001/03/24 theGeiyo earthquake Mj 6.7, 1998/05/23 Mj 5.3 and 2001/04/25 Mj 5.6, respectively.

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Fig. 12. Hypocenters for aftershock of the 2001Geiyo earthquake and the seismogeniczone upper surface viewing from N75»W.Circles indicate the hypocenters of theaftershocks occurred in one day after themainshock. Asterisks indicate the uppersurface of the seismogenic zone.

Fig. 13. Number of events per month in thefault area of slow thrust slip in 1997. Lowerlimit of magnitude is not considered.

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