著者

八木 勇治 菊地 正幸

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.112, no.6, pp.828-836, 2003-12-25 (Released:2009-11-12)

参考文献数

46

被引用文献数

1 5

---

Resent studies on relationship between sites for co-seismic slip and a-seismic slip are reviewed. Through analysis of seismic wave and Global Positioning System (GPS) data set, the sites for co-seismic slip and a-seismic slip have been obtained in Hyuga-nada and Sanriku region. Those studies show three important results : (1) the sites for co-seismic slip, episodic a-seismic slip, and continuous a-seismic slip do not overlap but complementarily share the plate boundary : (2) after-slip (post-seismic slip) may play an important role especially in the triggering large earthquakes : (3) the depth range of a-seismic slip may be controlled by not only a thermal effect but also some other factors such as lateral heterogeneity of fluidpressure and serpentinization.

著者

八木 勇治 菊地 正幸 吉田 真吾 山中 佳子

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.51, no.1, pp.139-148, 1998-07-03 (Released:2010-03-11)

参考文献数

26

被引用文献数

16 23

---

We investigate the rupture process of Hyuga-nada earthquake of April 1, 1968 (MJMA 7.5). Applying a multiple-time window inversion scheme to teleseismic body wave data, we obtained a detailed spatio-temporal distribution of moment release. The main source parameters are: the seismic moment=2.5×1020[Nm]; the rupture area=64×48[km2]; the stress drop=3.4[MPa]; the focal depth=15[km]. The rupture consists of three major asperities: the first asperity centring about 10km south and 20km west from the hypocenter and having a maximum slip of 4.0m, the second one centring about 8km north and 5km east from the hypocenter and having a maximum slip of 3.0m, and the third one centring about 50km west from the hypocenter and having a maximum slip of 3.2m. We compared the rupture area with that of a few large events (M>6.5) subsequent to the 1968 event. Then we found that the above three asperities of 1968 event coincide with the low seismicity area in the Hyuga-nada region, and do not overlap with the source area of the subsequent large events (M>6.5). This rupture pattern and the seismicity suggest that an area of slab bending as well as fracture of the slab can behave as barriers during earthquake rupture. These barriers may control the maximum size of earthquake source in this region.

著者

永井 理子 菊地 正幸 山中 佳子

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.54, no.2, pp.267-280, 2001-09-20 (Released:2010-03-11)

参考文献数

25

被引用文献数

29 54

---

In an attempt to examine the characteristic behavior of fault asperities (large slip areas), we comparatively studied two large earthquakes: the Tokachi-oki earthquake (M 7.9) of May 16, 1968 and the Sanriku-oki earthquake (M 7.5) of December 28, 1994, which have a partially common source area. Both the strong motion records at a regional network and the teleseismic body waves at global networks were analyzed to determine the detailed spatio-temporal distribution of moment release. The aftershock distribution, which may provide us with a more reliable location of asperity, was also re-examined using the same underground structure and the same algorithm for both events.The total seismic moment, Mo, and the source duration, T are obtained as: Mo=3.5×1021Nm; T=90s for the 1968 event, and Mo=4.4×1020Nm; T=60s for the 1994 event. It is also shown that the 1968 event consists of more than two asperities, one of which took a role of asperity again for the 1994 event. The distribution of relocated aftershocks, which fringe the major asperities, strongly supports this fact. A simple calculation indicates that the seismic coupling is almost perfect (100%) in this common asperity. We thus propose that there exist characteristic sites for asperities where fault slip occurs only as a seismic event, and that the individual asperities usually manifest M 7 class earthquakes but sometimes synchronize to cause M 8 class earthquakes.

著者

菊地 正幸 山中 佳子 纐纈 一起

出版者

学術雑誌目次速報データベース由来

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.2, pp.204-216, 2001

参考文献数

12

被引用文献数

1 20

---

A sudden collapse of the summit of Miyake-jima occurred on July 8, 2000, together with intermittent eruptions. This collapse generated long-period seismic waves with a dominant period of about 10s. Following this event, very long-period seismic pulses (VLP pulses) with a duration of about 50s were observed a few times a day until they ceased at the largest summit eruption on August 18. We analyzed these seismic pulses using waveform data recorded at several domestic stations for broadband seismographs and strong motion seismometers on Miyake-jima. The July 8 event is well characterized by a single-force directed initially upward and later downward during 12 sec. The single-force is interpreted as an abrupt collapse of massive rock. The total mass is estimated to be about 5 × 10<SUP>10</SUP> kg with fall of about 300 m. On the other hand, VLP pulses are modeled by moment-tensors with an isotropic component. They are located about 1 km southwest from the summit and 2 to 3 km deep. All three principal values are positive. The largest one is horizontal and the smallest one is near vertical. The total volume change due to 39 VLP pulses is 2.6× 10<SUP>8</SUP>m<SUP>3</SUP>, amounting to nearly one half of the total volume of the summit collapse. Based on theresults, we propose a buried geyser model. A large reservoir of hot water was formed just after the summit collapse on July 8. The ground water poured into the reservoir, being rapidly heated by hot rock underneath, and evaporated to form a highly pressurized steam, which pushed a lower conduit piston into the magma reservoir to generate VLP pulses. Non-isotropic expansion of the VLP pulses may be ascribed to the shape of the magma reservoir.

著者

橋本 徹夫 菊地 正幸

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.111, no.1, pp.118-125, 2002-02-25 (Released:2009-11-12)

参考文献数

17

被引用文献数

1 1

---

Subevents of the 1946 Nankai earthquake (Mjma8.0) were distinguished from seismograms recorded on smoked papers with strong motion seismograms (T0=6s, amplification=2) and seismological data of Japan Meteorological Agency. Location and origin time of each subevent were roughly estimated from a hypocenter determination with phase readings of P and S waves for the subevent. Rupture of the Nankai earthquake started at a point southward about 50km off Cape Shionomisaki (first subevent). The rupture propagated in the NNW direction with a rupture velocity of 1.9 km/s and a second subevent that grew to class M8 started after about 16 sec from the initiation of the first subevent. Then the rupture propagated westward, and third subevent of M8 occurred around Tosa Bay, located about 200 km from the second event, about 53 sec after the initiation of the second event. Seismic waves were excited near Tosa Bay during the 1946 Nankai earthquake.