著者

武尾 実

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.42, no.1, pp.59-66, 1989-03-25 (Released:2010-03-11)

参考文献数

24

被引用文献数

1

---

The 1974 Izu-Hanto-Oki earthquake is studied in detail using near-field strong motion seismograms. A waveform inversion method is applied to deduce the dislocation distribution and the characteristics of the rupture propagation during this earthquake.This earthquake involved right-lateral strike-slip motion on the almost vertical fault plane with a strike of N47°W. The rupture initiated at the central deepest part of the fault plane and propagated both sides smoothly, as a bilateral rupture propagation. The total source process time is about 11sec. A dislocation larger than 1m occurred in the region ranging from 3km to 10km in depth, and its horizontal span is about 20km around the hypocenter. Except for the southeastern end of the fault plane, dislocation smaller than 0.5m occurred in the shallower region of the fault plane. The average dislocation on the whole fault plane is 1.0m, and the total seismic moment is 7.6×1025dyne·cm. Few aftershocks took place in the area where dislocation larger than 2m occurred during the main event. Surface fractures, associated with this earthquake, appeared in the meizoseismal area. The dislocation distribution seismologically obtained in this study is consistent with the fault displacement along these surface fractures.

著者

橋本 学

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.43, no.1, pp.13-26, 1990-03-24 (Released:2010-11-17)

参考文献数

35

被引用文献数

5 22

---

Average horizontal crustal strain rates during interseismic period in the Japanese Islands, except the Hokkaido district, are deduced from precise geodetic survey, which has been conducted since the Meiji era. First, distance change rates between the first order triangulation stations are calculated using the weighted least squares method. In regions which suffered from disastrous earthquakes or volcanic eruptions, data before the events are neglected in the above calculation. Second, strain rates in arbitrary triangular regions are estimated.The estimated strain rates ate less than 6×10-7/yr, which is greater than those derived from seismic moment release rate or displacements of active faults. The Japanese Islands may be divided into several provinces on the basis of disstribution of strain rates. In the Tohoku district, a N-S to NE-SW extension is prevailing, and there is a zone of small strain rates which runs parallel to the Oga-Ojika tectonic line proposed by MOGI (1985). This zone may be a boundary between crustal blocks. In the northern Kanto district, strain rates are very small, while a NW-SE compression is predominant in the southern Kanto district. These facts indicate that compressional forces due to the subduction of the Philippine Sea plate may not be transmitted to the northern Kanto district. There is a zone of large compression which runs from Niigata to Gifu through Matsushiro. These high strain rates show a good correlation with seismic activity in the crust. Although compressional strain rates are prevailing in both the Chubu and the Kinki-Chugoku districts, principal axes abruptly change their directions from NW-SE to E-W at around lake Biwa. The westernmost Chugoku and the northern Kyushu districts are characterized by small extension, which suggests that compressional stress predominant in the Chubu and the Kinki districts may fade out. In the Shikoku district, NW-SE compressional strain rates are prevailing. However, temporal variations in baseline lengths show that the elastic rebound associated with the 1946 Nankaido earthquake did not occur in the western part of shikoku. It is considered that the coupling between the subducting Philippone Sea and the overriding Eurasia plates may vary significantly from east to west. Large N-S trending extensions appear in the central Kyushu district, which is consistent with the spreading of the Beppu-Shimabara graben.

著者

大竹 政和

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.50, no.appendix, pp.83-99, 1998-03-31 (Released:2010-11-17)

参考文献数

77

---

This article reviews observational and theoretical studies on seismic gap and seismic quiescence from the view-point of earthquake forecast. “Seismic gap of the third kind” by Y. Ishikawa is also briefly discussed. Seismic gap, which is recognized as an unruptured segment of plate boundary and large-scale geological structure in the present seismic cycle, is a strong tool for identifying the zone of high seismic potential in the near future. However, the concept of characteristic earthquake is not always valid. We need reliable information on the duration of seismic cycle, slip distribution of past earthquakes, and seismic coupling for effectively applying the seismic gap hypothesis to earthquake forecast. Seismic quiescence has been reported prior to major seismic events of a wide magnitude range, from great earthquakes to rock fractures in the laboratory, but the physical mechanism is not yet established, Among the several hypotheses so far proposed, the stress relaxation model based on the laboratory-driven friction law is of particular interest since it theoretically predicts the formation of seismic quiescence as a natural consequence of the physical process of earthquake preparation. For checking the validity of the model, we need detailed studies on the time-space development of quiescence, and focal mechanism change in the surrounding area. In spite of some limitations described above, past studies demonstrate that a combined use of the seismic gap and seismic quiescence is expected to provide a most useful tool to forecast the occurrence of a large earthquake for the time range of a year to several decades.

著者

岡野 健之助 木村 昌三

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.49, no.3, pp.361-374, 1996-11-23 (Released:2010-03-11)

参考文献数

45

被引用文献数

1

---

We propose a new idea for the crustal movements associated with past great Nankai earthquakes in Shikoku and the surrounding region, Southwest Japan. This study makes clear that the movements are not due to the oblique subduction of the Philippine sea plate, but due to superposition of strong seismic shaking to the uppermost crust in a compressional stress state in the E-W direction. The ground of this idea is as follows: at the time of the 1707, 1854 and 1946 Nankai earthquakes the Kochi plain subsided and the Muroto and Ashizuri peninsulas uplifted, whereas old documents show that the Kochi plain did not subside at the time of the 1605 Nankai earthquake and therefore no uplifting of the peninsulas is inferred because it is based on a set of subsidence of the Kochi plain and uplifting of the Muroto peninsula appearing at the time of the 1707, 1854 and 1946 Nankai earthquakes. This is explained by the reason why the 1605 (Keicho) earthquake was not accompanied with strong shaking of the ground owing to the tsunami earthquake. Next, because the uplifted peninsulas have anticline axes of the N-S direction, from unconsistency in stress direction it is difficult to attribute the uplifting to the subduction in the NW direction of the Philippine sea plate. On the other hand, it is easily explained that the uplifting was caused by the stress in the EW direction enhanced by strong seismic shaking. Tosa bay, which spreads between the Muroto and Ashizuri peninsulas, is characterized by depression. To be able to explain this depression is not by the elastic rebound theory, but by our idea. The undulation in the forearc zone composed of anticlines (peninsulas) and wide depressions (bays), which range alternately along the Japan island arc, can not also explained by the elastic rebound theory. In addition, we consider that the compressional strain variation in the NW direction of the ground surface observed at present in Shikoku does not result from the oblique subduction in the NW direction of the Philippine sea plate, but it is recovering the overdisplacement of the ground surface caused by the coseismic movement (2-3m at the ground surface) in the SE direction of the Muroto promontory by reverse faulting of the 1946 earthquake. Moreover, this study shows that unconsistency in directions of P axes between the 1946 Nankai earthquake and mantle earthquakes presently occurring in Shikoku and its vicinity is succesfully explained by taking account of constraint of the displacement in the direction along the Japan island arc.

著者

高波 鐵夫 村井 芳夫 町田 祐弥 斉藤 市輔 牧野 由美 勝俣 啓 山口 照寛 西野 実

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.57, no.3, pp.291-303, 2005-02-20 (Released:2010-03-11)

参考文献数

21

---

Nine OBSs (Ocean Bottom Seismographs) were deployed just before the 2003 Tokachi-oki earthquake (M8.0) in the source region, off Hokkaido island, Japan for the period from 7 August to 21 September 2003. The observation area was located about 50-100km landward of the southern Kuril trench where the Pacific plate is subducted toward Hokkaido island. The main shock occurred on 26 September 2003, just several days after this 50-days observation by OBS. Until this big earthquake, the seismic activity in this area has been extremely low in contrast with the adjacent trench areas. No big earthquake has occurred since the 1952 Tokachi-oki earthquake (M8.2). From the phase-reading data, 186 micro- and ultra micro-earthquakes were located in the regular seismic active zones. The b-value of 0.82 from the earthquakes with magnitude from -0.6 to 2.9 could support the fore-shocks of the 2003 Tokachi-oki earthquake. Several interesting phenomena were found in this study as follows. (1) Many clusters of events took place simultaneously in the surrounding source region after a short quiescence period of several days. (2) An enormous number of curious burst signals, which oscillated for only a few seconds, continued to appear at almost OBSs. In particular, at two OBSs DHH and DK being the shortest distance to the Kuril trench, over 3500 bursts of events with amplitudes of 10-7m/s were counted during the observation period. On the other side, OBS DD, which was close to the epicenter of main event and far away from the trench, around the beginning of September 2003, the rate of occurrence abruptly rose to over twice of the previous rate, which has kept the low rate until then. The start time of activation was almost the same time of the appearance of many widespread clusters of earthquakes. These results will be applied to a research project of the great earthquakes occurring along the trenches.

著者

木下 繁夫

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.50, no.4, pp.471-483, 1998-03-13 (Released:2010-03-11)

参考文献数

17

被引用文献数

4

---

Recently, servo-seismometers (negative feedback seismometers) are used for various fields in seismology and earthquake engineering. First, this report explains five fundamental negative feedback seismometers with a displacement or a velocity transducer. Next, composite negative feedback seismometers, such as STS, VSE and CMG seismometers, are explained. Also, new negative feedback seismometers that have two feedback paths, are discussed. Finally, seismometer noise, the representations of instrument noise and seismometer characteristics, and, some problems in borehole observation by using negative feedback seismometers are discussed.

著者

木村 昌三 岡野 健之助

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.50, no.4, pp.461-470, 1998-03-13 (Released:2010-03-11)

参考文献数

28

---

The source region of the 1946 Nankai earthquake is examined based on the recent studies of microearthquakes, focal mechanisms and the crustal structure in Shikoku, southwest Japan as well as the aftershock distribution just after the main shock. Although it is generally known that an aftershock distribution is nearly equal to the fault zone and the source area of tsunami, such an agreement has not been recognized in the case of the 1946 Nankai earthquake. In the present study, we point out the misinterpretation about the aftershock area and the focal plane in the previous works, and give a new interpretation indicating good agreement among the aftershock distribution, source area of tsunami, crustal movements, disaster area and fault region. In the previous works, the defect of the observation network in the west of Shikoku and the east of Kyushu was not taken into consideration on the estimation of the aftershock area. Furthermore, the focal plane was not properly estimated owing to misunderstanding of the crustal movements and the source area of tsunami. Since the focal plane exists in the focal layer in the uppermost mantle, we consider that the aftershocks occurred only in this layer and the crustal earthquakes were induced by stress release due to the occurrence of the main shock. Our microearthquake observations show that this layer has a thickness of about 5km from the west of Shikoku to the western part of the Kii peninsula and from the Median Tectonic Line in the north to the Nankai Trough in the south.

著者

太田 陽子 松田 時彦 長沼 和雄

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.29, no.1, pp.55-70, 1976-03-10 (Released:2010-03-11)

参考文献数

21

被引用文献数

1 1

---

Seven steps of marine terraces are well developed on the Ogi Peninsula, Sado Island. The seventh (lowest) one is a raised abrasion bench less than 2m high, emerged at the time of the destructive earthquake of 1802, hence it is named the 1802 terrace. The sixth terrace is about 2-4m high, and probably was formed at the time of Holocene transgression. Higher five terraces (Pleistocene terraces V-I) are well preserved over the most part of the peninsula and have the height of 32-40m, 70-55m, 94-118m, 123-137m and more than 165m, respectively.The height of former shorelines represented by shoreline angle of each terrace shows that all the terraces tilt northward. Generally, the higher the terrace, the larger the amount of tilting. However, the tilt of the lowest two terraces (the 1802 and Holocene terraces) is almost the same, ca. 1.5′. This indicates that the 1802 tilting caused by the earthquake was a first event after the formation of the Holocene terrace of ca, 6, 000 years old and the recurrence interval of the events was more than 6, 000 years. A uniform regional difference of 2m in height between these two terraces is probably interpreted as a superposed result of the eustatic lowering of sea level and a regional uplift during last 6, 000 years.It is possible to estimate the average intervals of earthquakes after the terrace formation by comparing the tilting rate of all Pleistocene terraces with that of the 1802's. Thus, the average recurrence intervals are estimated at about 8, 600 years since Terrace III was formed and about 5, 000 years since Terrace IV was formed. These values are consistent with the interval of more than 6, 000 years which is estimated from tilt of the 1802 and the Holocene terraces. It is concluded, therefore, that the earthquake has taken place repeatedly in a similar manner with a recurrence interval of about 5, 000-9, 000 years during at least last 105 years.Uplift and northward tilting of the Ogi Peninsula at the time of the 1802 earthquake (Magnitude 6.6) is significantly larger than those of Awa-shims at the Niigata earthquake of 1964 (M 7.4), though its magnitude was smaller.This fact and the limited areal deformation by the 1802 earthquake imply that the epicenter of this earthquake was located very close to the coast of the Ogi Peninsula, probably within a few kilometers off Shukunegi. A reverse faulting is inferred to have occurred along a northward-dipping fault plane at the 1802 earthquake.

著者

木村 昌三 岡野 健之助

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.48, no.2, pp.213-221, 1995-08-25 (Released:2010-03-11)

参考文献数

20

被引用文献数

1

---

We examine the decreasing of seismic activity which occurred in Shikoku, and the Wakayama and Tanba districts prior the 1946 Nankai earthquake, southwest Japan. Considering the effect of the Second World War, the seismic observation for that period by the Japan Meteorological Agency was not in sufficient conditions. Thus, the idea of seismicity lowering on the area has not been generally accepted, notwithstanding some studies on the subject were previously presented. This study proves however the lowering was not caused by the insufficient seismic observation. We give an explanation for the cause of lowering of seismicity. We also showed that the crustal seismic activity in Shikoku in terms of earthquake occurrence gradually decreased with time after the 1946 earthquake. The decrease in seismicity could be anticipated to continue till the next Nankai earthquake in the above mentioned districts. Accordingly, the lowering of seismic activity is expected as a precursor of the great Nankai earthquake, in particular in the Wakayama district where the usual seismic activity is comparatively high compared to other districts.

著者

和田 博夫 三雲 健 小泉 誠

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.32, no.3, pp.281-296, 1979-09-25 (Released:2010-03-11)

参考文献数

28

被引用文献数

2 3

---

Seismicity in the northern Hida region, Japan, has been routinely observed since May, 1977 at telemeter-network stations of the Kamitakara Geophysical Observatory, and about 1500 local shocks with magnitudes greater than 0.5 have been located. (1) The observation reveals high seismicity along the Atotsugawa fault, along the northern Japan-Alps, south of Mt. Norikura and Mt. Ontake, and near Hida-Osaka, with focal depths shallower than 20km. (2) Seismic activity along the Atotsugawa fault is high at the eastern and western portions, with an intermittent zone of low activity, extending over 70km. Epicenters are deviated about 2-3km north of the fault trace, and this deviation together with focal depth distribution suggests a slightly northwestwardd dipping fault plane. All these shocks are confined above 13km, suggesting either that the fault plane extends down to this depth, or that minor brittle fractures do not take place under the depth due to some flow propertities of rock materials there. (3) Nine shocks along the fault show focal mechanisms consistent with right-lateral strike-slip evidenced by geological and geomorphological surveys (MATSUDA, 1966). (4) Heavy damage along the fault region at the time of the 1858 Hida earthquake (M=6.9) appears to indicate that this large earthquake was caused by faulting motion of the Atotsugawa fault. Most of the present seismic activity along the fault might be associated with some readjustments of residual stresses around there.

著者

石橋 克彦

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.31, no.3, pp.342-345, 1978-10-31 (Released:2010-03-11)

参考文献数

12

著者

Kaoru TANIGUCHI Mitsuhisa WATANABE Yasuhiro SUZUKI Hiroshi SAWA

出版者

SEISMOLOGICAL SOCIETY OF JAPAN

雑誌

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) (ISSN:00371114)

巻号頁・発行日

vol.64, no.1, pp.11-21, 2011-08-25 (Released:2011-10-18)

参考文献数

28

被引用文献数

1 2

---

The 150 km long Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL) in central Japan is one of the most active fault systems in Japan. Paleoseismologcal studies 1980s have revealed that the most recent event and the average recurence interval of the ISTL. The approximately 7 km long portion of the fault system between Matsumoto and Okaya has been regarded as a gap without any active fault trace. The gap namely the “Shiojiri Pass Gap” has long been taken as a segment boundary owing to the geometric discontinuity. Recent geomorphological analyses of the gap have demonstrated a through-going left-lateral slip assocaited with recent earthquakes in this area, based on aerial photograph interpretation and excavation studies. Excavation study on this portion revealed that the latest faulting event occurred between 1,700 cal. B.P. to 1,310 cal. B.P. (255 A.D. -645 A.D.). The timing of the last faulting event at this study area coincides with the timing in the Gofukuji fault and Okaya fault. The active faults extending from the Matsumoto basin as far as the northwestern margin of the Suwa basin display the evidence for its recent reactivation at the same time.

著者

佐伯 龍男 梅田 康弘

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.41, no.2, pp.163-171, 1988-06-25 (Released:2010-03-11)

参考文献数

13

被引用文献数

2

---

The high accelerations exceeding the earth's gravity were observed on the small area near the main shock of the 1984 Western Nagano Prefecture Earthquake (M=6.8). Few aftershocks occurred in this small high acceleration spot of main fault. It is inferred that great destruction producing the high accelerations occurred within this small spot. For detecting the evidence of great destruction, we tried to investigate the crack orientation and density by using Shear-wave polarization method.We have identified that directions of polarization in and out of the high acceleration spot are N40-70°W. These directions are consistent with the axis of maximum compression obtained from earthquake fault mechanisms and geodetic surveys. There are no difference in polarizations between in- and out of the high acceleration spot. It suggests that open-cracks had been made before the main shock. Great destructions of the main shock seems to have little influences on S-wave polarization. The average time-lag between arrival of leading and slower shear-wave is 0.02S. It indicates that present open-cracks are distributed in a shallow depth.

著者

梅田 康弘 黒磯 章夫 伊藤 潔 飯尾 能久 佐伯 龍男

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.39, no.2, pp.217-228, 1986-06-25 (Released:2010-03-11)

参考文献数

27

---

Many boulders were thrown off out of their former sockets by the Western Nagano Prefecture Earthquake of September 14, 1984. Anomalous high accelerations of 5-30g, in the frequency range of 5-10Hz, are estimated from the displacement of these thrown out boulders.Almost all of the thrown out boulders were found on the tops, ridges and suddles of the mountains covered with the volcanic ash. The amplification effects by topography and soil sediment of the surface are estimated from the observation of the aftershocks recorded on the mountain-top, -foot and rock. The spectral ratios of seismic waves with the frequencies of 5-10Hz, namely mountain-top/-foot and soil/rock, are 2-7 and 2-10, respectively.The accelerations on the basement rock are obtained dividing the accelerations estimated on the mountain-tops. The high accelerations exceeding 1g distribute within a small area with a length of 3km and a width of 1km. This small area corresponds to the large dislocation portion of the assumed fault and the low active region of aftershocks.

著者

加藤 完 高橋 誠 吉川 清志

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.39, no.1, pp.47-55, 1986-03-25 (Released:2010-11-17)

参考文献数

16

被引用文献数

2

---

Weekly monitoring of radon concentration in soil gas in some active fault areas has been carried out using cellulose nitrate film as a part of earthquake prediction research. As the results of observation, we found increase of track density (tracks/cm2day) along the Median Tectonic Line (distance 100km) and the Matsushiro fault (_??_100km) since summer of 1982 and along the Ateta fault (_??_25km) since summer of 1983. We considered it as a precursor of the Western Nagano Prefecture Earthquake, 1984, by the reason that this increase just fit with the empirical formulas which relate the distance and apperance time of the precursory anomaly against the magnitude of an earthquake.At the same time, we found an annual variation of track density higher in summer and lower in winter. Supposing this annual variation being due to the seasonal variation of soil temperature, using uranium ore as source of random. As the result of experiment, it was clarified that track density increased with increased exposure temperature.

著者

杉崎 隆一 杉浦 孜

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.39, no.1, pp.99-109, 1986-03-25 (Released:2010-11-17)

参考文献数

31

被引用文献数

1

---

Remarkable anomalies in gas compositions were observed at a fumarole and three mineral springs before the Western Nagano Prefecture Earthquake, 1984. The epicentral distance are 9 km for the fumarole and 50 km, 71 km and 95 km for the springs, respectively. Deep seated gas emission induced by the compressive stress of the earth tide had been observed previously at the two mineral springs as the monitoring stations and at a lava lake in Hawaii. It is analogically inferred that the gas anomaly before the earthquake can be attributed to the variation in the emission rate of deep seated gas resulting from changes in pore pressure and magma-reservior volume, which are subjected to the tectonic stress that caused the earthquake. Another aspect is emissions of H2 gas at three mineral springs and gases from different sources at one spring. These indicate the formation of new cracks near monitoring stations and the permeation of groundwater into the cracks. The mechanical processes inferred from the present geochemical observation are compatible with those described by geophysical models based on fracturing experiments.

著者

今村 明恒

出版者

公益社団法人 日本地震学会

雑誌

地震 第1輯 (ISSN:00371114)

巻号頁・発行日

vol.7, no.4, pp.185-193, 1935-04-15 (Released:2010-03-09)

著者

今村 明恒

出版者

公益社団法人 日本地震学会

雑誌

地震 第1輯 (ISSN:00371114)

巻号頁・発行日

vol.10, no.6, pp.236-249, 1938-06-15 (Released:2010-03-09)

著者

今村 明恒

出版者

公益社団法人 日本地震学会

雑誌

地震 第1輯 (ISSN:00371114)

巻号頁・発行日

vol.2, no.2, pp.81-88, 1930-02-15 (Released:2010-11-17)

著者

谷口 薫 渡辺 満久 鈴木 康弘 澤 祥

出版者

SEISMOLOGICAL SOCIETY OF JAPAN

雑誌

地震. 2輯 (ISSN:00371114)

巻号頁・発行日

vol.64, no.1, pp.11-21, 2011-08-25

被引用文献数

2

---

The 150 km long Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL) in central Japan is one of the most active fault systems in Japan. Paleoseismologcal studies 1980s have revealed that the most recent event and the average recurence interval of the ISTL. The approximately 7 km long portion of the fault system between Matsumoto and Okaya has been regarded as a gap without any active fault trace. The gap namely the "Shiojiri Pass Gap" has long been taken as a segment boundary owing to the geometric discontinuity. Recent geomorphological analyses of the gap have demonstrated a through-going left-lateral slip assocaited with recent earthquakes in this area, based on aerial photograph interpretation and excavation studies. Excavation study on this portion revealed that the latest faulting event occurred between 1,700 cal. B.P. to 1,310 cal. B.P. (255 A.D. -645 A.D.). The timing of the last faulting event at this study area coincides with the timing in the Gofukuji fault and Okaya fault. The active faults extending from the Matsumoto basin as far as the northwestern margin of the Suwa basin display the evidence for its recent reactivation at the same time.