著者
武村 雅之
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.46, no.4, pp.439-455, 1994-03-14 (Released:2010-03-11)
参考文献数
29
被引用文献数
1

Aftershock activities for two days after the 1923 Kanto earthquake is reinvestigated from the data of S-P time, maximum amplitude, and its period, which are newly measured on the records at the Gifu observatory. 34 aftershocks with magnitude M from 5.0 to 7.5 are identified on the records. According to locations of epicenters of aftershocks inferred from the S-P times, most of large events concentrated in the vicinities of western or eastern ends of the rupture area of the main shock. The first aftershock activity started in the western region immediately after the main shock at about 12:00 on the 1st of September and included two M=7 class events, whose magnitudes are newly determined from maximum amplitudes in vertical component of these records. At about 12:40, aftershocks began to occur in the eastern region. The next activity began at 13:00 in the western region again. From 17:00 on the 1st to 11:00 on the 2nd of September, the activity was quiet. The largest aftershock of M=7.5 and its related events occurred in the eastern region after this quietness. Then, the activity returned to the western region again at about 21:00. It is found that the number of M≥6 aftershocks is 16 for two days, which is meaningfully larger than that for the other M=8 class events in and around Japan.The natural period of the vertical component of the seismograph was about 1s, while those of the horizontal components were about 5s. Therefore, the ratio γ of maximum amplitude in vertical component to those in horizontal components is used as a parameter which show a frequency characteristic of seismic waves from each aftershock. All the aftershocks in the eastern region have low-frequency characteristics, which show smaller γ values, comparing with those in the western region. We also found some aftershocks of which wave forms are similar to those of recent events occurring in the same region. These results indicate that the aftershock activities included the same type events as we can find in the recent seismicity and suggest that the locations of epicenters of the aftershocks inferred in the present study are proper.
著者
武村 雅之 諸井 孝文 八代 和彦
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.50, no.4, pp.485-505, 1998-03-13 (Released:2010-03-11)
参考文献数
96
被引用文献数
1 3

Two types of earthquakes have occurred in and around Japan, which have resulted in severe damages. They are inter-plate earthquakes in subduction zones around the Japan Islands, and intra-plate earthquakes in the upper crust beneath the mainland of Japan. The faults associated with intra-plate earthquakes usually exist within a depth of 20km. Six intra-plate earthquakes causing a toll of more than 1, 000 lives have occurred since the end of the 19th century. The faults responsible for these events were estimated from the trains of surface fault breaks and the measured crustal deformations. Three intra-plate earthquakes with magnitude M larger than 7, which killed more than 200 people, have also occurred in the same period. Their causative faults have also been estimated. Many reports of damage surveys of the above nine intra-plate earthquakes were examined to investigate conditions necessary for generating strong ground motion with seismic intensity I=VII (very disastrous) in the Japan Meteorological Agency (JMA) scale, and to elucidate characteristics of strong ground motion with I=VII. According to the original definition of the JMA scale, I=VII (very disastrous) corresponds to “Collapse of more than 30% of wooden houses”. The data on damage to wooden houses for the nine events show that the area of I=VII extends up to 5km on both sides of the fault in the case of faults through mountainous region. On the other hand, in the case of earthquake faults lying underneath basins, where sediments have accumulated from the Late Pleistocene to Holocene, the area of I=VII extends over a wider area and occasionally fills the whole basin. The ground condition is seen to be an important factor in generating strong ground motion with I=VII. A similar result is obtained from an examination of the 1923 Kanto earthquake near source region with the thrusting fault at a depth shallower than 20km. The Kanto earthquake is one of the typical disastrous inter-plate earthquakes in Japan. The data on the directions of simple bodies that have overturned and collapsed wooden houses in the proximity of earthquake faults indicate that systematically larger ground motions occur in a direction normal to the strike than parallel to it irrespective of the type of faulting. The effective periods for these simple bodies and wooden houses are estimated to be in the range of 0.3sec to 1.5sec. These results indicate that the dominant strong ground motion in the direction normal to the strike plays an important role in generating the severe damages in regions hit by intensity I=VII.
著者
武村 雅之 虎谷 健司
出版者
公益社団法人 日本地震工学会
雑誌
日本地震工学会論文集 (ISSN:18846246)
巻号頁・発行日
vol.15, no.7, pp.7_2-7_21, 2015 (Released:2015-12-25)
参考文献数
31

1944年東南海地震(MJ=7.9)の被害統計資料の整理と震度分布の評価を行った。その結果従来のデータの誤りを正し集計値と整合のある市区町村データを新たに整備することができた。それらに基づいて震度分布図を作成し地域毎の揺れの特徴をわかり易く表現することができた。またそれらのデータを用いて東南海地震の人的被害の要因を検討した。合計1183名の死者数のうち、静岡県袋井市周辺や愛知県西尾市の旧矢作川流域など震度7になった地域での住家倒潰による犠牲者ならびに三重県の熊野灘沿岸のリアス式海岸地域における津波による犠牲者が多くを占めることが分かった。それに加えて愛知県半田市や名古屋市南区では揺れは震度6弱程度であったにも係らず市区町村別の死者数ランキングで1位と3位の犠牲者が出ていることが分かった。両者を合わせるとその数は279名となり、愛知県全体の435名の実に64%に当る。その原因は、耐震性の欠如を放置して飛行機組立工場へと転用された紡績工場の存在があった。このような行為は場合によって、津波にも勝るとも劣らない被害要因となることが分る。
著者
諸井 孝文 武村 雅之
出版者
公益社団法人 日本地震工学会
雑誌
日本地震工学会論文集 (ISSN:18846246)
巻号頁・発行日
vol.4, no.4, pp.21-45, 2004 (Released:2010-08-12)
参考文献数
35
被引用文献数
11 11

関東地震の死者・行方不明者数は資料によって異なり, 総数14 万余名と言われることもあるがその根拠はよくわかっていない。また一般に, 人的被害の大半は東京市や横浜市などの大規模火災によるものとの認識がある。関東地震による住家被害については被害資料の再評価に基づく統一的なデータベースが作成されており, それによれば住家の全潰は歴代の地震災害の中でも最高位の数にのぼる。従って住家倒潰も火災と同様に関東地震における人的被害の大きな要因であったと考えた方が合理的であろう。本稿では住家被害数の再評価と同様に被害資料の相互比較によって死者数を整理した。用いた資料は震災予防調査会報告にある松澤のデータや内務省社会局が刊行した大正震災志に載せられたデータなどである。これらのデータを相互に比較し, 市区町村単位の死者数を評価した。次に住家の全潰率や焼失率と死者発生率の関係を検討し, 死者数を住家全潰, 火災, 流失・埋没などの被害要因別に分離した。その結果, 関東地震による死者・行方不明者は総数105385 名と評価された。そのうち火災による死者は91781 名と巨大であるが, 住家全潰も11086 名の死者を発生させている。また流失・埋没および工場等の被害に伴う死者もそれぞれ1 千名を超える。このように, 関東地震はあらゆる要因による人的被害が, 過去に起きた最大級の地震と同等もしくはそれ以上の規模で発生した地震であることがわかった。
著者
武村 雅之
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.377-396, 1998-03-13 (Released:2010-03-11)
参考文献数
28

Strong ground motion in Tokyo metropolis during the Sep. 1, 1923 Kanto earthquake was important factor for determining the seismic design code in Japan. However, it has not been clarified in detail, because of no strong motion record near the source region of the Kanto earthquake. The author collected the descriptions and made a data set of 548 personal experiences in and around the southern Kanto district to investigate the strong ground motion from the Kanto earthquake. Many descriptions in Tokyo metropolis indicated that three severely strong shakings arrived at this area during the Kanto earthquake. The first shaking was, of course, caused by the main shock (M=7.9) at 11:58AM (JST). The second and the third shakings were caused by the aftershocks occurring 3 minutes and 4.5 minutes after the main shock, respectively. These aftershocks were identified on a seismogram obtained at Gifu observatory and magnitudes of them were determined to be 7.2 and 7.3, respectively from this seismogram [TAKEMURA (1994)]. The results of the analyses of the personal experiences also indicated that the duration time of the first shaking due to the main shock was 30 to 40sec. The strength of the second shaking was as strong as the first shaking, of which seismic intensity was estimated to be VI in JMA scale, while duration time of the second shaking was shorter than the first shaking. On the other hand, the third shaking was weaker than the first and the second shakings, of which seismic intensity was estimated to be V. The sequence of strong shakings within 5 minutes after the occurrence of the main shock of the 1923 Kanto earthquake was elucidated in Tokyo metropolis in the present study.
著者
武村 雅之
出版者
北海道大学大学院理学研究院自然史科学部門(地球物理学)
雑誌
北海道大学地球物理学研究報告 (ISSN:04393503)
巻号頁・発行日
vol.73, pp.1-22, 2010-03-19

The Great Hanshin-Awaji Earthquake Disaster in 1995 killed more than 6000 people. At that moment, the Seismological Society of Japan and the Japanese government examined themselves and fully recognized that communications beneficial for disaster prevention were not sufficiently exchanged and valuable knowledge obtained from earthquake research were not efficiently applied to the general public and disaster prevention organizations. The Seismological Society of Japan established new committees in charge of publicity and education in 1996 to provide opportunities of the communication between seismologists and general public. The Japanese government also established the Headquarters for Earthquake Research Promotion as a direct system of accountability for government policies regarding earthquakes. Since 1880, the year of the establishment of the first Seismological Society in the world, the Japanese seismological community has experienced 4 big changes such as the Great Nobi Earthquake Disaster in 1891, the Great Kanto Earthquake Disaster in 1923, the Second World War, and this earthquake disaster. These histories are reviewed from the point of view of social activities in Japanese seismology. Finally, we will discuss achievement for the sake of the mitigation of earthquake disaster on the lessons learned from the history.
著者
武村 雅之 浜田 信生
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.2, pp.141-168, 1996-08-23 (Released:2010-03-11)
参考文献数
35

96 photographs of seismograms from the 1923 Kanto earthquake (M=7.9) have been preserved at JMA (the Japan Meteorological Agency). These seismograms were obtained at meteorological observatories in and around Japan and Tohoku Imperial University in Sendai, whose epicentral distances are within about 2000km. P-wave forms of the main shock are recorded at 37 observation stations, though larger shaking parts of the ground motions are saturated on the records at almost all the stations. First, a type of seismograph and its instrumental characteristics are examined for each record from old materials and old records preserved at JMA. It is found that the seismograms at 29 stations were recorded by Portable Seismographs with natural period T of about 5sec and magnification V of 10 to 100, and most of the others by Omori's Seismographs with T of 10 to 30sec and V of 10 to 20 and by Wiechert Seismographs with T of about 5sec and V of about 100. Secondly, P-wave characteristics of each record are examined. We reexamined polarities of initial P-wave motions and reviewed focal mechanisms obtained for the Kanto earthquake. We also found that P-wave forms at many stations have some characteristics in common. The P-waves begin with small amplitude and the first remarkable phase with a large amplitude appears after the duration of the initial small phases. The second remarkable phase is also identified several seconds after the first remarkable phase, irrespective of the epicentral distances of the stations. The duration time τ01 of the initial small amplitudes correlates with the epicentral distance at each station. It can be explained by the difference of calculated arrival times between Pn and Pg phases or between Pn and P* phases in addition to a duration of the initial small rupture near the hypocenter. The duration of the initial small rupture is evaluated about 4.5sec from τ01's at observation stations within the epicentral distance of 170km. In the meantime, the difference τ12 of arrival times between the first and the second remarkable phases correlates with the azimuth angle of each station from the epicenter. Geometry and difference of occurrence times of big subevents are evaluated on the fault plane of the main shock from the data of τ12's, if we assume the two remarkable phases are due to two big subevents, respectively. The second subevent is located about 40km distant from the first subevent in the direction of about N 100°E, and the time interval of the occurrences of the two subevents is about 14sec.
著者
武村 雅之 加藤 研一 八代 和彦
出版者
日本建築学会
雑誌
日本建築学会技術報告集 (ISSN:13419463)
巻号頁・発行日
vol.2, no.3, pp.269-274, 1996
被引用文献数
1 2

Historical damages due to intermediate and deep focus earthquakes are reviewed on the basis of earthquake catalogs in past 100 vears. These earthquakes occurred at North-Eastern and South-Western parts of the Japan islands at a rate of 4 to 5 times per year. The earthquake catalogs indicate that only the intermediate depth earthquakes, with magnitude exceeding 7.5 and focal depth shallower than 150 km, caused destructive damages. However, the damage rank is lower than the typical interplate earthquakes along the Japan trench such as the 1968 Tokachi-oki earthquake (M=7.9).
著者
武村 雅之
出版者
日本活断層学会
雑誌
活断層研究 (ISSN:09181024)
巻号頁・発行日
vol.2008, no.28, pp.53-63, 2008-03-31 (Released:2012-11-13)
参考文献数
37

Severe damage from the 1995 Hyogoken-Nanbu earthquake gave us a lesson that a cooperation of research of active fault and strong ground motion is very important for the prediction of strong ground motion from inland shallow earthquake. That is an ace of the scientific world for the mitigation of earthquake damage. In this paper we take up two subjects which are important to put the strong motion prediction to the practical use. One is to estimate the magnitude of a future big earthquake from the active fault data and the other is to estimate the upper level of strong ground motions caused by blind faults. Expectations for active fault research to realize a useful strong motion prediction are described.
著者
武村 雅之 池浦 友則
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.47, no.4, pp.351-364, 1995-01-24 (Released:2010-03-11)
参考文献数
28

Source process of the 1923 Kanto earthquake is investigated from descriptions of about 200 personal experiences near the focal region. The experiences of the strong shaking without a preliminary tremor in and around Odawara city and of the second strong vertical-shaking in the Miura Peninsula suggest that two major subevents occurred during the Kanto earthquake. We infer that the first subevent occurred under Odawara city and the second under the Miura Peninsula. The time interval between the two subevents is estimated to be about ten second from the interval between two prominent phases on an old strong-motion vertical record at Gifu observatory (Δ=226km), which was observed by a seismograph with a natural period of about 1sec. The above result of the two subevents can also explain the best documented account on the earthquake ground motion by Prof. Imamura at the Imperial University of Tokyo.
著者
武村 雅之
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.58, no.1, pp.41-53, 2005-06-10 (Released:2010-03-11)
参考文献数
32
被引用文献数
1 2

Three damaging inland earthquakes have occurred in the northern Miyagi Prefecture of the Tohoku District in Japan since 1900. Magnitudes M of the 1962 and the 2003 events were assigned to be 6.5 and 6.4, respectively, from local seismic records observed by the Japan Meteorological Agency (JMA). That of the 1900 event was 7.0, which was determined mainly from the seismic intensity data, while the rank of damage was very lower than other M=7 class inland shallow earthquakes in Japan. In the present study, damage rate data, seismic intensity data, and old seismograms were re-examined to evaluate the magnitude and the location of the focal region of the 1900 event. Re-evaluated magnitude of this event is almost the same as those of the 1962 and 2003 events. The seismic gap between the 1962 and 2003 events is filled with the focal region of the 1900 event obtained. The possibility of a big shallow earthquake occurrence must be very low in near future in the seismic zone of the northern Miyagi Prefecture.
著者
神田 克久 武村 雅之
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.58, no.3, pp.177-198, 2005-12-25 (Released:2010-03-11)
参考文献数
41

The inversion analysis using seismic intensity data is carried out to investigate the source characteristics of major damage earthquakes that have occurred in the sea off Miyagi Prefecture since 1861. It can evaluate short-period seismic wave radiation zones (SPRZ) on an earthquake fault plane. Comparison of the result of the inversion analysis for the May 26, 2003 Off-Miyagi earthquake with the source region deduced from reported aftershock activity verified sufficient accuracy of the present method. We compared the location of SPRZ among analyzed earthquakes and identified the kinds of earthquakes. The SPRZ of the June 12, 1978 earthquake was located near the seacoast of Miyagi Prefecture, adjacent to the large slip area obtained from the waveform inversion by Yamanaka and Kikuchi (2004). The SPRZ of the November 3, 1936 and July 27, 1937 earthquakes were located on the south of that of the 1978 earthquake, which may indicate that the rupture area of the 1936 and 1937 earthquakes were different from that of the 1978 earthquake. On the contrary, location of the SPRZ of the February 20, 1897 earthquake was similar to that of the 1978 earthquake. The SPRZ of the August 5, 1897 earthquake was located near the Japan Trench, while that of the April 23, 1898 earthquake was just beneath the coast of northern Miyagi Prefecture, suggesting that it was an intermediate-depth intraslab earthquake the same as the May 26, 2003 event. The location of the SPRZ of the 21 October, 1861 earthquake was mostly consistent with that of the 1978 earthquake. We consider that the 1861 earthquake should have occurred on the upper boundary of the subducting Pacific plate, not in the inland of northern Miyagi Prefecture.
著者
神田 克久 武村 雅之
出版者
公益社団法人 日本地震工学会
雑誌
日本地震工学会論文集 (ISSN:18846246)
巻号頁・発行日
vol.7, no.2, pp.68-79, 2007 (Released:2010-08-12)
参考文献数
20
被引用文献数
6 2

最近、高密度の地点で多くの地震のデータが得られるようになった計測震度をデータベース化して、短周期地震動に関する距離減衰特性や地点による揺れやすさの分析を行った。関東平野では、揺れやすさの尺度である相対震度についてマグニチュード (M) 依存性がみられ、埼玉東部などの低地ではM が大きくなると相対震度が大きくなり、千葉県南部などの丘陵地では逆の傾向が見られた。得られた震度の距離減衰特性や相対震度を用いて、1923 年大正関東地震と1703 年元禄地震の震度インバージョン解析を行い、短周期地震波発生域を求めた。大正関東地震は三浦半島を挟んで2 箇所に短周期地震波発生域があり、その重心は別に求められているアスペリティ (すべりの大きな領域) の終端部にあたる。元禄地震の短周期地震波発生域は、神奈川県から房総半島南部までは大正関東地震に類似し、加えて房総半島南東沖に広がっていることが分かった。
著者
武村 雅之 小山 順二
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.36, no.3, pp.323-336, 1983-09-25 (Released:2010-03-11)
参考文献数
37
被引用文献数
1

Classification of low-frequency earthquakes has been made quantitatively by using a diagram of seismic-moment factor Me versus characteristic period Tc. Tc and Me correspond to period of corner frequency and seismic-moment density at Tc of each earthquake. About 3, 000 earthquakes from 1926 to 1978 along the Kurile, Japan, and Ryukyu trenches have been analyzed and MS's of those earthquakes cover the range from 3 to 8.3. Most of earthquakes beneath the inner trench slopes have been classified into low-frequency events which show large Tc for the same Me, while ordinary earthquakes have been commonly found in the frontal arc regions. Relations among magnitudes and seismic moment: MS-MJMA, MS-mb, and MS-Mo, for low-frequency events are also different from those for ordinary events. The relations, therefore, cannot be explained by a scaling model for ordinary earthquakes. A scaling model has been derived for low-frequency earthquakes in a statistical manner, taking into account constraints based on the relations among magnitudes and seismic-moment mentioned above. Corner frequencies of source spectra of the low-frequency model are always one half of those of the ordinary model with the same seismic-moment. This model has been also justified by the data of so called tsunami earthquakes in other subduction regions in the world, suggesting a similarity relation among destructive tsunami earthquakes and low-frequency earthquakes with small magnitude.
著者
武村 雅之 池浦 友則 野澤 貴
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.52, no.4, pp.425-444, 2000-03-25 (Released:2010-03-11)
参考文献数
55

Magnitudes for the 1923 Kanto earthquake and its major aftershocks were determined in JMA (Japan Meteorological Agency) scale. The original definition of the JMA magnitude is a magnitude that is calculated by the Tsuboi's formula from the maximum amplitudes in horizontal components of seismograms obtained by the regional observation network of JMA. The used seismograms were recorded by standard seismographs, which were the displacement type with the natural period of about 5s and damping ratio of about 8. However, the seismometers had not been yet standardized before 1925 and various types had been used whose instrumental responses were quite different from those of the standard seismographs. The purpose of the present study was that the JMA magnitudes of the 1923 Kanto earthquake and its major 3 aftershocks were determined in consideration of the difference of the instrumental responses. Fortunately, unsaturated seismograms by the Imamura's type strong motion seismographs (displacement type) have been preserved at 7 stations of JMA. The natural period and damping ratio of each seismograph have been evaluated from the free oscillation records preserved at each station. The records for the main shock and aftershocks were digitized and corrected in the instrumental responses to calculate the seismograms with the instrumental response of the standard seismograph of JMA. After that, the maximum amplitudes were measured on the corrected records and the magnitude was determined for each earthquake following the definition of the JMA magnitude. The determined JMA magnitude was 8.1±0.2 for the main shock. All the results were consistent within the difference of 0.2 with the customary results, which were determined from the uncorrected amplitude and seismic intensity data. The standard deviations were smaller than 0.2 for all the events, which shows higher reliability of the present results, comparing with the past ones.
著者
神田 克久 武村 雅之 宇佐 美龍夫
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.57, no.2, pp.153-170, 2004-12-27 (Released:2010-03-11)
参考文献数
36
被引用文献数
1

An inversion analysis has been developed to evaluate short-period seismic wave radiation zones on an earthquake fault plane using seismic intensity data. It is a useful method for historical earthquakes, for which neither strong motion nor tsunami data have been observed by instruments. Since the accuracy of the present method is verified by using ground motion waveforms synthesized by the stochastic Green's function method, it is concluded that the effects of directivity of the fault rupture process and the radiation pattern on seismic intensity distribution can be neglected in the frequency range that is effective for seismic intensity. The present method is applied to great earthquakes that have occurred since 300 years ago in the Nankai Trough seismogenic zone in southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The reliability of their solutions is discussed based on a sensitivity analysis. It is noted that short-period seismic wave radiation zones are often adjacent to large slip areas, termed asperities, but don't always overlap them. Furthermore, they are sometimes located where fault rupture has stopped. Short-period seismic waves are not always radiated from the same zone. For example, fault zones in the Enshu-Nada Sea, the Kumano-Nada Sea, the Kii Channel and the Kochi seacoast have radiated short-period seismic waves at every event, but whether or not short-period seismic waves were radiated in the interior of Suruga Bay, off the Shiono Cape and off the Muroto Cape varied with the events. It is also concluded that crustal structures such as subducted seamounts and subducted ridges take an important role of radiation of short-period seismic waves.
著者
髙橋 利昌 浅野 彰洋 大内 泰志 川崎 真治 武村 雅之 神田 克久 宇佐美 龍夫
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.60, no.3, pp.193-217, 2008-02-15 (Released:2013-08-20)
参考文献数
40
被引用文献数
1

The major earthquakes of M=7 class, so-called Geiyo earthquake, have occurred in the subducted slab of the Philippine Sea plate under Aki-nada and Iyo-nada sea. The upper surface of the Philippine Sea plate is located in the depth from 40-45km in this region. Five Geiyo earthquakes can be found in 1649, 1686, 1857, 1905, and 2001 since 17th century. Magnitudes M of the older 4 events were larger than 7, while that of the last one is 6.7 in JMA (Japan Meteorological Agency) scale. Magnitudes and locations of focal regions of them were redetermined from seismic intensity data by the method of an attenuation curve fitting and of the seismic intensity inversion. Optimal magnitudes of old events were obtained between 6.7 to 6.9. This results show the old 4 events have almost the same magnitude as the 2001 event. One of the possible reasons why the former values were overestimated is that the empirical relation between magnitude and isoseismal area from inland shallow earthquakes was applied to determine the magnitude of the historical intraslab events such as Geiyo earthquakes.