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On April 13, 2013, a MJMA6.3 earthquake occurred in the Awaji Island. Epicenter located southwest of the active fault system ruptured by the 1995 Hyogo-ken Nanbu Earthquake. We installed the temporal seismic stations around the epicenter immediately after the mainshock. Using data of these temporal stations, we determined the focal mechanisms of aftershocks precisely. Although, we could find various types of mechanisms in the aftershock sequence, most of them occurred under the E-W compression stress field as like with other earthquakes in the Kinki Area. However, aftershocks in the area of southeast of the mainshock have the P axes of NE-SW direction.

2008年11月以降, 近畿地方北部に78点の臨時観測点を設置し，定常観測点を加えた150点のデータを解析した。同地域における一元化震源カタログの検知能力下限より大きい地震についてほぼ漏れなくメカニズム解を決定でき, 解析の空間・時間分解能が大幅に向上できることがわかった。丹波地域の応力逆解析結果には局地的な時間変化が見られるが，これは地震活動の偏りの影響であり，デクラスタ処理によりこれら「見かけ」の変化は除去される。逆に，M4級地震の余震などクラスター内部の詳細な解析が可能になった。琵琶湖西岸地域は逆断層型の応力場であるのに対して，丹波地域はσ2とσ3が分離できない場であり，両タイプの空間的境界は複雑な形状を呈する。丹波地域には小スケールの不均質構造が存在し，応力場推定に影響を与えていると考えられる。, In the northern Kinki district, we have done seismic observations using the dense seismic array stations since 2008. Total 150 temporal and permanent stations are used, and the average interval between the stations is about 5km. We get numerous mechanism data in a short time period, and the space and time resolution of the focal mechanism analyses are improved. Based on these data, we discuss about the feature of focal mechanisms, space and time variation of the regional stress field, and the detail mechanism distribution within aftershock sequence following the M4 class earthquakes.

2008年11月から，近畿地方中北部におけるひずみ集中帯において，稠密多点臨時地震観測を行っている。本稿では，この稠密多点観測における技術的な側面を報告する。45点の臨時観測点を5年間の長期に渡って維持し，かつデータ処理を行うためには，従来技術だけでは，人的・時間的な制約上困難があった。また，45点の観測網の維持管理や，約2TB/年の大量のデータを扱うことによって生じることが懸念される人的ミスをいかに軽減するかなどの問題もあった。この観測では，京都大学防災研究所付属地震予知研究センターと株式会社近計システムの共同で開発した次世代型地震観測システム及びデータ処理装置を使用した。次世代型地震観測システムの設置，メンテナンスから，データ処理装置を利用して，回収してきた大量のデータを短期間で処理するシステムの構築，さらに観測点情報や設置・メンテナンス・データの履歴を一元管理するデータベースの開発・運用に関することを報告する。, We installed 45 temporary seismic observation stations in the middle and northern Kinki district, where active faults are concentrated. The seismometers and data loggers used are newly developed. We perform the dense observation for about four years and analyze the data together with those of permanent stations to detect heterogeneous structures related to the strain accumulation process. The purpose of the project is to clarify the relation between the strain accumulation and the earthquake generation. In this report, we described about the observation system, the data processing system, and the database system for this project.

山陰地方の陸域と海域を含む西南日本背弧の地殻・マントル比抵抗構造モデルを構築するために， 2006年から海域・陸域での観測を連携させたMT観測を開始し，主に次の2本の測線に沿った観測を実施してきた。1) 鳥取・兵庫両県境付近沖の海域を含む測線(Profile SW100)と，2) 隠岐諸島周辺海域を含む測線(Profile SW200)である。これらの測線に沿って，海域では海底磁力電位差計(OBEM)，および海底地電位差計(OBE)を用いた観測を，また，陸域においては，広帯域MT観測，長周期MT観測を併用した観測を実施してきた。本報告では，2006年から2009年にかけて実施した，日本海での海底観測と，海底観測と連携した陸域での観測の概要と解析結果の概要を報告する。, In the San-in region, Japan (the Tottori and surrounding region in the northern part of Chugoku district, southwestern Japan), epicenters of seismic activities are distributing within a line belt with a width of about 4-9 km along the coastal line of the Sea of Japan. In order to investigate deep resistivity structure beneath the seismic belt and clarify the relationship between subducting Philippine Sea plate and the deeper resistivity structure beneath the San-in region, we have carried out not only seafloor EM measurements using OBEM and OBE but also longer period MT surveys on land since 2006 along two profiles, SW100 and SW200 passing through almost N-S lines of longitudes, 134.3E and 133.4E, respectively. We report the outline of the measurements in the region of sea and land beneath the San-in region, and preliminary results of the data analysis in this manuscript.

2006年10月に大都市大震災軽減化特別プロジェクト(大大特)の一環として，丹波山地から濃尾平野に至る東西方向の「近江測線」で大規模人工地震探査が行われた。我々はこの際行われた発破を，測線外の丹波山地において独自に観測することにより，丹波山地直下の下部地殻に存在する反射面の詳細を調べることを計画した。これらのデータの分析結果に加えて，同じく大大特の一環として新宮—舞鶴測線北部沿いにおいて2005年春より約2年間行われた臨時自然地震観測の結果も合わせて報告する。, Several dynamite explosions were performed along the E-W survey line from Kyoto to Gifu under the "DAIDAITOKU project" on OCT 16, 2006. We observed these explosions along another NNE-SSW trending survey line from Kameoka to Minoo to investigate the deep seismic reflector beneath the Tamba Plateau. Total 100 temporal stations were installed along the 30km long survey line. We can identify the evident later phases in the seismic records. These later phases are PxP reflection from the deep crustal reflector.

2002(平成14)年度から文部科学省の｢新世紀重点研究創生プラン～RR2000～｣の防災分野における都市再生プログラムとして，｢大都市大震災軽減化特別プロジェクト｣(大大特) が開始された。このプロジェクトのもと，京都大学防災研究所は近畿圏における地下構造調査を実施したが，制御震源による構造調査とともに，自然地震観測による調査も実施し，レシーバ関数解析結果と制御震源による構造を比較することで構造を精査した。そのため，近畿地方に35点の高感度臨時観測点を設置し，良好な記録を得た。また，周辺の高感度観測点を含めてデータの収録処理を行った。さらに，観測の効率化を目指して携帯電話を用いたデータ通信システムの開発を行った。, Underground structures in the Kinki district were surveyed from 2002 to 2006 under the Special Project for Earthquake Disaster Mitigation in Urban Areas. We carried out seismic observations with deploying 35 temporary stations across the Kinki district from the Pacific side to the Japan Sea side, in order to reveal subsurface structures, such as subducting Philippine Sea slab in southern Kinki and the continental Moho in northern Kinki. About 50, 000 event data were selected from continuously observed seismic records of both the temporary stations and nearby permanent stations during May, 2005 and Nov, 2006. We also try to develop a data communication system with a mobile telephone for a BREW application to improve the observation system.

2002(平成14)年度から文部科学省の｢新世紀重点研究創生プラン～RR2000～｣の防災分野における都市再生プログラムとして，｢大都市大震災軽減化特別プロジェクト｣(大大特) が開始された。このプロジェクトのもと，京都大学防災研究所は近畿圏における地下構造調査を実施している。地下構造調査は自然地震観測によるものであり，また，制御震源による構造調査(伊藤ら，2005)と相補的に構造の精査を行い，強震動予測の高度化を図ろうとするものである。2005年度までは，近畿地方に28点の単独高感度観測点を設置し，良好な記録を得ている。また，周辺の高感度観測点を含めてデータの収録処理を行っている。, Disaster Prevention Research Institute (DPRI), Kyoto University has conducted surveys of underground structure in the Kinki district under the Special Project for Earthquake Disaster Mitigation in Urban Areas since 2002. We carried out observations of earthquakes Kinki since 2004 deploying 28 observation stations along a control-sourced seismic survey line from Shingu City, Wakayama Prefecture to Kyoutango City, Kyoto Prefecture. This observation system worked well for obtaining high quality earthquake records with high sensitivity. In this report we describe the improvement of observation systems and the measures against snow.

新潟-神戸歪集中帯は，平成16年度からスタートした「地震予知のための新たな観測研究計画（第2次）」で重点ターゲットとして取り上げられ，そのほぼ中央に位置する跡津川断層周辺において，高密度の地震・GPS観測および比抵抗構造調査を合同観測により5ヶ年計画で実施する予定となっている。本論文では，2004年10月4日～10月30日の期間に実施された歪集中帯周辺での比抵抗構造探査のための広帯域MT観測の概要と，これまでに得られている解析結果の概要に関して報告する。これまでの解析から得られた跡津川断層周辺での比抵抗構造は，深さ7kmまでは断層をはさんで比較的高比抵抗領域となっている。さらに，跡津川断層北側では深部（20km程度）まで高比抵抗となっているが，断層の南側では，約7kmを境にして深くなるほど抵抗値が下がる傾向がある。地震の分布は断層北側の高比抵抗領域と良い対応が見られる。, The Niigata-Kobe strained belt pointed out by Sagiya et al.(2000), where crustal deformation is concentrated, becomes one of important target areas in “the 2nd new Program of Research and Observation for Earthquake Prediction” in Japan (e.g. Hirata, 2004). A multidisciplinary research for five years on the strained belt using dense GPS and seismological observations and investigation of crustal resistivity structure was started from 2004. In this paper, we will report outline of the MT measurements made in 2004 along a profile across the Atotsugawa fault, which is situated in the Niigata-Kobe strained belt, and also describe preliminary results on the data analysis. Obtained preliminary inversion result shows inhomogeneity in the crustal resistivity structure at depths from 7km to 18km around the Atotsugawa fault.

2002(平成14)年度から文部科学省の｢新世紀重点研究創生プラン～RR2000～｣の防災分野における都市再生プログラムとして ｢大都市大震災軽減化特別プロジェクト｣(大大特) が開始された。このプロジェクトのもとで京都大学防災研究所では近畿圏における地下構造調査を実施している。自然地震観測によって，地下構造の調査を行い，制御震源による構造調査と相補的に構造の精査を行い，強震動予測の高度化を図ろうとするものである。2004年度までに紀伊半島において，13点の単独高感度観測点を設置し良好な記録を得ている，また，周辺の高感度観測点を含めてデータの収録処理を開始した．, Disaster Prevention Research Institute (DPRI), Kyoto University has conducted surveys of underground structure in the Kinki district under the Special Project for Earthquake Disaster Mitigation in Urban Areas since 2002. We carried out observations of earthquakes in Kii Peninsula since 2004 deploying 13 offline stations along a control-sourced seismic survey line from Shingu City, Wakayama Prefecture to Kawachinagano City, Osaka Prefecture. At each station we set a three-component short period seismometer and a data logger to which power is supplied by solar panels and batteries. This observation system well worked for obtaining high quality earthquake records with high sensitivity. In this report we describe the observation and the data processing system.

最近、西日本においてM7クラスの地震を含む大・中規模が発生し、地震活動が活発化している。西日本内帯における代表的な活断層である山崎断層は、播磨の地震(M7クラス)の地震以後1100年以上M7クラスの地震が発生していないことに注目し、山陰地方および山崎断層における地震活動との関連を調べ、現在までの地震活動を時空間的に把握することを目的として解析をした。中国地方東部から近畿地方北部におけるM≧4.0の地震活動は、5～6年周期で繰り返して発生する傾向がみえる。また、M≧5.0の地震発生前後における地震活動は、発生前の地震活動の低下や発生後の移動、誘発、周辺領域での活発化および低下などの現象がみられることが分かった。山崎断層における地震活動は、2001年以後南東側の領域において、南東方向へ地震活動の移動がみられ、活発な地震活動として注目している。, Recently, several large and middle earthquakes were occurred, and seismicity became active in southwesternJapan. The Yamasaki fault system is typical in-land active fault in southwestern Japan. The last large earthquakearound the Yamasaki fault occurred more than 1100 years ago.We investigated the seismicity along the Yamasaki fault, and its surrounding areas. Seismic activity of theseareas shows periodicity of 5-6 years. Much seismic quiescence induced activation, migration were observedbefore/after large earthquakes (M>5). After 2001, active area along the Yamasaki fault migrated toward southeastdirection.

2002年9月16日，鳥取県中西部の地震(気象庁マグニチュードMj=5.3)が発生した。一連の活動は，前震，本震，余震のパターンで発生した。本震の震源断層は東北東-西南西走向であるが，その後の複雑な余震活動を経て，全体としてZ形の震央分布を示した。本報告は，今までのこの地域での地震活動の特徴を述べ，今回の地震活動および発震機構などの特徴について比較し考察する。, September 15, 2002, the Middle-western Tottori earthquake (Mj5.3) occurred. The seismic activities showed foreshock-mainshock-aftershockpattern. The rupture of the mainshock is ENE-WSW direction. Although, following aftershock activities are complicatedshowing Z shaped distribution. In this paper, we review the seismicity in the San-in area, and describe seismic activities around theMiddle-western Tottori earthquake precisely.

兵庫県北部域では, 2000年12月より集中的に群発地震活動が始まり, 2001年1月12日にはM5.4の地震が発生した。我々は同地域を南北に横切る測線において全9点の広帯域MT観測を行った。TM-modeを用いた2次元インバージョンを行い, 比抵抗構造を検出した。15km以浅は高比抵抗, 深部は低比抵抗を示し, 測線南側で低比抵抗部が浅部まで達している。地震活動は主に, 北側高比抵抗領域から低比抵抗部との境界部にかけて分布しているように見られる。, We carried out magnetotelluric measurements around the northern Hyogo Prefecture where many earthquakes occurred.The resistivity structure shows high resistivity at the shallow region (>15km) and lower resistivity at the deeper part.Moreover, lower resistivity area reach at the depth of 5km around the region from center to south of observed line.Earthquakes occurred around the area from the high resistivity zone to the resistivity boundary.

2001年1月12日, 兵庫県北部の地震(気象庁マグニチュードMj=5.4)が発生した。噌連の地震活動の範囲は東西に約6㎞, 南北に約7kmに及んだ。震源分布は, 時間的，空間的かつ発震機構的に四つないし五つのクラスターに分けることができる。本報告は, 今までのこの地域での地震活動の特徴を述べ, 今回の地震活動および発展機構の特徴について比較し考察する。, The Northern Hyogo earthquake (Mj5.4) occurred on JAN12 in 2001. Following activities expanded 6km along EW direction and 7km aiong NS direction.From space distribution and focal mechanisms, the hypocenter distribution can be divided into 4 or 5 clusters.In this study, we compare the seismicity in past and the activities in 2001, and discuss about the feature of seismicity and focal machanisms.

平成12年(2000年)10月6日13時30分頃, 鳥取県西部を震源とする気象庁マグニチュードMjma=7.3の地震が発生し, 気象庁により「平成12年鳥取県西部地震」と命名された。本報告では, 震源域とその周辺に展開された複数機関の微小地震観測網により収集されたデータをコンパイルした結果に基づき鳥取県西部地震の概要について報告する。, On October 6, 2000, an earthquake of Mjma=7.3 attacked western Tottori prefecture area, southwest Japan. This earthquake was named the 2000 Tottori-ken Seibu Earthquake by JMA. Micro seismic observation networks have been operated in this area by DPRI, JMA, and NIED, and thus many aftershocks data were obtained. In this paper, we will report the outline of the seismic activity of the 2000 Tottori-ken Seibu earthquake by compiling seismic data obtained by these seismic networks.

鳥取観測所の全磁力観測点の北側地点で, 1999年にはNTTDoCoMoのアンテナ鉄塔を含む携帯電話北園基地局, また, 2000年には中国セルラーの円護寺基地局の建設が行われた。基地局の建物およびアンテナ鉄塔建設に伴う人工擾乱の影響を調べ正確な補正量を算出するため, 鳥取観測所(TOT)から北へ約6kmに位置する鳥取大学乾燥地研究センター農場敷地内の一角に全磁力連続観測のための臨時観測点(TOS)を設置し, 全磁力の並行観測を実施した。基地局建設の影響は1999年と2000年の両結果とも, TOTのTOSに対する地点差の0.5-1nTの増加という結果になった。したがってこれを補正するために, 全磁力補正値を, 1999年では, 5月12目-6月10目の期間と8月1目.9月30日の期間のそれぞれにおける地点差平均値の差から, また, 2000年では, 4月28日-6月10目の期間と9月20日-11月30目の期間のそれぞれにおける地点差平均値の差から算出した。その結果, それぞれの工事に伴う擾乱の補正量は, -0.61nTと-0.93nTであった。, The geomagnetic total intensity has been observed at the Tottori Observatory (TOT) since 1967. Two antenna towers and relay stations for mobile phone companies constructed at places in distances of about 100 m located at the northeast of TOT in 1999 and 2000, successively. We made temporal continuous observations of the total intensity at a place in a distance of about 3 km in order to estimate correction values in the total Intensity observed at TOT, because observed data would be affected by the constructions.Changes in the total intensity differences between TOT and TOS observed in 1999 and 2000 are estimated as about 0.5 and 1.0 nT, respectively.

兵庫県南部地震以来、地方自治体等は地震情報の提供を望んでいる。地震活動の時間的空間的特徴を表現する方法に, 震央分布図, 時空間分布図, 積算曲線, b値変化などがある。情報の定量化を念頭において, これらの表現を再検討し改良を試みた。近畿南西部から山陰にかけて最近の22年間に震源が決められた地震について, メッシュデータ化を行い, 震央分布の新たな表示を行った。また, 時空間分布の数値的表現を行った。さらにM別地震積算曲線の詳細を調べた。最後にb値の時間変化の精度を検討した。, There are a lot of methods to express spatial and temporal features of seismic activity such as a hypocenter map, a spatial and temporal distribution map, a cumulative earthquake number curve and a b-value. Because they are not necessarily suitable methods to give citizens seismic activity information which is strongly needed since Hyogo-ken Nambu Earthquake, we reexamine these method and try to modify them. Using a seismic catalog which includes all microearthquakes occurred in south western Kinki district and Sanin district during the period from 1977 to 1998, we first made a mesh data set of seismicity and displayed a new distribution of seismicity. Next, we improved the spatial and temporal distribution map so as to express it in numerical way to detect quiescence of seismicity. Also we examined cumulative earthquake number curves for each M. Finally, we checked the temporal changes of b-values in view of accuracy of the method previously reported.

兵庫県南部地震の前後600日ずつの山崎断層域の地震活動の変化を調べた。その結果, 活断層の近傍に発生する規模以上の地震の活動度を見ることが, 広域の応力変化を捉えるために有効であることを認めた。⊿CFFの増減は地震活動変化のおおまかな傾向把握に適している。地殻をブロックに分けるような明確な不均質構造である大規模活断層の存在が内陸の地殻活動に大きく影響している。, The seismic activity for 600 days before the 1995 Hyogo-ken Nanbu earthquake was compared with that after the occurrence. The seismicity of the Yaniasaki fault zone became active after the occurrence of the 1995 Hyogo-ken Nanbu earthquake. Today's activity has already returned to the former level. On the contraly, seismic activity far from the fault zone was not so sensitive to the Nanbu earthquake. Concerning the local seismicity change, A CFF was not so severe facter. Large-scale active fault zone is sensitive to the wide tectonic stress variations.

Crustal movement change of the Yamasaki fault zone before and after the occurrence of 1995 Hyogo-ken Nanbu earthquake was investigated. The seismicity around the Yamasaki fault became active after the occurrence of the earthquake. On the other hand, that of the outside area of the fault zone did not show any significant increase.This means that an active fault zone is sensitive to the crustal stress concentration, The increasing rate of number of events with magnitudes greater than or equal to 1.6 is much higher than that of smaller ones. This means that there may exist some threshold level of magnitudes to detect the change of tectonic stress condition. For one to three months before the earthquake, seismic activity became lower over the whole area of the Yamasaki fault zone.This suggests the propriety of the block movement model of the crust. It is also shown that fracture zone is a sensitive area to the crustal stress conditions.

Seismic activity in the region from the eastern part of the Chugoku district to the western part of the Kinki district before and after 1995 Hyogo-ken Nanbu earthquake is investigated. Time series of earthquakes, epicenter distributions and b-values are shown in the 3 regions where earthquakes with magnitude more than 5 occurred from 1978 to 1995.3 regions are located around Tottori-Shimane border (A), along the Tottori Earthquake faults (B) and along the Yamasaki fault ?.The change of seismic activity is clear in the nearest region to the Hyogo-ken Nanbu earthquake. Little change of seismic activity is appeared in the (A) region. In (B) region, seismic activity from 1989 to 1991 is higher than that from 1992 to 1994. But seismic activity of the period from 1992 to 1994 is extremely low for earthquakes with magnitude more than 2.5. In ? region, seismicity of later period is higher than that of former period. For earthquakes with magnitude more than 2.5, seismic activity is high in the near area to the Hyogo-ken Nanbu earthqake.The characteristics that b-value become large before earthquakes with M5 class is shown in the 3 regions.Before Hyogo-ken Nanbu earthquake, b-value becoms large in (B) and ? regions.After Hyogo-ken Nanbu earthquakes, seismic activity becomes high only in ? region.

Hypocenters of earthquakes determined from the data recorded by Kamitakara, Hokuriku, Abuyama and Tottori Observatories, Kyoto University during 1976-1991 are collected to define the distribution of the thickness of the seismogenic layer of the crust in the inner zone of southwest Japan. Total of 63, 961 well-located earthquakes are selected and used to determine the cutoff depths in the crust. As a result, the thickness of the seismogenic layer varies 5-25 km from place to place and is closely related to the thermal structure of the crust. Further, active faults and large inland earthquakes are likely to occur at the steepchange in the cutoff depth. Thus, the thickness of the seismogenic layer is important to examine the rheological characteristics of the crust and eventually to reveal the relationship beteen the crustal heterogeneity and large inland earthquakes.

We carried out an seismic explosion survey at Taal volcano in March, 1993. The explosions were done at the west of the volcanic island and digital event recorders were deployed along a fan-shooting survey line at the east of the volcanic island and also along a short straight survey line across the island. Small-aperture arrays were also operated at two sites west of the volcanic island. We found that P waves that traveled just beneath the main crater strongly attenuated and showed later arrivals. This result suggests the existence of a low-velocity and low-Q region around the main crater at a depth of about 1.5km, although the location and extent cannot be determined exactly. P wave velocity of this abnormal region may be eatimated to be lower than that of the surroundings by much greater than about 25%, if we assume the region is restricted just beneath the main crater. We applied the NMO correction to the later part of seismograms and found a reflector around the east of the main crater at a depth of about 6km, which may suggest the top surface of the magma reservoir.

Temporal variations in water quality and temperature at 5 hot or mineral springs (Tamatsukuri, Yuda, Sekigane, Yoshioka and Yudani) in Tottori and Shimane Prefectures have been observed for geochemical research for earthquake prediction.Since October, 1989, the seismicity have been active near the border of Tottori and Shimane Prefectures.We carefully estimated the contribution of meteorological factors and earthquakes to the water quality and temperature and found the water quality at the Yoshioka hot spring and water temperature at the Yudani hot spring are sensitive to earthquake occurrences.Especially at Yoshioka hot spring, the C1￣concentration was anomalously increased just before the earthquake(M 5.9) on August 28, 1991 in the eastern part of Shimane Prefecture.We conclude that measurement of water quality and temperature at a carefully chosen hot or mineral spring should give information useful for earthquake prediction.

Since the occurrence of 27 October, 1989 earthquake (M 5.3), the seismic activity on the border of Tottori-Shimane prefecture have become remarkably high.On the west side of Tottori prefecture, five earthquakes (M ・.1) occurred along a line from southeast to northwest direction.This line with a length about 10 km may be an earthquake fault system in this area.To obtain the accurate earthquake foci, portable seismic observation systems were temporally settled at six stations on the border of two prefectures.We found that the lower limit of earthquake distribution were about 12km depth.The conspicuous "X" phases were found between P and S arrive on the seismogram for three stations. This phase may be a reflected wave from a liquid layer.

A new data acquisition system was developed for the routine observations of microearthquakes.The system consists of two personal computers, a 32-channel 12-bit A-D converter, a dualport RAM and a 600 MB magneto-optical disk unit. The system can be also equipped with a GPIB interface to the third computer, which enable automatic data processing or data transfer through computer net-works.The programs on the system attain real-time delay-trigger data acquisition, real-time monitoring of data on a display to watch the conditions of stations and manual processing of the stored data.The systems were intalled for test at the Hokuriku and the Kamitakara Observatories in 1992 and have been successfully operated for earthquake observations.The old analogue-recorder system at the Kamitakara, the Hokuriku and the Tottori Observatories will be replaced with this system in the near future.

Daily measurements by using GPS had been carried out at Kyoto, Kochi, Aso and Tottori sites in Southwest Japan from September 1990 to September 1991. The long term repeatability of the baseline lengths determined was 0.26 ppm, which was comparable to the short term repeatability of them on even as long baseline as about 200 km. The baseline lengths showed annual changes, amplitude of which were about 1 ppm. It seems that such changes were caused by insufficient consideration of tropospheric delay in computation of baseline vector.

Using tunnels and shafts in Ikuno mine, which was closed in 1974, we started in 1987 to observe seismic events and temporal change of 1) electric self-potential, 2) apparent resistivity, 3) underground water level, 4) water seepage from tunnel wall and 5) γ ray intensity. The main purpose of our comprehensive observation is to discuss correlation between the observed data in such a stable circumstance as that without excavation, taking account of the continuous observation of crustal movement which have been carried out before the mine was closed. Since all tunnels and shafts below the underground water table filled up with water now, we expect to catch any behavior of rock body in response to any change of underground water and also any preseismic or postseismic effect. From the observation of about five years, we found that the seismic events (S-P time < 0.3 s; M< 0) still occur even now in the vicinity of mine. From our preliminary analysis, the secular change of electric self-potential seems to correlate to the activity of the seismic event in the mine.

We investigated relations between seismic gaps in active seismic zones and occurrence of middle-size earthquakes by using of a microearthquake catalog compiled for 25 years by Tottori Observatory which belongs to Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University. Our study sites are active seismic zones of Tottori region, Tottori-Shimane border region and Yamasaki Fault region in the Inner zone of Southwest Japan. We examined preceding seismic activity in and around a focal area of 10 middle-size earthquakes in the active seismic zones.The result is that in most cases a middle-size earthquake(s) occurred in a seismic gap in the active seismic zones, some of which were preceded by small-size earthquakes. To point out present seismic gaps in active seismic zones can, therefore, be a clue of prediction of inland middle-size earthquakes. We investigated spatial distribution of the hypocenters in detail and found out 19 seismic gaps in the active seismic zones.After we pointed out the seismic gaps, earthquakes with magnitude 3.9 and 4.0 occurred in one of the seismic gaps.We intend to keep watch on seismic activity in and around these seismic gaps and try to detect precursory change of the seismic activity before occurrence of middle-size earthquakes.

Daily measurements by using GPS have been carried out at Kyoto, Kochi, Aso and Tottori sites in Southwest Japan since September 1990. The aims of the present measurements are to estimate a long term repeatability of the baseline vectors determined, to investigate effects of error factors and finally to improve the accuracy of GPS measurements.Results for the first six months are as follows:(1) The long term repeatabilities of the baseline lengths of about 150 to 500km were 0.2 to 0.4 ppm.(2) The baseline length in summer was determined longer than that in other seasons.(3) There were positive correlations among the changes of the baseline vectors.

We have continued the measurement of gamma-ray at 4 tunnels for crustal movement observation. We have also continued the measurement of chemical composition (or electrical conductivity) and temperature of the groundwater at 7 wells which are hot or mineral springs. The observation tunnels and wells are mainly located near Yamasaki, Shikano and Yoshioka faults.The observation periods range from 7 months to 14 years.In 1990, there were 2 neighboring earthquake activities whose magnitudes are greater than 5. There were also 2 typhoon landings accompanied with heavy rainfall and rapid change in atmospheric pressure. Although there were several typhoon-related changes in the gamma-ray intensity, the electrical conductivity or the water temperature at several observation tunnels or wells, we detected only one clear earthquake-related (coseismic) change in electrical conductivity of the groundwater at one observation well. As there clearly exist some sensitive observation stations to neighboring earthquakes, it seems important to investigate the conditions of such observation stations.

We have begun to observe seismic waves with very-broad-band and wide dynamic rangeseismometer (STS) since July 1988 at Tottori Observatory, Hokuriku Observatory and KamitakaraObservatory which belong to Research Center for Earthguake Prediction, Disaster PreventionResearch Institute, Kyoto University. The STS system has dynamic range of 140 dB and itsvelocity output is flat for the period range between 0.1 and 360 s. Therefore it can completelyrecord both body waves and surface waves that are too big for the microearthquake observationsystem to record them without scaleover. It is very important to record such complete wave-forms because they can be used in the studies of source mechanism and structure of the Earth'sinterior. In this paper we will introduce our STS system and some examples of waveformsrecorded with the system.