The 1998 Miyako fireball\u27s trajectory determined from shock wave records of a dense seismic array

A high velocity passage of a meteoroid through the atmosphere generates a shock wave with a conical front. When the shock front arrives at the surface, it causes high frequency ground motions that are registered on the seismograms. We can use seismological data to determine the trajectory of the meteoroid in the atmosphere. A strong shock wave from the 1998 Miyako fireball is recorded by more than 20 stations in a dense array of seismographs installed in the northeastern region of Honshu Island, Japan. We determine the velocity and the trajectory of the fireball in the upper atmosphere using the arrival times of the shock wave at the stations

Check-Sheet for the MeSO-net in Trouble

＜報告

Development of a Spectrogram Analysis Tool for Seismic Waveform Data and its Application to MeSO-net for Noise Survey

We developed a spectrogram analysis tool working on Matlab to check on a seismic waveform data easily. This tool can display waveforms, power spectrum densities (PSD), and PSD spectrograms of ground motion recorded in WIN format. It can be used to evaluate a temporal variation of ground motion, and compare waveforms and spectrums for data from different channels. In this paper, we outline this tool and show its application to MeSO-net (Metropolitan Seismic Observation network) data recorded at Yayoi station (YYIM). From the analysis of whole day waveform, it is found that the noise level from AM to PM is more than ten times larger than that in the midnight because of a higher human activity, and the noise caused by train is recorded from 5 AM to 0:30 AM, which are trains’ operation hours. We then compare ground noise levels between YYIM, YYI and ASO. YYIM station is located in Tokyo University and its sensor is installed about 20m beneath the ground. The sensor at YYI is installed at the surface just beside the YYIM borehole. ASO is a permanent seismic station operated by Tokyo University, and one of the quietest sites in Kanto plain. From the analysis using this tool, YYIM (20m deep) is 10 to 20 dB quiet than YYI0 in the range more than 5 Hz, and ASO is more than 20 dB quiet than YYIM in the range lower than 20 Hz.＜論説

History of Shin\u27etsu Seismological Observatory

＜短報

Development of Support System for Selecting Observational Stations

＜短報

Conclusions and Suggestions on Low-Dose and Low-Dose Rate Radiation Risk Estimation Methodology

Background: For radiological protection and control, the International Commission on Ra- diological Protection (ICRP) provides the nominal risk coefficients related to radiation expo- sure, which can be extrapolated using the excess relative risk and excess absolute risk obtained from the Life Span Study of atomic bomb survivors in Hiroshima and Nagasaki with the dose and dose-rate effectiveness factor (DDREF).Materials and Methods: Since it is impossible to directly estimate the radiation risk at doses less than approximately 100 mSv only from epidemiological knowledge and data, support from radiation biology is absolutely imperative, and thus, several national and international bodies have advocated the importance of bridging knowledge between biology and epidemiology. Be- cause of the accident at the Tokyo Electric Power Company (TEPCO)’s Fukushima Daiichi Nu- clear Power Station in 2011, the exposure of the public to radiation has become a major concern and it was considered that the estimation of radiation risk should be more realistic to cope with the prevailing radiation exposure situation.Results and Discussion: To discuss the issues from wide aspects related to radiological protec- tion, and to realize bridging knowledge between biology and epidemiology, we have established a research group to develop low-dose and low-dose-rate radiation risk estimation methodology, with the permission of the Japan Health Physics Society.Conclusion: The aim of the research group was to clarify the current situation and issues relat- ed to the risk estimation of low-dose and low-dose-rate radiation exposure from the viewpoints of different research fields, such as epidemiology, biology, modeling, and dosimetry, to identify a future strategy and roadmap to elucidate a more realistic estimation of risk against low-dose and low-dose-rate radiation exposure.Keywords: Radiation Risk Estimation, Low Dose and Low Dose Rate, Epidemiology, Biology, Modeling, Dosimetr

Determination of the fireball trajectory using dense seismic arrays

Fireballs, which are caused by high-velocity passages of meteorites through the atmosphere, generate shock waves. It has been known that such shock waves are often recorded on seismograms. It is possible to determine the trajectories and the sizes of a fireball using seismological records. We have searched shock wave signals from many bright fireballs observed in the period from September 1996 to November 1998, and the 1999 Kobe meteorite. The shock waves from one large fireball, which is called the Miyako fireball, and the Kobe meteorite are clearly recorded on many seismograms. In particular, the shock waves from the former fireball are widely recorded by the dense seismic array of 1997-98 joint seismic observations in the Tohoku Backbone Range. We determine their trajectories. Amplitudes of the shock waves are found to be possibly correlated with the masses of the meteorites. It is also indicated that the shock waves from fireballs, which are darker than brightness magnitude -10, are too weak to be recognized on the seismograms of ordinary seismic stations in Japan

Source Process, Characteristics of Associated Seismicity and Seismotectonic Implications of the 1986 Omachi Earthquake of 5.9 in the Northwestern Part of Nagano Prefecture, Central Japan

北部フォッサマグナの糸魚川・静岡構造線に長野盆地西縁断層(善光寺地震断層系)及び千曲川構造線のそれぞれの延長がぶつかる地域において発生した1986年12月30日の地震の震源パラメータや余震活動および先駆的活動の特徴,テクトニクスとの関連について調べた.震源域直上の1臨時観測点を含む近傍の観測点のデータを用いて余震の高精度震源決定を行い,さらに本震の震源についても定常観測点に基づく結果を補正した.この際,深発地震データから推定した走時の観測点補正時間を導入した.本震の深さは5.5kmで,その近傍に集中した余震(狭義の余震)の発生域はN15～20°Wの走向をもち,僅かに西に傾いた,ほぼ垂直な面上にあり,水平に6km,深さ方向に4kmの広さに収まる.この余震分布は初動の押し引きから得られた断層面の一つ(走向N19°W,傾斜角73°,すべり角26°)にほぼ一致する.この狭義の余震の外に点在する広義の余震は東西,南北にそれぞれ20kmの広さに分布する.気象庁の観測点の変位地震計記録の初動P波から推定した震源断層の破壊は,本震の震源付近から,余震が密集している南の領域へ向けて3km/sの速度で伝播した.その全面積は6km2,平均的な変位は75cm.変位の立ち上がり時間は0.5sである.また,地震モーメントは1.3×1024dyne・cm,応力降下は220barである.本震の破壊領域は既存の断層上にはなかったが,広義の余震は,2本の新第三紀層中の断層(小谷-中山断層,持京断層)が会合する地点,両断層に画された東南側の領域一帯,北部の両断層に挾まれた地域や,孤立的に東部の一地点に分布する.活動の範囲は時間とともに,拡大縮小の変化が認められた.最大余震はM3.5(広義の余震)で,本震の大きさに比べ,極めて小さく,余震回数も多くはなかったが,その減衰の定数はp=1で,通常と変わらない.この地震に先行した微小地震活動があった.その震源域は広義の余震の一つのクラスターとほぼ一致する.また,周囲半径100km以内の地震活動が1～2年前から1年後にかけて活発であった.直前の5～9日前には,飛騨山地を隔てた跡津川断層でも,目立った活動があった.大町市付近の系魚川・静岡構造線に沿った地域には,過去にも度々M6程度の地震が発生している.その中で1958年の地震の震央は,今回の地震の活動域にある.このときにも跡津川断層の活動が連動した(1858年飛越地震,M6.9).糸魚川・静岡構造線等を含む広域のネオテクトニクスの枠組みのなかに今回の地震の活動域が位置づけられるとともに,小規模の地殻ブロックの役割も注目される.A remarkable earthquake of Af 5.9 occurred at 09:38 on December 30, 1986, 10km northeast of Omachi city, Nagano Prefecture. This earthquake was accompanied by precursory microearthquake activity from one year before, at nearly the same place with one of the clustering spots of aftershocks, which is located in the vicinity of the meeting point of the Tertiary faults: the Otari-Nakayama and Mochigyou faults. The historical earthquake of M 5.7 in 1858 took place around this spot. Adjacent to this area, there were two other historical events with magnitude around 6 in 1890 and 1918. Synchronized seismic activity between the Omachi region and the Atotsugawa fault region, about 60 km apart from each other, was found in this 1986 event as in 1858. The surrounding seismicity within 100 km from the epicenter of the 1986 event had been active from several years before

Some Characteristics of the Earthquake Sequence with the Main Shock of M 4.9 on August 24, 1986, at Maruko Town, Eastern Part of Nagano Prefecture, and Its Seismotectonic Implications

北部フォッサマグナの中央隆起帯を横断する千曲川構造線の東端に位置する長野県小県郡丸子町付近で1986年8月24日,M4.9の地震が発生した.ここは2つの火山前線がぶつかる点のすぐ背後でもある.通常の地震活動レベルは低いが,過去には1912年の上田市付近の地震(M5.2)がある.丸子町の地震活動は前震・本震・余震系列と本震の10日後から始まった群発地震が重なったものであった.2回の主要な活動ピークをもつ例は,北部フォッサマグナ地域では少なくなく,ピーク間の間隔は1918年大町地震の13時間,1969年焼岳の地震の2日,1912年上田の地震の5日,今回の地震の12日,1963年燕岳の地震の20日,1897年上高井の地震の104日というように様々である.2回目が群発地震であったのは丸子の地震と,燕岳の地震,上田の地震である.現地における臨時観測によって精密な震源分布が得られた.震源域は時間とともに拡大したが群発地震後最終的には東西3km,南北2km,深さは6kmを中心に3kmの幅をもつ拡がりであった.定常観測網で求めた震源との比較を行い,観測網に依存する震源の系統的なずれやその値のバラツキから震源の絶対精度と相対精度を推定した.MO～4.5の間のM別頻度分布はGutenberg-Richterの関係から少しずれる.群発地震の回数の減衰(p～2)は本震直後の余震のそれ(p～1)と比べ大きい.燕岳の地震ではどちらもp～2であった.本震の震源断層は発震機構及び余震分布の特性から西上り東落ちの高角逆断層である.これは中央隆起帯東縁でのテクトニックな変動と調和する.1986年の千曲構造線の地震活動はそのピークが東南東から西北西へ約150km/yearの速度で伝播した.1912年～1918年にもこの構造線の両端付近で地震があった.約70年の間隔を置いて同じような活動を繰り返したことになる.The earthquake of M 4.9 which occurred at Maruko town, eastern part of Nagano prefecture, at 11 h 34 m on August 24, 1986, was accompanied by foreshocks, ordinary aftershocks just after the main shock and peculiar swarm-like aftershocks that began 10 days after the event. Seismic sequences with double high adtivity peaks have occurred frequently in and around the northern Fossa Magna region; the intervals between the two peaks ranege from 13 hours to 104 days

Structural Features of the Precursory Seismic Gap and Aftershock Region of the 1990 Southern Niigata Earthquake of M5.4

A moderate-sized earthquake of M5.4 occurred at 18:38 on December 7, 1990 (JST), at the border of Kashiwazaki City and Takayanagi Town, Niigata Prefecture, in the region of the southern end of the Uetsu active fold system and the northern end of the Fossa-Magna. After 2 minutes this earthquake was accompanied by an event with a nearly identical magnitude, M5.3. An area of 30km in linear dimension covering the aftershock area had been a so-called seismic gap of the second kind since about one year before the earthquake. A detailed structure of the seismicity bordering the gap is disclosed by the method of volume visualization in computer graphics. The several linear peripheral active zones form a so-called doughnut-shaped pattern suggesting activation of the preexisting tectonic structure. Far surrounding seismic zones along the Shinano-gawa River and the Ogi-Kashiwazaki line in the Strait of Sado became active before the formation of the gap. When the after shock activity revived after a half year, the Ogi-Kashiwazaki zone was also activated.新潟県南部東頸城丘陵,羽越活褶曲地帯において1990年12月7日,双子地震(MJMA5.4, MJMA5.3)が発生した.震源域を囲む長径30kmの地域は約1年間,微小地震レベルの活動もない明瞭な第2種空白域であった.地震活動のVolume Visualizationというコンピュータグラフィックスの手法を用い,空白域の生成過程と周囲の地震分布の構造を調べた.空白域とそれを囲む活動域(いわゆるドーナツ・パターン)の境界は線状である.これは,潜在的な断層ないし構造線が活性化したものと考えられる.周囲の活動線(小木-柏崎線)と余震との同期的活動も確認された.本震の翌日に2点,約2週間後にも2点,合計4点の地震観測点を震源域近傍に設けた.常設の観測点にこの臨時観測点を加えた観測網によって1km程度の震源精度をもつ余震分布を得た.震源域付近の観測網による震源データを基準にして,広い観測網による震源の系統的なずれを求め,40kmのスパンをもつ常設観測網による震源データを補正した.余震の震央分布には,最初から活動した直径10kmの円内にだいたい収まる核の部分(狭義の余震域)とやや時間をおいてその周辺の直径18kmに広がった領域(広義の余震域)がある.前者は本震の震央を中心にして走向N30°Eをもつ棒状の分布とこれに直交し北東側にて線状配列をなす分布が鈎型の形状を示す.本震の断層とその副断層を意味していると考えられる.これら二組の断層のそれぞれに平行な線状分布が1～2kmの幅をもつ低活動域を挟んで広義の余震域に存在する.震源の深さは上部地殻と下部地殻の境界付近の9～13kmに集中している.余震は上下に薄く(4km)水平に広く(10km)拡大したことになる.本震はその最下部域に位置し,本震および主な余震は本震などの発震機構の断層面解のうち高角の逆断層面に一致した分布を示す.なお,震源域一帯の地殻構造については,自然地震の走時を解析して震源域周辺の上部地殻と下部地殻の境界の深さを12kmに求め,余震のデータからVp/Vs値1.67を得た.余震はだいたい改良大森公式(近似的にベキ関数)に従い, p=1.2で減衰しだが,最初の10時間は時定数3.3時間の指数関数的減衰を示した