著者

Fukao Yoshio

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo (ISSN:00408972)

巻号頁・発行日

vol.48, no.5, pp.707-727, 1970-12-05

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The focal process of a deep shock which occurred at 580km depth in the Banda Sea region in 1964 is investigated by using long period P and S waves from WWSSN seismograms. The two nodal planes of this earthquake are given by dip direction=319°, dip angle=45°, and dip direction=180°, dip angle=53°, respectively. Assuming that a rectangle represents a far field pulse form, the S wave seismograms are synthesized for various pulse widths and epicentral distances. The effects on the wave form due to passage through the mantle, crust and instrument are removed. Trials are made to find the pulse width, Ts, of the original rectangular pulse which fits the observed S wave form. The variation of Ts with respect to the fault plane orientation is investigated. Ts is remarkably well correlated with the emergence angle measured from the slip vector on the nodal plane with dip direction=319°, dip angle=45°. This result, leads to the following conclusions: (1) The source is plane-like rather than spherical; a fault, model is more preferable to a phase transition model. (2) The nodal plane with dip direction=319° and dip angle=45° is the fault plane. (3) The fault surface is not circular but elongated. (4) The direction of elongation coincides, with the slip direction. (5) The fault width W should not exceed 15km. It may. probably be smaller than 10km. (6) The fault length L is estimated to be 40.km. (7) The process of the body wave radiation took place almost simultaneously, within about two seconds, over the entire fault surface; this model is significantly different from the ordinary propagating fault model. On the basis of this model, the seismic moment M0 is estimated to be 5.8×1026 dyne cm from the P wave amplitude and width. Assuming W=8km, the average dislocation, stress drop and elastic energy released when the fault surfaces become free, are. estimated to be 150 cm, 290 bars and 7.0×1022 ergs, respectively. The stress drop obtained here is one order of magnitude greater than that estimated for shallow earthquakes.|1964年バンダ海で起った深さ580kmの深発地震の震源過程を世界標準地震計観測網が記録した.長周期P波およびS波を用いて調べた.この地震の2つのP波節面はdip direction=319°dip angle=45°およびdip direction=180°, dip angle=53°で与えられる.S波の遠方での波形が矩形で近似できると仮定して種々のパルス巾,震央距離に対してP波理論記象を作った.その際,マントル,地殻および地震計を通過するときの波形への影響を考慮した.これらを実際の記録と比較して,もとの矩形波のパルス巾,Ts,を求めた.Tsの方位に関する著しい規則性より次の結論を得た.(1)震源は球状ではなく面状であり,従って相転移モデルより断層モデルの方が適当である.(2)dip direction=319°,dip angle=45°のP波節面が断層面である.(3)断層面は円ではなく,slip方向にのびた細長いものである.(4)断層の巾は15kmを越えず,恐らく10km以下と思われる.(5)地震は断層面全体にわたって殆ど同時に起ったと考えられる.この間約2秒である.普通の伝播性断層モデルでは結果を説明できない.(6)断層の長さは約40kmである.これらの結論はP波の実際の波形とも矛盾しない.P波理論記象と実際の記録との比較から地震モーメントは5.8×1026dyne cmと推定される.断層の巾を8kmと仮定すると,断層面上でのくい違いの大きさ,stress dropおよび断層面がfreeになったと仮定したとき解放される弾性エネルギーは,それぞれ150cm,290 barsおよび7.0×1022 ergsとなる.