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Ocean Bottom Seismometer Study of the Kuril Trench Area

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Ocean Bottom Seismometer Study of the Kuril Trench Area Zol NOAA-JTRE-166 07 REi OVE HIG-76-9 -- - -· -~ - -· - --' OCEAN BOTTOM SEISMOMETER STUDY ·' OF THE KURil TRENCH AREA By JUNZO KASAHARA and ROBERT R. HARVEY .. ' I 1 / ; . i . AUGUST 1976 . , .. •. HAWAII INSTITUTE OF GEOPHYSICS UNIVERSITY OF HAWAII, HONOLULU and JOI NT TSUNAMI RESEARCH EF FORT PACIFIC MARINE ENVIRONMENTAL LABORATORY ENVIRONMENTAL RESEARCH LABORATORIES, NOAA Distribution of this document is unlimited. Reproduction of this report in whole or in part is permitted for any purpose of the United States Government. HAWAII INSTITUTE OF GEOPHYSICS UNIVERSITY OF HAWAII ·. NOM-JTRE-166 HIG-76-9 OCEAN BOTTOM SEISMOMETER STUDY OF THE KURIL TRENCH AREA By Junzo Kasahara 1 and Robert R. Harvey AUGUST 1976 Hawaii Institute of Geophysics University of Hawaii, Honolulu and Joint Tsunami Research Effort Pacific Marine Environmental Laboratory Environmental Research Laboratories, NOM ~~uJ Director Director Joint Tsunami Research Effort Hawaii Institute Partial support for this research was contributed by Office of Naval Research contract N00014-75-C-0209. 1 ON LEAVE FROM EARTHQUAKE RESEARCH INSTITUTE, UNIVERSITY OF TOKYO. I i - ! ABSTRACT An ocean bottom seismograph was deployed on the seaward side of the Kuril Trench off Hokkaido ~ Japan in 5460 m of water~ in llugust during the 1975 Joint Soviet-American Tsunami Expedition. During the seven- day record" S-P times were distributed in three groups : 19- 24 sec ~ corresponding to aftershocks of the 10 June and 13 June 19?5 earthquakes southeast of Nemuro ~ Japan and to earthquakes east of Sanriku~ Japan; rv30 sec ~ from south of Erimo Peninsula ~ Hokkaido ; and rv100 sec~ from the Izu-Bonin Islands . Seven earthquakes ~ with hypocenters well determined by the land seismic net~ are studied in detail . A shallow focus earthquake yields typical oceanic mantle velocities shallower than 50 km in the slab which dips unde r the Japanese archipelago. However" I ! deeper foc~s earthquakesreveal anomalously high velocities (V > 8. 50 km/sec ~ V > 4. 80 km/sec) averaged over the upper p s - I 230 km~ in agreement with the models of Utsu and Oliver and Isacks. Two deep earthquakes" whose paths lie in the Pacific Ocean asthenospher e" suggest a velocity 3% lower than that predicted by Jeffreys - Bullen~ in agreement with the above models . Spectral analysis of S arrivals suggests Q values of 1000-1500 for nearby s earthquakes (S-P < 43 sec) and 4000-6000 for longer distance earth- quakes ~ implying an unusual attenuation mechanism for long travel path~ which enhances the high frequencies . I I TABLE ' OF CONTENTS ABSTRACT iii LIST OF TABLES vi LIST OF FIGURES vii I INTRODUCTION 1 ! OBS DESCRIPTION l I S-P TIME DISTRIBUTION AND TYPICAL FEATURES 4 I I TRAVEL TIME ANOMALY IN THE SEISMIC ZONE · 8 I Q STRUCTURE 13 I OBS NOISE 15 I CONCLUSIONS 17 REFERENCES 19 TABLES unpaged (follow page 24) FIGURES unpaged (follow Table 3) . I I v LIST OF TABLES l. Arrival times, S- P times, and amplitudes of P-, S-, and T-phases for OBS observations duri ng the 1975 Soviet-American Tsunami Expedition. Amplitudes are taken from the vertical, low-gain channel 2. Hypocenter parameters for the seven earthquakes used in the present analysis, determined from U.S . Geological Survey and Japan Meteorological Agency data. Japan Meteorological Agency I · data are in parentheses 3 . Summary of results for the seven earthquakes used in the present analysis. Origin times and focal depths are from the U.S. Geological Survey Epicenter Determination Record i i ... .-........ ... --.-..-.. m:. !.~~·~--~ ___ ., ....... ~ -.. ~, LIST OF FIGURES 1 . Instrument locations during the 1975 Soviet-American Tsunami Expedition . 2 . Pop-up o c ean bot tom seismograph on board the Soviet vessel R. V. Valerian Uryvaev . 3 . Block diagram of ocean bottom seismog raph. 4 . Estimated response curve for geophone, amplifier, tape recording system, and playback system . 5. Playback scheme. Time compression ratios for original OBS tape recording and Hewlett-Pa ckard 3960 playback are 1:400 and 1:200, r espectively. 6 . S- P time distribution for (a) all detected earthquakes and (b) those with S-P time between 0 and 37 seconds. Shaded area indicates those earthquakes a ccompanied by T-phase activity. 7 . Seismograms as displayed on Visicorder. P, P , S, and T denote 1 P- phase arrival on vertical channel , P-phase arrival on horixontal channel, S- phase arriv al and T- phase arrival, respectively. Top and bottom traces are the time channel, with altered wave f orm of the pr i mary 1-se c pu lses containing a BCD code with day, hour, a n d minute information . Earthquakes (c), (g), (h), (j), and (k) show T-phases . Note that the P-phase arrives earlier on the vertical than on the horizontal c hannel for most earthquakes. 8 . Epic entral distribution o f aftershocks of the 10 June and 13 June 1975 earthquakes (after Japan, Hokkaido University, 1976). vii 9. T-phase amplitude versus P and S wave amplitudes as recorded on the low-gain vertical channel for all earthquakes with S-P time between 19 and 24 sec. T-phase generating earthquakes occurred in the aftershock region of the 10 June and 13 June 1975 earth­ quakes and near the Japan Trench. 10 . Chart showing epicenters of the seven earthquakes used in the present analysis, as determined by Japan Meteorological Agency and the U.S. Geological Survey. ll. Schematic ray paths from the hypocenter to the OBS for (a) earth­ quakes with negative travel time anomalies and (b) earthquakes with normal arrivals. The ray path for (a) lies entirely in the high velocity (HV) , high Q (HQ) zone, whereas for (b), the path lies predominantly in the low vel ocity (LV), low Q (LQ) zone. 12. S- P time versus P arrival t i me for the OBS, vertical channel, using U.S. Geological Survey epicenter determinations. 13. S-P time versus P arrival time for individual earthquakes~ using U. S. Geologi cal Survey epicenter determinations. Land station arrivals are denoted by circles , OBS arrivals by squares. 14. Travel time curves for earthquakes with early OBS arrivals. Land station arrivals are denoted by circles, OBS arrivals by squares. The solid lines represent standard Jeffreys-Bullen travel time curves . 15. Travel time curves for earthquakes with normal OBS arrivals. Symbols are the same as in Fig. 14. 16 . Crustal model used to calculate average mantle velocities, based on the results of Lw:iJJJig et al. [1966] and Den et al. [1971). viii 17. Sonograms as recorded on Sona-Graph. Contours correspond to 6-dh amplitude intervals P, S, and T refer to P- phase, S-phase, and T- phase arrivals; and Hand V refer to horizontal and vertical channels. 18. S-wave spectra for selected earthquakes, obtained as a cut through a sonogram at the peak in S-phase energy and corrected for the ' OBS system response. Numbers on curves refer to S-P time in seconds. 19. Q variation as a function of S-P time. The ordinate refers to the slope of logarithmic amplitude decay with frequency. The solid and dotted lines are calculated using a model with a flat velocity amplitude source spectrum with varying Q for P and S waves, respectively. INTRODUCTION The 1975 Soviet-American Tsunami Expedition i~tended to s tudy tsunami generation, propagat i on, and runup. The program comp ris ed a cooperative e ffort b e tween scientists of the Joint Tsunami Re s earch Ef f ort, NOAA; Hawaii Institute of Geophysics , Uni versit y of Hawaii; and the Sakhalin Complex Scientific Research I nstitute and the Hydrometeorological Service of the U.S.S.R. The field effort [ Soloviev ~ Miller et a l .~ 1976] involved a two-month cruise a long the PaciEi c coast of Hokkaido and the southern Kuril Islands , duri ng whi ch the f ollowing f ree-vehicle instruments were deployed : nine bottom pressure r e corders, one vertical electric f ield recorder, and one ocean bott om seismograph (OBS). Figure 1 s hows the i nstrument locations. This pa per reports on the analysis of t he OBS da ta. Water wave results are being published separ:'ltely [Efimov et a l. ~ in press]. OBS DESC RIPTION The OBS was built at t he Hawaii Institute of Geophysics and is a mo dified ve rsi on of that designed by the Earthquake Research l ns t j Lut e , Un iversi t:y of Tokyo [Ka .s ahm'a et; al . ~ 1973, 1974a, 1974b; 2 Sut t on et al.. ]. The instrument operates as a free vehicle and consists basically of data acquisition and recovery systems . 2 S UT TON , G . H., J . KA S AHA R A, W. N. ICHINOSE, ANO D. A. BYRNE . MS, OCEAN B OTTOM S EISMOGRAPH DEVEL OPMENT AT HAWAII IN S TITUTE OF GEOPHYSICS, IN PR EP ARATI ON, 2 Figure 2 depicts the assembled OBS. Obvious components f rom top to bottom are the flotation module, pressure case, release, and d ispos able ~nchor. Fl ota tio n consists of five 10- inch diameter g lass spheres which provide 8 kg o f net positive buoyancy upon release of the ballast . The 18-cm outside diameter aluminum pressure vessel :is fabric ated of 7075-T6 alloy and is capable of resisting external pressure to 10-km water depth. Dual explos ive bolts separate the a nchor from the i n strume nt p a ckage at the preset release time, which is con tro l led by two independent clocks . I n the photograph a safety shield covers the release mechanism. Fina l l~ a 45-kg core weight serves as ballast; the complete assembly weighs 37 kg in water and sinks at roughly 1 m/sec.
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