Wide-angle seismic experiment crossing the Sofu-gan tectonic line in the Izu-Ogasawara arc KY0507 cruise

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1 JAMTEC Report of Research and Development, Volume, March, Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise eiichi Miura, Takeshi ato, Tetsuo o, arumi Takahashi, huichi Kodaira and Yoshiyuki Kaneda Abstract We conducted a wide-angle seismic experiment using a large volume (l) airgun array and ocean bottom seismographs (OBs) in the central and southern Izu-Ogasawara region. The experiment was KY- cruise using R/V Kaiyo of Japan Agency for Marine-Earth cience and Technology (JAMTEC) during July and August,. Objectives of the experiment are to reveal a velocity structure of the central and southern Izu-Ogasawara region, and to constrain the structural variation along the profile, especially crossing the ofu-gan tectonic line. The total shot number of the large volume airgun array was, which were recorded by the OBs and a -ch hydrophone streamer towing simultaneously. In this paper, we summarize information of the experiments and introduce OB and streamer data. Keywords: crustal structure, seismic, wide-angle data, OB, Izu-Ogasawara. Introduction An oceanic island arc is one of the best examples to study a process of the crustal growth, because the crustal growth had been started by a subduction of an oceanic crust beneath the other oceanic crust and the tectonic history is simpler than that of a continental arc, which had been separated from the continental margin with complex structure. The oceanic arc is one of products formed by subduction factory ). The Izu-Ogasawara arc is a typical oceanic island arc and has formed by the subduction of the Pacific Plate beneath the Philippine ea Plate since Eocene time ). Recently, the wide-angle refraction and reflection experiment was conducted in the northern Izu- Ogasawara arc transect, so that the layer with -km/s of P-wave velocity is found and interpreted as the granitic middle crust )). However, there is bathymetric variation along the Izu-Ogasawara arc: bathymetric high as islands and seamounts, and bathymetric low as deep region between the seamounts. The volume of volcanic material also varies along the northern arc ). These results indicate that the igneous activity in the Izu- Ogasawara arc is not occurred homogeneously in space and time but varied along the arc. And, the velocity model along the northern Izu-Ogasawara arc reveals that the crustal thickness has the large variation and the Moho depth is tend to be shallower in south than those of north ). Moreover, two boundaries separating the Izu- Ogasawara arc are confirmed by the bouguer gravity anomaly: one is located on southern Tori-shima and another is located around Kinyo seamount ). It is inferred from large variation of the crustal structure along the arc that the nature of the crustal growth varies in the space. The ofu-gan tectonic line is crossing the Izu- Ogasawara arc in E-W direction and proposed as the boundary divided into the north and south parts of the arc )). At this tectonic line, four structural characteristics (the distribution of bathymetric high, chemical compositions of rock samples in volcanic front, hypocenter distribution associated with the Pacific Plate subduction, and back arc depressions) differ at the south and north parts )). Yuasa ) mentioned that these differences are reflected by the different development histories between two parts at the tectonic line. Moreover, on the basis of topography characters, this arc has topographic gap between umisu-shima and Tori-shima, which is divided into northern and central parts of the arc ). However, to clarify the variation of the crustal structure, the seismic survey has not been conducted around this tectonic line. The objectives of this study are to reveal the structural variation along the Izu-Ogasawara arc, especially in both sides of ofu-gan tectonic line, and to understand the nature of crustal growth of subduction factory.. Experiment We conducted a wide-angle seismic survey using an airgun array and ocean bottom seismographs (OBs) to achieve above objectives (Figure ). This cruise using R/V "Kaiyo" of Japan Agency for Marine-Earth cience Japan Agency for Marine-Earth cience and Technology, Japan Agency for Marine-Earth cience and Technology, - howa-machi, Kanazawa-ku, Yokohama, -, JAPA

2 Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise Figure : Map of the experimental area. A black line is the seismic line (IBr). Red circles and three-digit numbers indicate OB locations and OB numbers, respectively. Blue circles with three-digit numbers are example OBs shown in Figures -. and Technology (JAMTEC) were from July to August,. The R/V Kaiyo departed from Yokosuka-shinko at July and started OB deployment from northern part of the seismic line until July. Airgun shooting carried out until July. And, we recovered OBs in two periods from July to and from July to because of a typhoon. Then, we carried out a high-density shooting reflection experiment using G-gun array from July to. Finally, we transited to JAMTEC and arrived at August. The detailed activities are shown in Table and Figure. The seismic line of the cruise was about -km long along the Izu-Ogasawara arc traversing the main axis of volcanic front from the umisu-shima to north of the Kita-Iwo-shima. To clarify the structural variation of whole crustal scale, we need a long line to receive the deep penetrating signals with long offset. Moreover, the Figure : Map for ship's track line. Cross marks indicates ship position of every hours. northern part of the seismic line was overlapped about -km with the former seismic line conducted in ) for understanding continuous structural variation of whole Izu-Ogasawara arc scale. The northernmost and southernmost shot points of the seismic line were -., -. E and -., -. E, respectively. We used OBs and an airgun array with, cubic inches (l) capacity as the source to reveal deep profiling of the crust and uppermost mantle. Along the seismic line, we towed a -channel hydrophone streamer to constrain the shallow structure because of the severe variation of the topography. Airgun signals were recorded by not only the OBs but also the hydrophone streamer. Moreover, the high-density reflection experiment was conducted using small volume array (G-gun) and a -ch. hydrophone streamer from the north of the ofu-gan to Kayo seamount to reveal the shallower structure than acoustic basement. JAMTEC Rep. Res. Dev., Volume, March,

3 . Miura et al. Table : Activity log during KY cruise. Date July July July July July July July July July July July July July July July July July July July July July July July July August Remarks Departure from JAMTEC, transit to OB# OB deployment (OB#-OB#) OB deployment (OB#-OB#) OB deployment (OB#-OB#) and airgun shooting from south to north Airgun shooting from south to north and OB retrieval and re-deployment of OB# Airgun shooting from south to north Airgun shooting from south to north Airgun shooting from south to north and from north to south Airgun shooting from north to south Airgun shooting from north to south Airgun shooting from north to south Airgun shooting from north to south and OB retrieval (OB#-#) OB retrieval (OB#-OB#) OB retrieval (OB#-OB#) OB retrieval (OB#-OB#) OB retrieval (OB#-OB#) and transit to Tokyo Bay to escape the typhoon taying in Tokyo Bay to recover the sea condition taying in Tokyo Bay to recover the sea condition taying in Tokyo Bay to recover the sea condition Transit to survey area OB retrieval (OB#-OB#) OB retrieval (OB#-OB#) and high-density reflection experiment High-density reflection experiment and OB retrievals Transit to JAMTEC Arrival to JAMTEC This experimental method is basically same as those described by Takahashi et al. ).. Ocean Bottom eismographs We deployed OBs on the seismic line in this cruise with km interval (Figure, Table ). OB locations were calculated by -D ray tracing using expected velocity model referring to those of the northern Izu- Ogasawara arc of previous studies ))). However, six OBs were not recovered. All OBs were equipped with three-component geophones (vertical and two horizontal components perpendicular each other) using gimbal-leveling mechanisms and a hydrophone sensor. atural frequency of these geophones was. Hz. Our OBs and the digital recorder system were originally designed by Kanazawa and hiobara ) and hinohara et al. ). The digital recorder used a -bit A/D converter and stored data on digital audiotape or a hard disk sampling continuously with original format ). The electronic power for the recorder system of OB is supplied by rechargeable lithium-ion or alkali batteries. Above geophone sensors with gimbal-leveling mechanism, batteries and a recorder system are installed in inch glass sphere by Benthos, Inc. The glass sphere is stored in the yellow hard hat. To enable easy OB retrieval after arriving at sea surface, each OB is attached to a flash light and a radio beacon with coded signals. An OB is deployed by free fall and retrieved by melting releaser composed of stainless steel plates connecting the OB with a weight when a transponder system receives acoustic signal sent from a vessel. This acoustic communication between the OB and the vessel was performed using transducers installed on the vessel. Positions of OBs on sea bottom are estimated by BL of the vessel s positioning system during the cruise. After the cruise, we edited the continuous OB data with length of s and EG-Y format. At the same time, calibration of the OB clock for GP time was carried out using difference times between OB clock and GP time, which measured just before OB deployment and just after OB retrieval.. Large volume airgun shooting To obtain the good quality OB data and observe the phases from the deep structure, we carried out the high spatial shooting with m interval (about - s interval depending on the ship speed) by twice shooting on a same line (Figure ). Table shows the shooting log. The airgun array with total capacity of, cubic inches consists of eight Bolt long life airguns with, cubic inches capacity each. The air pressure sent to JAMTEC Rep. Res. Dev., Volume, March,

4 Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise Table : OB information. Each recorder using or hard disk is shown by each abbreviation of or "". The "" and "" means that makers of the hydrophone sensor are Benthos Inc. and High Tech Inc., respectively. Deployment Retrieval ITE Time UTC Lat () Coodinate Lon (E) Dep (m) Time UTC Lat () Coordinate Lon (E) Dep (m) type hyd maker / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : JAMTEC Rep. Res. Dev., Volume, March,

5 . Miura et al. Table : (continued). Deployment Retrieval ITE Time UTC Lat () Coodinate Lon (E) Dep (m) Time UTC Lat () Coordinate Lon (E) Dep (m) type hyd maker / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / :. : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : / : JAMTEC Rep. Res. Dev., Volume, March,

6 Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise chamber was, psi. The geometry of the seismic experiment is shown in Figure. The two floats with two airguns each were deployed from port and starboard sides, respectively. The airgun array s size is. m length x. m width. Airgun s position was located. m behind the ship position (distances from ship antenna to tail of ship, and from tail of ship to center of the airgun array, are. m and. m, respectively). As the differential global positioning system (DGP) of the ship navigation system, kyfix system was used (aha as the nearest station). The accuracy of shooting position was about m. Figure : Map of airgun shooting. Blue arrows show a direction of the shooting.. Multichannel hydrophone streamer and recording system To reveal the shallow structure, we towed a -channel hydrophone streamer (TEALTHARRAY T-) during airgun shooting. (Figure a). The detailed descriptions are reported by Takahashi et al. ). The interval of each channel was m. The lengths of active section and read-in cable until the ship end are m and m, respectively. The sampling rate was ms and the record length was. s. A seismic system of R/V "Kaiyo" consists of a navigation system with software PECTRA, a recording system (tratavisor X Marine) and a gun controller system (GC), and these systems are connected via RTµ (Figure b). Table : Airgun shooting log. IBr_obs- Time (UTC) Latitude () Longitude (E) Depth (m) P First shot // :.'.' top shot // :.'.' Restart shot // :.'.' top shot // :.'.' Restart shot // :.'.' Last shot // :.'.' IBr_obs- Time (UTC) Latitude () Longitude (E) Depth (m) P First shot // :.'.' Last shot // :.'.' IBr_obs- Time (UTC) Latitude () Longitude (E) Depth (m) P First shot // :.'.' Last shot // :.'.' IBr_obs- Time (UTC) Latitude () Longitude (E) Depth (m) P First shot // :.'.' Last shot // :.'.' IBr_G Time (UTC) Latitude () Longitude (E) Depth (m) P First shot top shot Restart shot Last shot // : // : // : // :.'.'.'.'.'.'.'.' JAMTEC Rep. Res. Dev., Volume, March,

7 . Miura et al. (a) ide View. m. m R/V "Kaiyo" Lead-in cable Tail bouy m Airgun Airgun Airgun Airgun ch ch ch ch ch ch ch ch ch ch ch ch m Up View. m R/V "Kaiyo" Hydrophone solid streamer. m. m (b) tar Fire MEA-GGA (time, ship position, heading etc.) kyfix & Terminal erver OJ data (time, ship position, heading etc.) Master Clock (for OB) RT avigation data ystem start Linux PC installed PECTRA hot data Trigger signal Trigger signal data hot signal Airgun controller GC First Time Break Airgun Airgun Airgun Airgun Recording start Recording system E Tape drive tratavisor X Marine Analog seismic data treamer cable Depth controller ch ch ch ch ch ch ch ch ch ch ch ch Digicource data Figure : MC system (after Takahashi et al., ). (a) Geometry of airgun system and the hydrophone streamer. (b) Flow chart for the MC recording system. High density shooting with small volume airgun array (G-gun) A small volume airgun array was composed of four cu. in. airguns (G-gun: Bolt Inc.) in order to reveal the fine shallow structural images (Figure ). The four G-gun were shot simultaneously in every -m, which was about s. Prior to the survey recording, a shooting test was conducted to design the preferable source signature with varied shooting delays, and the adopted pattern was no delay and simultaneous shooting. G-gun s position was located. m behind the ship position (distances from ship antenna to tail of ship, and from tail of ship to center of the airgun array, are. m and. m, respectively). The controlling and recording system was same as that of ordinary shooting and recording mentioned above.. Data In this chapter, we introduce examples of the seismic data obtained by OBs and MC. Vertical components Figure : Photograph for G-gun array. G-gun array is composed of four cu. in. G-guns fixed with a metal flame. White dotted rectangle shows a G-gun. of OB#, OB# and OB#, and horizontal components of OB# are described section.. MC data are described below section.. JAMTEC Rep. Res. Dev., Volume, March,

8 Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise. OB We retrieved OBs and did not retrieved OBs. Except some OB data with recorder troubles, the data quality of available OBs is basically good and we can trace the first phases on vertical records until about km distance from each OB. Horizontal records also show good quality despite of poorer / ratio than the vertical, and we can see converted arrivals until about km from the OB. We describe characteristics of OB data using vertical record sections of OB# (Figure ), OB# (Figure ) and OB# (Figure ) as follows. OB# was deployed in deep between Tori-shima and ofu-gan. We can trace first arrivals until an offset of km from the OB (Figure ). The apparent velocities of first phases in north sides are.,.,.,.,. and over km/s in offsets of -, -, -, -, - and over km, respectively. In southern side, apparent velocities of first arrivals are.,. and. km/s in offsets of -, - and - km, respectively. Variation of these apparent velocities drawing concave shape can be seen at offsets of - km in northern side and -, - and - km due to some seamounts. Reflections from the Moho (PmP) with high amplitudes can be also seen at an offset of - km. OB# was deployed in deep near Kayo seamounts. On northern side, the apparent velocities are.,. and over km/s in offsets of -, - and over km, respectively (Figure ). In southern side, the apparent velocities of these phases are.,. and over. km/s in offsets of -, - and over km, respectively. We can see the apparent velocities variation of first phases at a distance of - km in northern side and - and - km in southern side. Reflections from the Moho (PmP) with high amplitudes can be seen at a northern offset of - km. OB# was deployed between Kaikata and Kaitoku seamounts. We can trace first phases in northern side with apparent velocities of.,. and. km/s in offsets of -, - and - km, respectively (Figure ). In southern part, the apparent velocities of first phases are.,.,. and. km/s in offsets of -, -, - IBr_OB (V) T-D/ (s) Filter: Band Pass (.Hz -.Hz) Reduction Velocity:. Km/sec. Distance (km) Figure : Vertical record section recorded by OB#. All traces are filtered by - Hz. Vertical and horizontal axes are offsets from OB and reduced traveltimes by km/s. IBr_OB (V) T-D/ (s) Filter: Band Pass (.Hz -.Hz) Reduction Velocity:. Km/sec. Distance (km) Figure : Vertical record section recorded by OB#. The details are same as for Figure. JAMTEC Rep. Res. Dev., Volume, March,

9 . Miura et al. and - km, respectively. These variation is also effected the topography near the Kaitoku seamount. Figures a and b indicate two horizontal components of OB# crossing perpendicularly each other. We can see many phases with same apparent velocities to P-wave. This record section seems to have much PP phases. At an offset of - km and - s in reduced time by. km/s in southern side, we can see some phases with slow apparent velocities and these phases probably correspond to P phases.. MC The reflection data recorded by a -channel hydrophone streamer has also enough quality to pick the acoustic basement (Figure a). Applied flows were a collection of spherical divergence, editing bad quality IBr_OB (V) T-D/ (s) Filter: Band Pass (.Hz -.Hz) Reduction Velocity:. Km/sec. Distance (km) Figure : Vertical record section recorded by OB#. The details are same as for Figure. (a) IBr_OB (H) T-D/. (s) Filter: Band Pass (.Hz -.Hz) Reduction Velocity:. Km/sec. Distance (km) (b) IBr_OB (H) T-D/. (s) Filter: Band Pass (.Hz -.Hz) Reduction Velocity:. Km/sec. Distance (km) Figure : Horizontal record sections recorded by OB#. All traces are filtered by - Hz. The reduced velocity is. km/s. (a) Horizontal component-. (b) Horizontal component-. JAMTEC Rep. Res. Dev., Volume, March,

10 Wide-angle seismic experiment crossing the ofu-gan tectonic line in the Izu-Ogasawara arc KY cruise (a) P ( P = km) TWT(s) (b) P ( P = km) TWT(s) Figure : (a) MC profile along the seismic line. Vertical and horizontal axes are two-way traveltime (s) and shotpoint (P), respectively ( P = km). Dotted rectangle indicates the G-gun shooting location. (b) G-gun profile along the seismic line. Vertical and horizontal axes are two-way traveltime (s) and shotpoint (P), respectively ( P = km). traces, a predictive deconvolution filtering using an operator length of ms and an predictions distance of ms, stacking at shot points, a time variant bandpass filter of - Hz and the auto gain control. For the G- gun data, the range of bandpass filter is - Hz since the high frequency signals (Figure b). Because of the channel interval of m and the shot interval of m and the fold number was less., we stacked the traces not the CDPs but the shot points. The profiles show that rough topographic features of seamounts (Figure ). The seamounts have little sediments on the summits. Bathymetric deep between seamounts are sediments recognized part, and thick sediments are distributed in limited area as northern part of the seismic line.. ummary In this paper, we summarize the seismic experiment in the central and southern Izu-Ogasawara arc area and introduce the seismic data of OB and MC data. The recording condition is good enough to trace the first arrival phases about km offsets. From the data, we will reveal the structural variation of central and southern Izu-Ogasawara arc area especially crossing the ofu-gan tectonic line. Moreover, we will estimate the volume and rate of continental crustal growth. Acknowledgement We greatly appreciate to following members of KY cruise shipboard party described following, and would not be able to get succession of this seismic experiment without their efforts. We thank Dr. T. Tsuru, Dr. Y. Kaiho, Dr. M. Yamashita and Dr. J-O Park for planning discussion of this cruise. Marine technicians Chief Makoto Ito Masato ugano Yui Hashimoto Ikumasa Terada JAMTEC Rep. Res. Dev., Volume, March,

11 . Miura et al. Crew Captain Chief Officer Chief Officer econd Officer Third Officer Assistant Officer Chief Engineer First Engineer econd Engineer Third Engineer Chief Radio Officer econd Radio Officer Third Radio Officer Boatswein o. Oiler Oiler Oiler Oiler Oiler Chief teward teward teward teward teward Hidenori hibata Ichiro ara obuo Kojima Kozue Kurihara Maki Ito Mai Komura Miho Ido Tomoaki Kudoh Kimiko erizawa Hitoshi Tanaka Kouji amejima Rikita Yoshida Akihisa Tsuji Kazunori Kamiya Atsushi Takao Tatsuo Jidozono Kazuhiko Kaneda Chikara Onohara Takafumi Tominaga Hiroyasu aitake Kiyotaka Yamashita Youhei Yamamoto Kingo akamura Kouzou Yatougo eiichi Goto Kinnya houji Hatsuo Oda Yasuo Konno Takumi Yoshida Kazuaki akai Hideo Hatakeyama Hiroyuki Ohishi Keita Funawatari Masanori ato Takeshi Miyauchi Tomohide onoda Yukio Tachiki Kouji Kirita Isao Matsumoto Reference ) Y. Tatsumi, Magmatological approach to subduction zone process: towards a comprehensive understanding of arc evolution, J. Geol. oc, Japan,, -, () (in Japanese with English abstract). ) D. E. Karig, and G. F. Moore, Tectonic complexities in the Bonin arc system, Tectonophysics,, - (). ) K. uyehiro,. Takahashi, Y. Ariie, Y. Yokoi, R. Hino, M. hinohara, T. Kanazawa,. Hirata, H. Tokuyama, and A. Taira, Continental crust, crustal underplating, and low-q upper mantle beneath an oceanic island arc, cience,, - (). ). Takahashi, K. uyehiro, and M. hinohara, Implications from the seismic crustal structure of the northern Izu-Ogasawara arc, The Island Arc,, - (). ) K. uga, and K. Fujioka, Volume of volcanic materials along northern Izu-Bonin arc, Bull. Volcanol. oc. Jpn. er.,, - () (in Japanese with English abstract). ). Kodaira, T. ato,. Takahashi, A. Ito, and Y. Kaneda, Crustal evolution process of an oceanic island arc inferred from crustal/uppermost mantle structures results from seismic imaging along the volcanic front in the Izu arc, Abst. the eismological ociety of Japan Fall Meeting, p. () (in Japanese). ) Y. Ueda, Bouguer gravity anomalies (ver. ) of Japanese island arcs and its adjacent seas, Rept. Hydrgr. Res.,, - () (in Japanese with English abstract). ) M. Yuasa, orth-south geomorphological discrepancy observed in Izu-Ogasawara Island arc, Monthly Earth,, - () (in Japanese). ) M. Yuasa, ofugan Tectonic Line, a new tectonic boundary separating northern and southern parts of the Ogasawara (Bonin) arc, northwest Pacific, in Formation of Active Ocean Margins,. asu, K. Kobayashi,. Uyeda, I. Kushiro, and H. Kagami, Eds. (TERRAPUB, Tokyo,), pp. -. ) M. Yuasa, The reason for the difference in geologic phenomena between northern and southern parts of the Izu- Ogasawara arc, J. Geography,, - () (in Japanese with English abstract). ) M. Yuasa, and F. Murakami, Geology and geography of the Ogasawara arc and the ofugan tectonic line, J. Geography,, - () (in Japanese with English abstract). ). Takahashi,. Kodaira, T. ato, A. Ito, T. o and Y. Kaneda, Wide-angle seismic profiling of arc-arc collision zone in the northern Izu-Ogasawara arc KY cruise, JAMTEC Report of Research and Development,, - (). ) T. Kanazawa, and H. hiobara, ewly developed ocean bottom seismometer, Abst. Japan Earth and Planetary cience Meeting,, (). ) M. hinohara, K. uyehiro,. Matsuda, and K. Ozawa, Digital recording ocean bottom seismometer using portable digital audio tape recorder. J. Jpn. oc. Mar. urv. Tech.,, - () (in Japanese with English abstract). (Received December, ) JAMTEC Rep. Res. Dev., Volume, March,