Uppermost mantle and crustal P and S waves structure beneath the northwestern Pacific Basin using airguns, OBSs and borehole broadband seismometer

Author(s): Fukano, T.; Shinohara, M.; Nakahigashi, K.; Yamada, T.; Mochizuki, K.; Kanazawa, T.; Araki, E.; Suyehiro, K.
Author Affiliation(s): Primary:
University of Tokyo, Earthquake Research Institute, Tokyo, Japan
Japan Agency for Marine-Earth Science and Technology, Japan
Volume Title: AGU 2006 fall meeting
Source: Eos, Transactions, American Geophysical Union, 87( Fall Meeting Suppl.); American Geophysical Union 2006 fall meeting, San Francisco, CA, Dec. 11-15, 2006. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0096-3941 CODEN: EOSTAJ
Note: In English
Summary: In August 2000, the borehole broadband seismometer, WP-2 was installed in the northwestern Pacific basin. Geomagnetic studies in this area indicate the lithosphere was formed 129Ma. The seismic experiments with ocean bottom seismometers (OBSs), WP-2 and airguns were performed around the WP-2 in July 2001, July 2002 and July 2005. To detect a seismic anisotropy in the uppermost mantle, the experiments had four profiles with different directions. Line1 was perpendicular to the magnetic lineations, Line3 was almost parallel to the lineations. In addition, there are two profiles whose directions are different from those of Line1 and Line3. Shallow seismic velocity models just below OBS were derived from using tau-p method and records of a hydrophone streamer. The obtained seismic models were confirmed using the ODP logging data. Deep structures were estimated by forward modeling using a two dimensional ray tracing method. The estimated crustal structures beneath each line are almost identical. In a sediment layer, P wave structure is 1.6km/s, S wave is about 0.2km/s, the thickness of this layer is about 0.4km. P and S wave velocities at the top of layer 2A are 4.6km/s and 2.7km/s, respectively. Layer 2B has P and S wave velocities of 5.3km/s and 3.1km/s at the top of layer, respectively. Total thickness of Layer 2 is about 1.4 km. The data from the WP-2 were useful to estimate detailed structure of the Layer 2. Uppermost Layer 3 has P and S wave velocities of 6.8km/s and 3.8km/s, respectively. The Layer 3 is about 5km thick. To explain large amplitude of Pn phase, a crust-mantle transition layer is required at the bottom of the crust. The Pn velocities are different in each profile. Average velocities of Pn and Sn are 8.2 km/s and 4.7 km/s, respectively. The velocity variations are about 5% for P wave and about 4% for S wave. From this result, a seismic anisotropy of the uppermost mantle is suggested, and the fast direction seems to be perpendicular to the magnetic lineations.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 19 Geophysics, Seismology; Air guns; Anisotropy; Body waves; Crust; Elastic waves; Geophysical methods; Hydrophones; Instruments; Lithosphere; Mantle; Models; North Pacific; Northwest Pacific; Ocean Drilling Program; Ocean bottom seismographs; P-waves; Pacific Basin; Pacific Ocean; Pn-waves; S-waves; Seismic methods; Seismic waves; Seismographs; Sn-waves; Upper mantle; Velocity; Velocity structure; West Pacific
Record ID: 2009066830
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.

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