HR: 1340h
AN: T33A-1360    [Abstracts]
TI: Detailed Structure of the Upper Mantle Discontinuities Around the Japan Subduction Zone Imaged by Receiver Function Analyses
AU: * Tonegawa, T
EM: tonegawa@eps.nagoya-u.ac.jp
AF: Nagoya university, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Hirahara, K
EM: hirahara@eps.nagoya-u.ac.jp
AF: Nagoya university, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Shibutani, T
EM: shibutan@rcep.dpri.kyoto-u.ac.jp
AF: Kyoto university, Gokasho, Uji, Kyoto, 611-011 Japan
AB: We applied receiver function (RF) analyses to P wave coda potions of teleseismic events observed at 138 broadband stations in Japan and Korea to procure the Ps phases converted at the upper mantle seismic velocity discontinuities. These broadband stations are consist of F-net, J-array, IRIS, Korean ones, which realize enough high-density spacing to reveal the detailed structure around the Japan subduction zone. After careful inspection, we retrieved 7,725 RFs with good SN ratios from 389 teleseismic earthquakes with the magnitudes of 5.5 and greater. The numbers of used RFs and stations are far larger than those in previous studies, which enables us to investigate the more details in the discontinuity structure. RFs are conventionally constructed through frequency domain division of radial components by vertical ones with a water level of 0.01. Gaussian filters of 1.0, 0.3, 0.17, 0.1 Hz low-pass filter are also applied, respectively. Assuming the phases in RFs are generated by Ps converted ones at depths, we transformed the time domain RFs to the depth domain ones referring to 1-D IASP91 velocity model. Further, in addition to this model, examining 3-D tomographic one and 1-D JMA one, we investigated whether lateral variation in velocity around the subduction zone shifts the discontinuities largely. In our obtained RF images, remarkable positive RF amplitudes corresponding to the Pacific plate (PAC), the 410 km and the 660 km discontinuities appear beneath the whole Japan Islands. The PAC can be traced from the Japan trench to a depth of about 200 km remarkably, and further down to a depth of about 400 km. Phase boundary of the 410 km discontinuity is locally elevated by 30 km inside the cold PAC where the PAC penetrates the 410 km. Positive RF amplitudes dipping westwards near a depth of 660 km describes the phase boundary affected by the stagnation of the subducted PAC on the 660 km discontinuity. These undulations corresponding to the 410 and 660 km discontinuity varies from 380 km to 410 km locally and from 650 km to 700 km gradually with 1-D velocity migration. To investigate the sharpness of the 410 km discontinuity, we examined the frequency dependence of RFs by changing the width of Gaussian filter. Our resultant RF images show that the phase boundary near a depth of 410 km varies in a relatively local scale, in other words, the thickness of the 410 km one beneath Korea spreads out broader than the one beneath Japan. In contrast, the 660 km discontinuity can be seen everywhere and its transition seems to be sharp. Comparison of the 1-D JMA velocity-migrated RFs with the ones referring to recent 3-D tomographic velocity models confirms these obtained undulations are not apparent ones caused by existing lateral seismic velocity variations. Indeed, RF amplitudes of dipping layer in the cross-section referring to 3-D velocity model would be traced well compared with the ones using 1-D JMA one. Also, the absolute depths of the 410 km and 660 km discontinuity with 3-D and 1-D JMA velocity models are imaged deeper than the one with 1-D IASP91 one. They were estimated from 390 km to 420 km and 680 km and 730 km. This is caused by that 1-D JMA and 3-D velocity models include the lower velocity portion than 1-D IASP91 one above a depth of 400 km.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting