HR: 0830h
AN: S11E-0354    [PDF]
TI: Imaging Inhomogeneous Crustal Structure Related with Arc Magma and Fluids beneath Northern Miyagi Prefecture, Northeastern Japan, by an Envelope Inversion Method
AU: * Asano, Y
EM: asano@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Ten-nodai 3-1, Tukuba, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Ten-nodai 3-1, Tukuba, 305-0006 Japan
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki-aza-aoba, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki-aza-aoba, Sendai, 980-8578 Japan
AB: We have estimated 3-D distribution of scattering coefficients by an envelope inversion method [Asano and Hasegawa, 2003] in the northern part of Miyagi prefecture, northeastern (NW) Japan. In this region, Nakajima and Hasegawa [2003] imaged a clear seismic low velocity zone (LVZ) extending from the uppermost mantle to the crust below an active volcano, Naruko volcano, and to the crust in a seismically active region, the focal area of 1965 M6.5 Northern Miyagi earthquake. We have tried to image short-wavelength inhomogeneous crustal structure by using a large number of observed envelopes of S coda. It is known that the seismic coda is a superposition of seismic waves scattered by such a short-wavelength inhomogeneous structure. Estimated distribution of scattering coefficients provides us information about the short-wavelength inhomogeneous structure, which is hardly imaged by travel-time tomography. We analyzed 1299 three-component seismograms recorded at 44 stations, whose separation is about 10 to 20 km. In the target area we set 2471 grids with a spacing of 7.5-km in the central part, and with a spacing of 15-km in the surroundings. Scattering coefficient values in each grid is unknown parameter to be estimated. The results show that there exist several zones with large scattering coefficients (LSZs). Predominant LSZs are distributed beneath active volcanoes in the backbone range, which indicates a strongly inhomogeneous structure related to the volcanoes. A clear LSZ is also detected around the mainshock fault of 1965 M6.5 Northern Miyagi earthquake. This LSZ in the upper crust suggest the existence of fault-damaged zones along the mainshock fault. The LSZ extends to the deeper extension of the fault and connects to another LSZ beneath Nuruko volcano in the lower crust. This characteristic distribution of the LSZ is well correlated with that of the LVZ imaged by Nakajima and Hasegawa [2003]. The LVZ is characterized by high V$_{p}$/V$_{s}$ in the uppermost mantle and the lower crust, but by slightly low V$_{p}$/V$_{s}$ in the upper crust. They interpreted that the LVZ in the uppermost mantle and the lower crust is caused by the existence of partial melt and that in the upper crust by the existence of H$_{2}$O supplied from the partial melting zone in the lower crust. The present observations suggest that the LSZs reflect the path of fluids from the lower crust to the upper crust, and such distribution of fluid filled cracks can effectively scatter seismic waves.
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Seismology [S]
MN: 2003 Fall Meeting