Variation in Lithospheric Deformation of the Northern Manila Subduction Zone, in

Light of the Seafloor Seismicity Detected by Two OBS Networks

Emmy T.Y. Chang

Institute of Oceanography National Taiwan University, Taipei Taiwan


Based on the global teleseismic observations, the shallow interface seismicity is rather in fragmentary distribution in the Manila subduction system, which relfects the vigorous convergence in-between the South China Sea basin and the Philippine Sea. In order to characterizing the local seismic parameters in the northern Manila trench, two oceanbottom-seismometer (OBS) experiments were carried out in 2005 and 2006, with 7 and

9 instruments (MicrOBS) respectively. Even though each of the OBS experiments held at the seafloor in less than one week, numerous local events have been retrieved, which are little recorded by the seismic observations on land. With a precisely location by a careful examining the local layered velocity models, these events can be grouped into a few seismic clusters in space. It is interesting to see that these events form the continuous distributions through the very shallow layers to the upper mantle. We further performed the near-field waveform to determine focal mechanisms with the events of



>2.5 (program: FASTMECA, now renamed as FMNEAR). The resultant focal mechanism determinations show a strong variation in lithospheric deformation in the study area.