Realtime source inversion using W-phase and GRiD MT for regional tsunami early warning

For tsunami early warning purposes we have implemented a W-phase source inversion algorithm (Kanamori and Rivera, 2008) into the GRiD MT system which had been developed earlier at the Earthquake Research Institute (Tsuruoka et al., 2009). W-phase is a very long-period (200-1000s) phase starting right after the P-wave arrival, and is suitable for fast source parameter retrieval. The GRiD MT system currently in use continuously monitors long-period seismic wavefield at a period of 20-50s and automatically and simultaneously determines the origin time, location and seismic moment tensors within 3min of the event occurrence. The new system using W phase works at a much longer-period band and consists of modules that continuously perform the following tasks: (1) receiving data, (2) decimation, deconvolution and filtering (retrieval of W-phase), (3) moment tensor inversion, (4) detection of seismic events. When broad-band seismograms are available at regional distances (i.e. delta <= 10deg), we can detect seismic events and determine satisfactory solutions within 6 min after the earthquake occurrence. We performed a systematic test on several recent Japanese events to investigate the performance of the system. We will discuss several examples (e.g.Ibaraki-oki, 2008; Iwate-oki, 2008) in detail. The preliminary results suggest that this system provides rapid and robust source parameters useful for tsunami warning purposes.