About the SWIFT system

SWIFT (Source parameter determination based on Waveform Inversion of Fourier Transformed seismograms) developed at NIED by Nakano et al. (2008), estimates both the moment rate function and centroid moment tensor (CMT) of earthquakes based on waveform inversion of long-period seismic waveforms in the frequency domain, enabling efficient and rapid computations. SWIFT assumes a point source and a pure double-couple source mechanism in order to stabilize the solution when using data from a small number of stations. A spatial grid search of strike, dip, and rake angles identifies the best-fitting fault parameters and the source centroid that minimizes the normalized residual between the observed and synthetic displacement seismograms in the frequency domain. The SWIFT system is triggered by receipt of an email providing initial earthquake information determined by the SeisComP3 system at NIED (https://www.seiscomp3.org/). This information is used as an initial source location for the spatial grid search, which is performed by an adaptive grid spacing procedure (Nakano et al. 2008). We adopt the fault parameter and source location showing the minimum residual.

Currently the SWIFT system implemented at NIED performs the automatic and manual estimation of source mechanism parameters for large seismic events (M>6) in South-East Asia (Indonesia and Philippines) and Latin America. Manual CMT solutions for smaller events (M>4.5) are also routinely obtained for Indonesia and Philippines.

Data selection

Waveform data at individual stations within 1500 km of the epicenter are retrieved, checked for amplitude saturations and data gaps, and a band-pass filter is applied to the data that are free of saturations and gaps. Frequency range for the inversion analysis is set between 50 and 100 s for events with magnitude less than or equal to 8, and between 50 and 200 s for events with magnitude above 8. The band-passed waveforms with signal-to-noise amplitude ratios greater than 4 are selected. SWIFT inversion is performed only if data from 2 or more stations is available. The waveform data are decimated to a sampling frequency of 0.5 Hz and a total length of 512 s (30 s before and 482 s after the origin time) is used. A further waveform data screening for the automatic process is applied based on event magnitude and station distance from the epicenter; for events with magnitude above 8 only data for stations with epicentral distances above 1000 km are used, for magnitudes between 7 and 8 only data from stations farther than 500 km are used, and all data are used for events with magnitudes below 7.

References

Nakano M, Kumagai H, Inoue H (2008) Waveform inversion in the frequency domain for the simultaneous determination of earthquake source mechanism and moment function. Geophys J Int 173:1000-1011. doi:10.1111/j.1365-246X.2008.03783.x

Inazu D., Pulido, N. and E. Fukuyama, J. Senda and H. Kumagai, Near-field tsunami forecast system based on near real-time seismic moment tensor estimation in the regions of Indonesia, the Philippines, and Chile, Earth Planets and Space, 68, 1-18, 2016.