SWIFT (Source parameter determination based on Waveform Inversion of Fourier Transformed seismograms) developed by Nakano et al. (2008) estimates both the moment function and centroid moment tensor (CMT) based on waveform inversion of long-period seismic signals in the frequency domain, which enables 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. Because band-passed waveforms are used in the inversion, the resultant moment function is a band-passed form. The seismic moment is estimated from the moment function deconvolved from its band-passed form (Nakano et al. 2008). SWIFT has been used to study source mechanisms of earthquakes in various regions: Indonesia (Nakano et al. 2008, 2010a, 2010b), the Philippines (Punongbayan et al. 2015; Bonita et al. 2015), Turkey (Görgün 2013; Görgün and Görgün 2015; Nakano et al. 2015), and Japan (Ando et al. 2012; Kumagai et al. 2012). The SWIFT system is triggered by receipt of an email providing earthquake information determined by the SeisComP3 system (https://www.seiscomp3.org/) if an estimated earthquake magnitude (M) is greater than 4.0. Waveform data at individual stations within 1500 km of the epicenter are retrieved between 10 min before and 9 min after the earthquake origin time. The unprocessed waveform data are first checked for amplitude saturations and data gaps and a band-pass filter between 50 and 100 s is then applied to the data that are free of saturations and gaps. The noise and signal amplitudes are estimated from peak-to-peak amplitudes during periods 200 s before and 482 s after the origin time, respectively, in each band-passed waveform. The band-passed waveforms with signal-to-noise amplitude ratios greater than 4 and the source amplitude ratios (Sakai et al. 2016) less than 11 are selected. If the selected waveforms are from more than 2 stations, the SWIFT inversion is performed. 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. The hypocenter given by SeisComP3 is used as an initial source location for the spatial grid search in which we use the adaptive grid spacing of Nakano et al. (2008). At each source node, a grid search of fault parameters (strike, dip, and rake angles) is conducted, and waveform inversion in the frequency domain is performed to estimate the moment function (band-passed form) for each combination of angles. We adopt the fault parameter and source location showing the minimum residual, in which the deconvolved form of the moment function is estimated to determine the seismic moment and moment magnitude (Mw). If the estimated Mw is greater than 7.0, the above processes are repeated for the waveform data band-passed between 50 and 200 s.
This project was supported by the JST-JICA SATREPS project in the Philippines (2010-2014) entitled “Enhancing Earthquake and Volcano Monitoring Capabilities and Promoting Effective Utilization of the Disaster Information in the Philippines.”. The earthquake source component (SWIFT) is a product of joint collaboration with Nagoya University, Japan and National Research Institute for Earth Science and Disaster Prevention (NIED), Japan. The system is sustainably maintained by PHIVOLCS up to present and used as one of our tool for earthquake monitoring and tsunami warning.
SWIFT CMT is intended to aid PHIVOLCS earthquake monitoring and tsunami warning operations and for it's related research. If you use the PHIVOLCS - SWIFT CMT in research that you submit for publication, we ask only that you cite these three papers:
Bonita JD, Kumagai H, Nakano M (2015) Regional moment tensor analysis in the Philippines: CMT solutions in 2012–2013. J Disaster Res 10:18-24
Punongbayan J, Kumagai H, Pulido N, Bonita JD, Nakano M, Yamashina T, et al. (2015) Development and operation of a regional moment tensor analysis system in the Philippines: Contributions to the understanding of recent damaging earthquakes. J Disaster Res 10:25-34
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