DETERMINATION OF THE SEISMIC MOMENT TENSOR USING SURFACE WAVES RECORDED BY

The Provisional Technical Secretariat (PTS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) has been ramping-up the installation of the International Monitoring System (IMS) consisting of a network of seismic, hydroacoustic, infrasound, and radionuclide stations, since its inception in March 1997. Data from this network are automatically processed at the International Data Centre (IDC) to produce, within a few hours, a series of automatic bulletins called the Standard Event Lists (SEL1, SEL2, SEL3). After analyst review and correction as necessary the Reviewed Event Bulletin (REB) is produced. Additional information about characterization of an event as an earthquake or otherwise is also available in the Standard Event Bulletin (SEB) shortly after production of the REB. The Comprehensive Nuclear-Test-Ban Treaty (CTBT) states that the IDC will apply standard event screening criteria to each event formed. The objective of this process is to filter out events that are considered to be consistent with natural or non-nuclear man-made phenomena, leaving a reduced set of events that may require further examination. In Annex 2 of the Protocol to the Treaty, the focal mechanism is listed as possible event screening parameters. In order to provide a focal mechanism and increase the number of elements potentially useful as screening attributes, we have in the last two years implemented two methods for moment tensor (MT) inversion. One method is based on the P body waves and the other on surface waves. Implementation of these sophisticated inversion methods has led to improvements in the calibration of the broad-band seismic network, notably quality control of the instrument responses. We report here on the results obtained from the implementation of the surface-wave MT inversion, which uses both Rayleigh waves and Love waves. We are presenting results of the application of this inversion scheme on selected events as well as statistics showing the results of automatic use of the method. Compared to using body waves, we find that using the surface waves allows us to lower the magnitude at which it is possible to obtain an automatic MT solution.