QuickTime Movies: Juxtaposition of Seismograms to Assess Aftershock Viability Within the Mainshock Coda
Goal: When a large earthquake (mainshock) occurs, the following smaller earthquakes (aftershocks) are often obscured by the large decaying amplitude of the mainshock's seismic waves (coda). This makes it difficult to identify early aftershocks. Here, we use visualization techniques to quantify when early aftershocks are identifiable within the mainshock coda.
Method: We linearly add seismograms from a magnitude 3.2 earthquake to the continuous waveform data of the 2001 sequence. We start with an enforced 60 second time shift between the two earthquakes. For this large time lag the magnitude 3.2 earthquake is easily identified in both the time domain and the spectragram for all stations within 70 km of the mainshock. We interactively reduce the lag time between the two events by 1 second and assess when the magnitude 3.2 earthquake is no longer visible within the mainshock coda. A Hanning window taper is initially applied to the waveform data. Using a Hilbert transformation, the envelope of each seismograms is computed and then smoothed using a centered 50-point moving average (equivalent to a 0.5 second time window). The spectrograms are computed using 2.0 second windows with an overlap of 0.01 seconds. Conversion from the time-domain to the frequency domain is performed using a multitaper method (Thomson, 1982; Vernon, 1989) using the MATLAB routine PMTM with the time-bandwidth product set to 4.
Component HHZ [~36 MB; Quicktime movie file Kilb_HHZ_small_corrected.mov].
Component HHE [~28 MB; Quicktime movie file Kilb_HHE_small_corrected.mov].
Component HHN [~31 MB; Quicktime movie file Kilb_HHN_small_corrected.mov].
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