Blue: seismogram for the event that triggered Japan’s earthquake early warning system on January 5th, from this station in central Japan. It is probably the hybrid of two events, and looks weird compared to a more normal earthquake (red). Data from IRIS, plotted using Obspy.
A seismogram that is the hybrid of two events arriving at the seismometer together, compared to a more normal seismogram from a single event. The former lacks the clear sequence of body and surface waves arrivals seen in the latter.