EARTHQUAKE NEAR-FIELD AND W-PHASE OBSERVATIONS AT TELESEISMIC DISTANCES

Near field phases can be observed on individual broadband seismograms at teleseismic distances from great earthquakes. Even at distances of 20 degrees-30 degrees, such phases can have amplitudes of several tent hs of a mm. While previous studies have identified near field phases i n vertical component data, I show that such phases can be much more ea sily observed using the transverse component. Although observed effect s agree well with complete waveform calculations using point moment te nsor sources, they do not satisfy some of the assumptions commonly mad e in seismic data analysis. It is therefore suggested that care be tak en in the analysis of very broadband seismograms which may contain the se effects. Re cent W phase observations are shown to be a case in poi nt.