T-Phase Observations in Global Seismogram Stacks

Abstract:

We show that the T-phase — conversion of acoustic to seismic energy — can be observed in broad-band waveform stacks of earthquakes recorded by the Global Seismic Network. T-phases arrive late at seismic stations or hydrophones as they travel as an acoustic wave through the SOFAR channel for much of their propagation paths. Attenuation of T-phases along continental paths can be high, but by stacking seismograms from many earthquakes we can enhance weak arrivals. To further increase the visibility of the phase arrivals in stacks, we apply STA/LTA filtering to individual traces before stacking. Our stacks show that the T-phase can clearly be observed at stations up to 4° inland from the coast. We also demonstrate that a change in apparent velocity of the T-phase can be observed in the stacks if we select data depending on the continental path length on the receiver side, and how these changes agree with the theory of acoustic to seismic coupling of energy at continents. In addition, analysis of the spatial distribution of energy in the stacks shows clearly that mostly oceanic paths have large amplitudes in the T-phase window. We are attempting to include more stations from regional arrays in order to analyze areas of T-phase blockage and amplitude variability with propagation paths in more detail in order to increase understanding of T-phase generation and propagation processes.