T phase observations in global seismogram stacks

Abstract

The T phase, conversion of acoustic to seismic energy, is typically observed as a high-frequency wave train at hydrophones or coastal seismic stations. Here we show that the T phase can be observed in broadband waveform stacks of ∼5200 earthquakes recorded by the Global Seismic Network. To enhance the phase arrivals in stacks, we apply short-time window average over long-time window average filtering to individual traces before stacking. Although the T phase arrival is visible in stacks from seismograms filtered at 0.5-5 Hz, it appears much stronger at higher frequencies (2-8 Hz) and is further enhanced by only stacking seismograms from oceanic paths. Stacking only subsets of the data depending on continental path lengths on the receiver side shows that the T phase can be observed at stations up to 4° inland from the coast, and changes in the T phase arrival time correspond to reasonable crustal velocities.