Frequency-dependent coherence of T waves from repeating earthquakes

Monitoring deep ocean temperature is critical for understanding the global warming, as over 90% of the excessive heat is trapped in the ocean. Wu et al. (2021) showed that travel time changes of T waves from repeating earthquakes can be used to monitor the large-scale warming of the deep ocean . Like the widely studied surface waves in seismology, T waves at different frequencies have sensitivities to ocean properties that peak at different depths, so their frequency-dependent travel time anomalies can reveal the vertical structure of ocean temperature anomalies . A problem in measuring frequency-dependent travel times, however, is the decreased waveform coherence of T waves at high frequencies. At ~4 Hz, the coherence drops sharply and prohibitively. Although some of this drop in coherence can be explained by source property discrepancies, data from high-quality repeating events indicate a major contribution from ocean property changes . We attribute the sharp drop in coherence at ~4 Hz primarily to the appearance of higher modes of the ocean waveguide, which experience different travel time changes from the fundamental mode. Numerical simulations of T-wave propagation replicate this frequency-dependent coherence, supporting our interpretation. To make use of frequencies above ~4 Hz, where T waves usually have a much higher signal-to-noise ratio than at lower frequencies, we propose to deploy vertical hydrophone arrays, with which modes could be separated for improved depth resolution and weak signals from smaller earthquakes would become usable for seismic ocean thermometry.