Figure F7. Hydroseismograms for (A, B) distant (10 s sampling) and (C) local earthquakes (1 Hz sampling using the NEPTUNE cable connection). Comparison of borehole and seafloor signals in A and B shows P-waves to be attenuated in the formation relative to the seafloor by the same amount as the tidal loading signal, whereas the seismic Rayleigh waves produce large pressure signals in the formation. The remarkable match of the seafloor pressure record in C with that of a nearby broad-band seismometer (blue and red traces, respectively) demonstrates the high fidelity of recording afforded with the NEPTUNE connection, allowing both high-frequency oceanographic (e.g., before the earthquake) and low-frequency seismic loading to be studied.
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