When the Hydrophone Works as an Accelerometer

Abstract

We show the equivalence of earthquake-induced ground acceleration and water-pres- sure waveforms for the case of collocated hydrophones and seafloor seismometers installed in shallow water. In particular, the comparison of the waveforms and ampli- tude spectra of the acceleration and water-pressure signals confirms the existence of a frequency range of “forced oscillations” in which the water-pressure variations are pro- portional to the vertical component of the ground acceleration. We demonstrate the equivalence of the acceleration and water-pressure signals for a set of local earthquakes (epicenter distance of a few tens of kilometers) and regional earthquakes with a wide range of magnitude (2:7 < M w < 6:8), recorded by seismometers and hydrophones oper- ating in shallow water (depth less than 80 m) in the Campi Flegrei caldera (southern Italy). We describe the “forced oscillations” theory, and we demonstrate the signals equivalence in the frequency range 0.1–10 Hz, thus extending the frequency range of application of the hydrophones as accelerometers. The high correlation between the ground acceleration, derived from the ground velocity, and hydrophone pressure signals in the mentioned frequency range enables the use of the hydrophone wave- forms for standard seismological studies (i.e., earthquake source). The calibration of hydrophones by comparison with collocated accelerometers, or seismometers, is also enabled in a range of frequencies that is very difficult to reproduce in a laboratory. The results of our work also open the possibility of hydrophones being more exten- sively used in place of accelerometers in marine environments where accurate installa- tion of seismic sensors is difficult or unaffordable.