ALBERT

Beschreibung: We show the equivalence of earthquake-induced ground acceleration and water-pressure waveforms for the case of collocated hydrophones and seafloor seismometers installed in shallow water. In particular, the comparison of the waveforms and amplitude 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 proportional 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

Beschreibung: We present the equivalence of the ground acceleration and water pressure variation induced by earthquakes, using data from co-located seafloor instruments in shallow water. The equivalence is evident from a strong similarity of the waveforms, which completely and persistently overlap along the entire duration of the recordings, and it is supported by a high cross-correlation value. Moreover, the comparison of the amplitude spectra of the acceleration and pressure signals confirms the existence of a frequency range of ‘forced oscillations’, where the pressure variations are proportional to the ground vertical component of the acceleration. We demonstrate the equivalence of the signals for a set of local and regional earthquakes on a wide range of magnitude (2.7〈M〈6.8), recorded by seismometers and hydrophones operating in a coastal monitoring infrastructure, at less than 80 m of water depth, located in the Gulf of Pozzuoli (Campi Flegrei caldera, Southern Italy). The infrastructure consists of four marine platforms which integrate a wide set of sensors, in particular a seafloor multi-sensor module, hosting a broad-band seismometer (120s – 100 Hz), a low frequency hydrophone (down to 0.01 Hz) and a bottom pressure recorder. The simple linear relation between sea bottom pressure variation and vertical seafloor acceleration measurement is widely reported in the scientific literature and well verified in previous works. All these works use data from low frequency waveforms of large magnitude tsunamigenic earthquakes, recorded by pairs of bottom pressure recorders and accelerometers. Here, for the first time, short period data recorded by pairs of hydrophones and seismometers are used expanding the theory validity to a larger frequency range. The high correlation value between the ground acceleration, derived from the ground velocity, and hydrophone pressure signals allows to calibrate hydrophones by comparison with co-located accelerometers, or seismometers, used as a reference in a range of frequencies that is very difficult to reproduce in lab. The proved similarity of the recordings underpins the possibility that hydrophones can be more extensively used in place of accelerometers in those marine environment, where seismic sensor accurate installation is usually not easy and not always affordable.

Beschreibung: Unpublished

Beschreibung: San Francisco

Beschreibung: 1IT. Reti di monitoraggio e sorveglianza

Beschreibung: 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.

Beschreibung: Published

Beschreibung: 365–377

Beschreibung: 4V. Processi pre-eruttivi

Beschreibung: JCR Journal