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Acoustic detection of a seafloor spreading episode on the Juan de Fuca Ridge using military hydrophone arrays

Christopher G. Fox, W. Eddie Radford

Pacific Marine Environmental Laboratory, NOAA, Hatfield Marine Science Center, Newport, Oregon

Robert P. Dziak, Tai-Kwan Lau, Haruyoshi Matsumoto, and Anthony E. Schreiner

Oregon State University/Cooperative Institute for Marine Resources Studies, Hatfield Marine Science Center, Newport, Oregon

Geophysical Research Letters, 22, 131-134 (1995)
Copyright ©1995 by the American Geophysical Union. Further electronic distribution is not allowed.

Gallery of Figures

Fig. 1. The real-time detection system is designed to display combined spectrograms based on two formed acoustic beams overlapping at preselected sites along the spreading center axis. Spectra for 1-minute data segments are translated to source time based on calculated travel times (t1, t1) from each site to each receiving array. The spectra are then combined by selecting the minimum power (P) from each array for each frequency (f). Where an event occurs outside the location of interest (A), its energy appears on only one array and the combined spectrum displays the other member of the pair. When an event occurs within the location of interest (B), its energy appears in both arrays and is displayed.

Fig. 2. Spectrogram based on crossed SOSUS beams of initial onset of seismic activity from the central Juan de Fuca Ridge on June 26, 1993. Dark colors represent high acoustic power at a given time and frequency. Low frequency noise, interpreted to represent harmonic tremor, with small embedded earthquakes begins at 21:43 GMT. Shipping noise appears as parallel vertical lines.

Fig. 3. Three-dimensional relief of the north rift zone of Axial Seamount and the CoAxial segment, showing the location of earthquake epicenters detected by NOAA/SOSUS real-time detection system during the period June 26-July 15, 1993. The neovolcanic zone of the CoAxial segment is shown as a black line. Seismicity associated with an inferred lateral dike injection originates on the volcanic rift zone of Axial Seamount and migrates NNE to an eruptive site on the CoAxial segment (triangle, near right). The high relief of the volcanic rift zone may influence the location of epicenters.


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