Methods

The ship's position was determined with a GPS NAVSTAR satellite navigation system. Morphobathymetry was obtained with a Hollming Echos 625 multibeam system, consisting of 15 (12.5 kHz) beams covering a swath of seafloor roughly 2/3 water depth in width. Rock and sediment sampling were carried out by conventional dredging and coring methods. Seismic reflection data were obtained using a Sodera GI gun as sound source. It operated in harmonic mode configuration, with the capacity of 105 cubic inches for the generator as well as for the injector, at the pressure of 2000 psi. The receiving streamer, made by Teledyne, employed 24 channels (each with 20 hydrophones) spaced 25 m apart. Seismic source and nearest channel were spaced 150 m apart. Shot interval was 50 m, allowing six fold coverage. Digital acquisition was carried out with a Geometrics ES2420. with a sampling rate of 1 ms, record length of 11 s, and an antialias filter of 180Hz. Seismic data have been processed at the Institute of Marine Geology of the CNR of Bologna using an industry-standard package (DISCO) made by Cogniseis.

Figure 3: Simplified map of the Romanche area, based on the data by [Heezen et al.(1964)Heezen, Bunce, Hersey, & Tharp], [Chermak(1979)], [Honnorez et al.(1991)Honnorez, Mascle, Basile, Tricart, Villeneuve, & Bertrand], [Monti & Mercier(1991)], [Searle et al.(1994)Searle, Thomas, & Jones] and our own data. Earthquake epicenters (L-DGO data bank) are superimposed.