Project Summary

In January 1999, six autonomous hydrophones, designed and built by NOAA’s Pacific Marine Environmental Laboratory (PMEL), were moored on the flanks of the Mid-Atlantic Ridge (MAR) between 15 and 35N to monitor the seismicity of a portion of this slow-spreading ridge [Smith et al., 2001].

The region of the MAR within the hydrophone array is part of the French American Ridge Atlantic (FARA) project, which extends from 15-40N. This section of the MAR has been subject to intensive study over the past several years. Multibeam bathymetry data have been collected along the entire length of the ridge axis, and several surveys have extended well off-axis. The FARA region includes several sites of multi-disciplinary study: well-known examples of which include the MARK area near 23N, the TAG hydrothermal vent field near 26N, the segments between the Hayes and Oceanographer transforms (33-35N), and the FAMOUS area near 37N.

There have been a number of detailed studies of teleseismic earthquakes on the MAR. In addition, a number of local microseismicity experiments using ocean bottom seismometers (OBS) have been conducted at various locations along the MAR. All of these studies provide a framework within which we can place the results of the hydrophone monitoring. The differences in temporal and spatial scales of the various types of seismic studies (teleseismic, OBS, and hydrophone) require that they be integrated to understand more fully the seismic character of this portion of the slow spreading MAR.

Preliminary results from the first year of hydrophone data [Smith, 2002] showed some general seismic patterns that were previously unknown. The magnitude of completeness of the hydrophone data set is ~3.0, with a number of smaller events (magnitudes < 2.5) being recorded. A significant variability in event rate along the axis of the MAR was observed within the array. Groups of neighboring segments appear to behave similarly, producing an along-axis pattern with high and low levels of seismic activity. We suggested that this broad scale pattern is likely influenced by the axial thermal regime. In addition, data from off-axis indicate that most seismic faulting occurs within 15 km of the axis center. Finally, several earthquake sequences were detected that have variable temporal characteristics, suggesting fundamental differences in the causes of their seismicity.

An additional year of data have been added since the initial study. The ~2 years of monitoring (day 52, 1999 – day 78, 2001) yielded a total of 4922 hydroacoustic events from throughout the North Atlantic basin recorded by four or more hydrophones. A total of 3485 of these events were located within the hydrophone array. This data set provides us with an unparalled view of the seismicity of the ridge over a broad region and over a broad range of event magnitudes. We are in the process of submitting a paper on the details of the relationship between the spatial and temporal patterns of the seismic activity recorded by the hydrophones and the structure of the ridge, especially with regard to ridge axis segmentation

References

1. Smith, D. K., M. Tolstoy, C. G. Fox, D. R. Bohnenstiehl, H. Matsumotu, M. Fowler, Hydroacoustic monitoring of seismicity at the slow-spreading Mid-Atlantic Ridge, Geophys. Res. Letts., 29, 2002.

2. Smith, D. K., J. Escartin, M. Cannat, M. Tolstoy, C. G. Fox, D. R. Bohnenstiehl, S. Bazin, Spatial and temporal distribution of seismicity along the northern Mid-Atlantic Ridge (15°-35°N), J. Geophysic. Res., in press, 2003.