Time-clustering behavior of spreading-center seismicity between 15 and 35 degrees N on the Mid-Atlantic Ridge: observations from hydroacoustic monitoring

An earthquake catalog derived from the detection of seismically-generated T-waves is used to study the time-clustering behavior of moderate-size (greater than or similar to3.0 M) earthquakes between 15 and 35degreesN along the Mid-Atlantic Ridge (MAR). Within this region, the distribution of inter-event times is consistent with a non-periodic, non-random, clustered process. The highest degrees of clustering are associated temporally with large mainshock-aftershock; however, some swarm-like activity also is evident. Temporal fluctuations characterized by a power spectral density P(f) that decays as 1/f(alpha) are present within the time sequence, with alpha ranging from 0.12 to 0.55 for different regions of the spreading axis. This behavior is negligible at time scales less than similar to5 x 10(3) s, and earthquake occurrence becomes less clustered (smaller alpha) as increasing size thresholds are applied to the catalog. A power-law size-frequency scaling foe Mid-Atlantic Ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes, or source levels. Although fractal seismic behavior have been linked to the structure of the underlying fault population in other environments, power-law fault size distributions have not been observed widely in the mid-ocean ridge setting. (C) 2003 Elsevier B.V. All rights reserved.