HR: 0800h
AN: T21C-0540    [Abstracts]
TI: Seismicity rates of slow, intermediate, and fast spreading ridges: Insights from long-term hydroacoustic monitoring
AU: * Dziak, R P
EM: robert.p.dziak@noaa.gov
AF: Oregon State University/NOAA, 2115 SE OSU Dr, Newport, OR 97365
AU: Haxel, J H
EM: joe.haxel@noaa.gov
AF: Oregon State University/NOAA, 2115 SE OSU Dr, Newport, OR 97365
AU: Bohnenstiehl, D R
EM: del@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, P.O. Box 1000 207C Oceanography, Palisades, NY 10964
AU: Goslin, J
EM: jgoslin@univ-brest.fr
AF: U.Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Plouzane, 10964 France
AB: Ocean basin earthquakes recorded on NOAA/OSU and U.S. Navy hydrophone arrays are used to evaluate long-term volcano-tectonic seismicity levels from segments of the fast-spreading rate East Pacific Rise (EPR) from 20$\deg$S-20$\deg$N, intermediate-spreading rate Juan de Fuca Ridge (JdFR) from 39$\deg$-52$\deg$N and Galapagos Rift (GR) from 90$\deg$-103$\deg$W, and the slow-spreading northern Mid-Atlantic Ridge (MAR) from 5$\deg$-60$\deg$N. The hydrophones record the acoustic energy of seafloor earthquakes that propagate along the ocean sound channel with little attenuation over large distances. Frequency-magnitude relationships (Bohnenstiehl et al., 2002; Dziak et al., 2004) indicate the hydrophone catalogs are complete in these regions to body-wave magnitude $\sim$2.5 (EPR and GR), 2.5 (JdFR), and 3.0 (MAR), an improvement of 1.5 to 2 units over the land-based seismic catalogs for mid-ocean ridge systems. Using the hydrophone earthquake catalog, we will compare seismicity rates of the JdFR (12 years of data), to seismicity rates along the GR (6 years) and EPR (6 years) and MAR (4 years of data from 5$\deg$-39$\deg$ N; 16 months from 39$\deg$-60$\deg$ N). During these monitoring periods, five confirmed seafloor spreading events (four of which were associated with magmatic activity) were recorded on discrete JdFR segments, while 6 possible magmatic events were observed on the EPR, one on the GR, and one on the MAR. Empirical orthogonal functions will be used to elucidate the space-time patterns of seismicity and compare between the various spreading rates ridges, as well as to investigate the recurrence rate of seafloor spreading events present. In addition, single-link cluster analysis (SLC; Frolich and Davis, 1990) will be used to de-cluster the earthquake databases to reduce the effects of aftershock sequences and magmatic swarms, allowing us to evaluate how overall plate motion and changes in spreading rate effect levels of seismicity between ridge segments and different ridge systems. Preliminary results indicate the distribution of seismicity at the JdFR "super"-segments (between transforms) are positively skewed, indicating there are significant, yet brief periods of time (1-2 months) when seismic activity is well above the mean. Transforms along the JdFR, however, exhibit a random distribution.
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 4259 Ocean acoustics
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting