HR: 0800h
AN: T21C-0543    [Abstracts]
TI: Long-term Spatial and Temporal Variations in Seismic Activity Along the Juan de Fuca Plate System (Northeast Pacific Ocean) Recorded on Military Hydrophone Arrays
AU: * Haxel, J H
EM: joe.haxel@noaa.gov
AF: Oregon State University/ NOAA, Hatfield Marine Science Center 2115 SE OSU Drive, Newport, OR 97365 United States
AU: Dziak, R P
EM: robert.p.dziak@noaa.gov
AF: Oregon State University/ NOAA, Hatfield Marine Science Center 2115 SE OSU Drive, Newport, OR 97365 United States
AU: Bohnenstiehl, D R
EM: del@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W Box 1000, Palisades, NY 10964-8000 United States
AU: Fowler, M J
EM: matt.fowler@noaa.gov
AF: Oregon State University/ NOAA, Hatfield Marine Science Center 2115 SE OSU Drive, Newport, OR 97365 United States
AB: Since August 1991 U.S. Navy hydrophones have been used by NOAA/OSU to monitor seismicity from seafloor spreading centers in the northeast Pacific Ocean. Over the last decade, more than 10,000 earthquakes have been located using the Tertiary (T-) wave of these seafloor events recorded by this hydro-acoustic monitoring system. Before a reliable analysis of the long-term behavior of northeast Pacific seismicity could be undertaken, a magnitude completeness filter (~208 dB) as well as a single link cluster (SLC) algorithm (Frolich and Davis, 1990) were applied to each segment of the ridge and plate systems. The SLC technique reduces clusters of earthquakes caused by large tectonic sequences or volcanic eruptions to single events in the catalog. This removes any bias due to large events from the monthly average of seismicity over the record length. From analysis of the long-term monthly average of events, a substantial increase in seismicity is observed in the entire northeast Pacific earthquake database beginning in 1998 with a peak in late 2000 and trailing back down during 2001. Events per month during this period of heightened seismicity often double the mean value ($\mu$ = 43.8 $\pm$ 2.0 events/month). The entire record was also broken down into the geologic segments of the Explorer Ridge, Sovanco Transform, Juan de Fuca Ridge, Blanco Transform, and Gorda Ridge revealing similar increases in seismicity along each segment contributing to the overall increase observed in the composite time series. First order statistics reveal the Blanco ($\mu$$_{Bl}$ = 20.1 $\pm$ 0.9 events/month) as the most active component followed by the Gorda ($\mu$$_{Gr}$ = 8.3 $\pm$ 0.5 events/month) and Juan de Fuca segments ($\mu$$_{Jf}$ = 6.7 $\pm$ 0.7 events/month). In addition, the distribution of seismicity at ridge segments are positively skewed, indicating there are significant, yet brief periods of time when seismic activity is well above the mean whereas transforms exhibit a more random distribution. Empirical orthogonal functions (EOF) will be used to further decompose the time-space modes of variability in seismicity both within and between ridge and transform segments, as well as to investigate the recurrence rate of seafloor spreading events present in the non-declustered earthquake database. We will also explore correlations of bottom pressure with long-term signals in seismicity as well as any correlation with the slow earthquakes at the Cascadia Subduction Zone documented by {\it Szeliga et al., 2004}.
DE: 7220 Oceanic crust
DE: 5475 Tectonics (8149)
DE: 4259 Ocean acoustics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting