HR: 1340h
AN: T23B-1416    [Abstracts]
TI: Spatial and temporal patterns of microearthquake frequency-magnitude distributions (b- values) within the axial region of the East Pacific Rise near 9-50 N
AU: * Bohnenstiehl, D R
EM: drbohnen@ncsu.edu
AF: Department of Marine Earth and Atmospheric Sciences North Carolina State University, Campus Box 8208, Raleigh, NC 27695, United States
AU: Waldhauser, F
EM: felixw@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Box 1000, Palisades, NY 10964,
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Box 1000, Palisades, NY 10964,
AB: Relocated hypocentral data from a seven-month deployment of ocean bottom seismometers provides an opportunity to map microearthquake frequency-magnitude distributions (FMDs) along the 9 49-52 N region on the East Pacific Rise. These analyses, which incorporate more 9000 earthquakes, represent the first investigation of the 3-D spatial and temporal patterns of FMD's along any mid-ocean ridge spreading center. The data are well described by a Gutenberg-Richter model, indicating a power-law or fractal relationship between earthquake size and frequency. The scaling exponent, or b-value, shows significant spatial variability, exceeding a value of 2.0 at the shallowest depths (less than 0.5 km) on axis and dropping below 1.0 away from the axial trough. This spatial pattern is consistent with an inverse relationship between b-value and effective stress, with the lowest stress levels at shallow depths and relatively high stress levels (or low pore pressures) observed away from the hydrothermal cell. Intermediate b-values are observed in the 0.5-1.5 km depth range above the axial magma chamber; however, within this region significant variability also exists along axis. This heterogeneity suggests that the effective stress may vary at sub-kilometer scales within the hydrothermal reservoir. Although the rate of seismic activity increases by roughly a factor of three throughout the deployment (Oct. 2003-Apr. 2004), the frequency-magnitude distribution shows no temporal trend. As a by-product of this b- value analysis the detection capabilities of the array are assessed empirically in both space and time.
UR: http://web.mac.com/drbohnen/iWeb/MG_NCSU/Welcome.html
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting