Frequency-magnitude distribution of microearthquakes beneath the 9°50′N region of the East Pacific Rise, October 2003 through April 2004

Relocated hypocentral data from a 7-month deployment (October 2003 to April 2004) of ocean bottom seismometers provide an opportunity to map microearthquake frequency-magnitude distributions (FMDs) along the 9 degrees 49-52'N region on the East Pacific Rise. These analyses, which incorporate more than 9000 earthquakes, represent the first investigation of the 3-D spatial and temporal patterns of FMDs along any mid-ocean ridge spreading center. The data are described well 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 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 ambient stress conditions, with the lowest stress levels at shallow depths and relatively high stress levels (or low pore pressures) observed away from the axial zone. Intermediate b values are observed on-axis above the ridge system's melt lens; however, within this region there also exists significant spatial variability. This indicates that stress conditions and/or structural heterogeneity may vary at subkilometer scales within the hydrothermal circulation cell. Although the observational period is characterized by increasing seismicity rates, building toward an eruptive episode in January 2006, the first-order spatial pattern of b values is sustained, with no overall temporal trend. As a byproduct of this b value analysis, the detection capabilities of the array are assessed empirically.