Microearthquake activity on the Orozco Fracture Zone: Preliminary results from Project ROSE

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Abstract
We present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi‐institutional network of ocean bottom seismometers and ocean bottom hydrophones in the Orozco Fracture Zone in the eastern Pacific during late February to mid‐March 1979. The network was deployed as part of the Rivera Ocean Seismic Experiment, also known as Project ROSE. The Orozco Fracture Zone is physiographically complex, and the pattern of microearthquake hypocenters at least partly reflects this complexity. All of the well‐located epicenters lie within the active transform fault segment of the fracture zone. About half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the Cocos‐Pacific relative plate motion; these events appear to be characterized by strike‐slip faulting. The second major group of activity occurred in the central portion of the transform fault; the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading, and several are not obviously associated with distinct topographic features. Hypocentral depth was well resolved for many of the earthquakes reported here. Nominal depths range from 0 to 17 km below the seafloor.