Low frequencies with a 'dense' OBS array: The Atlantis‐Green Canyon earthquake data set

We extracted 2.5 hours of continuous data from the Atlantis OBSnode array around the time of the 10 February, 2006 magnitude-5.2 Green-Canyon earthquake, for the purpose of characterizing the location, mechanism, and significance of this unusual event. The array, consisting of about 500 active 4C nodes, was several tens of kilometers to the South of the earthquake. Typical earthquakes radiate the bulk of their energy at frequencies much lower than the 10Hz geophones used in the nodes were designed to record, so the dataset also served as a testbed for understanding how standard exploration-seismic geophones might be used at frequencies below 5Hz. At traditional frequencies of 10Hz and above the Atlantis airgun signal completely dominates the data. However, filtering away frequencies above 2Hz removes the Atlantis airgun signal and reveals a strong series of arrivals from the earthquake. At “earthquake” frequencies of 2Hz and below, the Atlantis array becomes densely sampled in space (i.e., with more than 2 receivers per wavelength), allowing it to be formed into a powerful directional antenna. Beam steering the array reveals many interesting signals in the data. Examining our own airguns, we find that ”inconsequential” differences in our airgun arrays had unexpected and significant effects. Beam steering resolves a complex sequence of distinct arrivals from the Green-Canyon earthquake spanning 8 minutes of time, and two other mysterious events subsequent to the Green-Canyon earthquake that appear to be temporally related to it, but cannot be traditional aftershocks because they arrive from different azimuths.