January 2006 seafloor-spreading event at 9°50′N, East Pacific Rise: Ridge dike intrusion and transform fault interactions from regional hydroacoustic

An array of autonomous underwater hydrophones is used to investigate regional seismicity associated with the 22 January 2006 seafloor-spreading event on the northern East Pacific Rise near 9 degrees 50'N. Significant earthquake activity was observed beginning 3 weeks prior to the eruption, where a total of 255 earthquakes were detected within the vicinity of the 9 degrees 50'N area. This was followed by a series of 252 events on 22 January and a rapid decline to background seismicity levels during the subsequent 3 days. Because of their small magnitudes, accurate locations could be derived for only 20 of these events, 18 of which occurred during a 1-h period on 22 January. These earthquakes cluster near 9 degrees 45'N and 9 degrees 55'N, at the distal ends of the young lava flows identified posteruption, where the activity displays a distinct spatial-temporal pattern, alternating from the north to the south and then back to the north. This implies either rapid bilateral propagation along the rift or the near-simultaneous injection of melt vertically from the axial magma lens. Short-duration T wave risetimes are consistent with the eruption of lavas in the vicinity of 9 degrees 50'N on 22 January 2006. Eruptions on 12 and 15-16 January also may be inferred from the risetime data; however, the locations of these smaller-magnitude events cannot be determined accurately. Roughly 15 h after the last earthquakes were located adjacent to the eruption site, a sequence of 16 earthquakes began to the north-northeast at a distance of 25-40 km from the 9 degrees 50'N site. These events are located in vicinity of the Clipperton Transform and its western inside corner, an area from which the regional hydrophone network routinely detects seismicity. Coulomb stress modeling indicates that a dike intrusion spanning the known eruptive zone to the south (9 degrees 46'-9 degrees 56'N) would act to promote normal faulting or a combination of normal faulting and transform slip within this region, with stress changes on the order of 1-10 kPa.