Analysis and Preliminary Results of the Seafloor Earthquake Array–Japan-Canada Cascadia Experiment (SeaJade)

Abstract:

SeaJade is a collaborative project involving research organizations from Japan, Canada and the US to study the Cascadia subduction zone off the west coast of Vancouver Island. The purpose of the experiment is to better understand the seismic behavior of plate boundaries and associated faults, and the source characteristics of earthquakes that result from the interaction and build-up of tectonic stress during earthquake cycles. The first phase of SeaJade was initiated in July 2010, with the deployment of 32 short-period ocean bottom seismometers (OBS) from JAMSTEC for 3 months and 10 broadband OBS’ from WHIO for one year. Phase II began in October 2013 with the deployment of 8 broadband seismometers in the Nootka Sound region of Vancouver Island, followed by the deployment of 35 OBS’ in December for 9 months with an observational emphasis on the Nootka fault zone between the Juan de Fuca plate and the Explorer plate. Using SeaJade I data, we have identified and located 1276 local earthquakes (including origin times, hypocentral locations and magnitudes), analyzed converted phases to constrain the locations and depths of seismic interfaces, and developed a seismic tomography model via joint hypocentral relocation and velocity model inversion. Preliminary results indicate that most seismic events were associated with the Nootka fault zone. At least two seismic interfaces within the Cascadia fore-arc could be identified. A comprehensive study of the M_w 6.6, April 24^th 2014, Nootka earthquake sequence using the SeaJade II dataset is currently underway. Relocations of aftershocks confirms that the Nootka earthquake probably involved two seismogenic structures. In addition to the NE–SW Nootka fault zone, a NW–SE-striking fault within the Explorer subducted slab might also be involved in the rupture process.