Seismic structure of the Haida Gwaii region using tomography and multiple migration of OBS data

English Abstract:

The second largest (M7.7) instrumentally recorded earthquake in Canada occurred on Oct 27^th 2012 off the coast of Moresby Island, Haida Gwaii. The epicenter of this thrust event was located in the region of Queen Charlotte terrace, which was formed as a result of 20 degree oblique convergence between the Pacific and the North American plates. Earlier studies indicate more than 20,000 aftershocks, and tsunami wave heights of 3-5 m in the Moresby Island region. Accurate location of these events was not possible as all the land stations on Haida Gwaii are located east of the main aftershock area. As well, detailed seismic velocity structure needed to accurately locate these events is currently not available. Therefore, to obtain the sediment velocities to accurately locate these events and to obtain detailed crustal structure of the area, a small active source ocean bottom seismometer (OBS) experiment was conducted in January 2013 around the main aftershock area in the Queen Charlotte terrace region. Additionally, six single-channel seismic (SCS) profiles were acquired across these six OBS stations. Preliminary results of first arrival travel time tomography velocity analyses show clear refractions with apparent velocities up to 6.0 km/s, which are believed to be from the upper crust. SCS data show clear images of the Queen-Charlotte Transform Fault, and the 520 cubic inch airgun was strong enough to generate reflections from the upper crust, i.e. the top of the Pacific Plate. Additionally, a bottom-simulating reflector (BSR) is identified for the first time in the Queen Charlotte Terrace region, which indicates the presence of gas hydrate. OBS refraction velocity analyses corresponding to the BSR reflection indicate high-velocity sediment with P-wave velocities of 2.0-2.5 km/s. Furthermore, OBS multiple migration technique which was previously applied on the north Cascadia margin, is being used to obtain the detailed crustal structure of the aftershock region.