Last deployment

In the morning of Day 12 of the cruise we successfully deployed our last OBS – WD54, 32 hours ahead of schedule! (The deck looks so empty now.) This was not, however, an intended location for this OBS. Half way through this cruise we decided to move one of the OBSs to near the aftershock zone of the M7.9 Offshore Kodiak earthquake.  It struck ~300 km offshore Kodiak Island in the early morning hours of January 23, 2018, in the outer rise region of the Alaska-Aleutian subduction zone. It triggered tsunami warnings and prompted evacuations of thousands of people in Alaskan coastal communities. While the source parameters (such as seismic moment tensor) for the earthquake suggested strike-slip faulting (hence no significant tsunami generated), the true complexity of the source has only become evident through analysis of multiple datasets. At least 4 conjugate strike-slip faults were involved in the earthquake rupture. However, distant location of the aftershock source region to the land-based stations made the data analysis and interpretation difficult. Previously on Leg 1 cruise, a couple of OBS stations were already serendipitously placed near or in the aftershocks zone (the OBS placement was decided before the earthquake occurred). Leg 2 was supposed to add a couple more, but none immediately on top of the aftershock cluster. So, after consultations with the PI group we moved WD54 from its originally intended location to the aftershock cluster. This enhanced network of OBS sensors in the aftershock zone will give seismologists a much better control over the earthquake locations and will help characterize the aftershock sequence with much better accuracy. This will further improve our understanding of tectonics offshore Kodiak Island and aid in earthquake and tsunami hazard characterization for the region.

Since we were ahead of schedule, as an additional bonus we decided to collect more bathymetry and sonar data from the 2018 M7.9 earthquake source region. We designed a grid of profile lines crisscrossing the aftershock region that we were planning to be sailing along until the captain said it was time to head back. The winds and wave patterns, however, did not cooperate with our design and we had to modify direction of our survey a little. The plan still is to arrive in Seward in the early morning hours on Wednesday, July 25.

Map with the January 23, 2018 M7.9 earthquake aftershocks and nearest OBS sites. Legend: yellow circles – background M>=3 seismicity for the past 25 years; white squares and corresponding beach balls – previous earthquakes with moment tensor solutions; black circles – 2018 aftershocks; red circles – M>=4 aftershocks; white circle and large beach ball – GCMT solution for the M7.9 earthquake; white lines – fracture zones; white triangles with labels – AASCE OBS locations.  Inset map: NAP – North American Plate, PP – Pacific Plate (the arrow shows direction of convergence), YT – Yakutat Terrain. Red starts show locations of the 1987 M7.8 and 1988 M7.7 Gulf of Alaska earthquakes.

Last OBS is prepped for deployment on the deck.

Last OBS is afloat.

Good bye! See you next summer!

By Natalia Ruppert

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